aof.c

/*
 * Copyright (c) 2009-2012, Salvatore Sanfilippo <antirez at gmail dot com>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   * Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of Redis nor the names of its contributors may be used
 *     to endorse or promote products derived from this software without
 *     specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include "server.h"
#include "bio.h"
#include "rio.h"
#include "functions.h"

#include <signal.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/resource.h>
#include <sys/wait.h>
#include <sys/param.h>

/* redis7 的 aof 功能和之前版本的 aof 功能在文件管理方面有了很大的改变
 * redis7 对 aof 文件进行了分类，分为以下 4 类文件（临时文件不用管）
 * 1) manifest: 清单文件，该文件的内容对应 aofManifest 结构，记录所有其他类型文件的文件信息，并进行了分类
 * 2) base: 基础文件，就是在执行 bgrewrite 的时候，当前数据的 RDB 快照或者 AOF，该文件的文件名后缀也是以 rdb 或者 aof 结尾，取决于 aof-use-rdb-preamble 配置项
 * 3) incr: 增量命令文件，执行 bgrewrite 之后到的写命令，会存入该文件
 * 4) history: 历史文件，执行 bgrewrite 之后，之前存在的 incr，base 文件就是历史文件，会通过 bio 关闭文件或者删除文件
 */

void freeClientArgv(client *c);
off_t getAppendOnlyFileSize(sds filename, int *status);
off_t getBaseAndIncrAppendOnlyFilesSize(aofManifest *am, int *status);
int getBaseAndIncrAppendOnlyFilesNum(aofManifest *am);
int aofFileExist(char *filename);
int rewriteAppendOnlyFile(char *filename);
aofManifest *aofLoadManifestFromFile(sds am_filepath);
void aofManifestFreeAndUpdate(aofManifest *am);
void aof_background_fsync_and_close(int fd);

/* ----------------------------------------------------------------------------
 * AOF Manifest file implementation.
 *
 * The following code implements the read/write logic of AOF manifest file, which
 * is used to track and manage all AOF files.
 *
 * Append-only files consist of three types:
 *
 * BASE: Represents a Redis snapshot from the time of last AOF rewrite. The manifest
 * file contains at most a single BASE file, which will always be the first file in the
 * list.
 *
 * INCR: Represents all write commands executed by Redis following the last successful
 * AOF rewrite. In some cases it is possible to have several ordered INCR files. For
 * example:
 *   - During an on-going AOF rewrite
 *   - After an AOF rewrite was aborted/failed, and before the next one succeeded.
 *
 * HISTORY: After a successful rewrite, the previous BASE and INCR become HISTORY files.
 * They will be automatically removed unless garbage collection is disabled.
 *
 * The following is a possible AOF manifest file content:
 *
 * file appendonly.aof.2.base.rdb seq 2 type b
 * file appendonly.aof.1.incr.aof seq 1 type h
 * file appendonly.aof.2.incr.aof seq 2 type h
 * file appendonly.aof.3.incr.aof seq 3 type h
 * file appendonly.aof.4.incr.aof seq 4 type i
 * file appendonly.aof.5.incr.aof seq 5 type i
 * ------------------------------------------------------------------------- */

/* Naming rules. */
/* AOF 各种类型的文件命名规则 */
/* 基本文件，做 bgrewrite 的时候 redis 数据快照文件 */
#define BASE_FILE_SUFFIX           ".base"
/* 增量命令文件，做 bgrewrite 之后到达的命令写入该文件 */
#define INCR_FILE_SUFFIX           ".incr"
/* 使用 rdb 编码格式在 bgrewrite 的时候存储 base 文件的文件后缀标识 */
#define RDB_FORMAT_SUFFIX          ".rdb"
/* 不使用 rdb 编码格式在 bgrewrite 的时候存储 base 文件的文件后缀标识，和 incr 文件使用的后缀标识 */
#define AOF_FORMAT_SUFFIX          ".aof"
/* 清单文件后缀，该类型的文件会存储所有其他类型文件的文件名 */
#define MANIFEST_NAME_SUFFIX       ".manifest"
/* 临时文件前缀 */
#define TEMP_FILE_NAME_PREFIX      "temp-"

/* AOF manifest key. */
/* 下面三个宏在生成 aof 文件信息字符串的时候被使用 */
/* 文件名 */
#define AOF_MANIFEST_KEY_FILE_NAME   "file"
/* 文件的序列号 */
#define AOF_MANIFEST_KEY_FILE_SEQ    "seq"
/* 文件的类型 */
#define AOF_MANIFEST_KEY_FILE_TYPE   "type"

/* Create an empty aofInfo. */
aofInfo *aofInfoCreate(void) {
    return zcalloc(sizeof(aofInfo));
}

/* Free the aofInfo structure (pointed to by ai) and its embedded file_name. */
/* 释放 aofInfo 实例空间 */
void aofInfoFree(aofInfo *ai) {
    serverAssert(ai != NULL);
    /* 释放其文件名空间 */
    if (ai->file_name) sdsfree(ai->file_name);
    zfree(ai);
}

/* Deep copy an aofInfo. */
/* 深拷贝一份 aofInfo 实例 */
aofInfo *aofInfoDup(aofInfo *orig) {
    serverAssert(orig != NULL);
    aofInfo *ai = aofInfoCreate();
    ai->file_name = sdsdup(orig->file_name);
    ai->file_seq = orig->file_seq;
    ai->file_type = orig->file_type;
    return ai;
}

/* Format aofInfo as a string and it will be a line in the manifest. */
/* 将 aofInfo 实例格式化成字符串，
 * 这个主要用于将内存中的清单信息写入文件的时候使用 */
sds aofInfoFormat(sds buf, aofInfo *ai) {
    sds filename_repr = NULL;

    if (sdsneedsrepr(ai->file_name))
        filename_repr = sdscatrepr(sdsempty(), ai->file_name, sdslen(ai->file_name));

    /* 格式：file xxx seq xxx type xxx */
    sds ret = sdscatprintf(buf, "%s %s %s %lld %s %c\n",
        AOF_MANIFEST_KEY_FILE_NAME, filename_repr ? filename_repr : ai->file_name,
        AOF_MANIFEST_KEY_FILE_SEQ, ai->file_seq,
        AOF_MANIFEST_KEY_FILE_TYPE, ai->file_type);
    sdsfree(filename_repr);

    return ret;
}

/* Method to free AOF list elements. */
/* 释放 aofInfo 列表中的一个元素 */
void aofListFree(void *item) {
    aofInfo *ai = (aofInfo *)item;
    aofInfoFree(ai);
}

/* Method to duplicate AOF list elements. */
void *aofListDup(void *item) {
    return aofInfoDup(item);
}

/* Create an empty aofManifest, which will be called in `aofLoadManifestFromDisk`. */
/* 创建一个 aofManifest 结构 */
aofManifest *aofManifestCreate(void) {
    aofManifest *am = zcalloc(sizeof(aofManifest));
    am->incr_aof_list = listCreate();
    am->history_aof_list = listCreate();
    /* 给 incr aof 信息列表和历史 aof 文件列表绑定释放函数和复制函数 */
    listSetFreeMethod(am->incr_aof_list, aofListFree);
    listSetDupMethod(am->incr_aof_list, aofListDup);
    listSetFreeMethod(am->history_aof_list, aofListFree);
    listSetDupMethod(am->history_aof_list, aofListDup);
    return am;
}

/* Free the aofManifest structure (pointed to by am) and its embedded members. */
/* 释放 aofManifest 实例 */
void aofManifestFree(aofManifest *am) {
    if (am->base_aof_info) aofInfoFree(am->base_aof_info);
    if (am->incr_aof_list) listRelease(am->incr_aof_list);
    if (am->history_aof_list) listRelease(am->history_aof_list);
    zfree(am);
}

/* 获取 AOF 清单文件名 appendonly.aof.manifest */
sds getAofManifestFileName() {
    return sdscatprintf(sdsempty(), "%s%s", server.aof_filename,
                MANIFEST_NAME_SUFFIX);
}

sds getTempAofManifestFileName() {
    return sdscatprintf(sdsempty(), "%s%s%s", TEMP_FILE_NAME_PREFIX,
                server.aof_filename, MANIFEST_NAME_SUFFIX);
}

/* Returns the string representation of aofManifest pointed to by am.
 *
 * The string is multiple lines separated by '\n', and each line represents
 * an AOF file.
 *
 * Each line is space delimited and contains 6 fields, as follows:
 * "file" [filename] "seq" [sequence] "type" [type]
 *
 * Where "file", "seq" and "type" are keywords that describe the next value,
 * [filename] and [sequence] describe file name and order, and [type] is one
 * of 'b' (base), 'h' (history) or 'i' (incr).
 *
 * The base file, if exists, will always be first, followed by history files,
 * and incremental files.
 */
/* 将 aofManifest 清单格式化成字符串 */
sds getAofManifestAsString(aofManifest *am) {
    serverAssert(am != NULL);

    sds buf = sdsempty();
    listNode *ln;
    listIter li;

    /* 1. Add BASE File information, it is always at the beginning
     * of the manifest file. */
    /* 格式化 base 文件信息，其实就是调用 aofInfoFormat 函数，后面两种文件信息也一样 */
    if (am->base_aof_info) {
        buf = aofInfoFormat(buf, am->base_aof_info);
    }

    /* 2. Add HISTORY type AOF information. */
    listRewind(am->history_aof_list, &li);
    while ((ln = listNext(&li)) != NULL) {
        aofInfo *ai = (aofInfo*)ln->value;
        buf = aofInfoFormat(buf, ai);
    }

    /* 3. Add INCR type AOF information. */
    listRewind(am->incr_aof_list, &li);
    while ((ln = listNext(&li)) != NULL) {
        aofInfo *ai = (aofInfo*)ln->value;
        buf = aofInfoFormat(buf, ai);
    }

    return buf;
}

/* Load the manifest information from the disk to `server.aof_manifest`
 * when the Redis server start.
 *
 * During loading, this function does strict error checking and will abort
 * the entire Redis server process on error (I/O error, invalid format, etc.)
 *
 * If the AOF directory or manifest file do not exist, this will be ignored
 * in order to support seamless upgrades from previous versions which did not
 * use them.
 */
/* 从磁盘中加载 aof 清单文件 */
void aofLoadManifestFromDisk(void) {
    server.aof_manifest = aofManifestCreate();
    if (!dirExists(server.aof_dirname)) {
        serverLog(LL_DEBUG, "The AOF directory %s doesn't exist", server.aof_dirname);
        return;
    }

    /* 获取 aof manifest 文件名 */
    sds am_name = getAofManifestFileName();
    /* 获取 aof manifest 文件路径 */
    sds am_filepath = makePath(server.aof_dirname, am_name);
    if (!fileExist(am_filepath)) {
        serverLog(LL_DEBUG, "The AOF manifest file %s doesn't exist", am_name);
        sdsfree(am_name);
        sdsfree(am_filepath);
        return;
    }

    /* 从磁盘中加载 aof manifest 文件，并根据其内容生成 aofManifest 结构 */
    aofManifest *am = aofLoadManifestFromFile(am_filepath);
    /* 释放之前的 aofManifest 结构实例，更改为现在的 am */
    if (am) aofManifestFreeAndUpdate(am);
    sdsfree(am_name);
    sdsfree(am_filepath);
}

/* Generic manifest loading function, used in `aofLoadManifestFromDisk` and redis-check-aof tool. */
#define MANIFEST_MAX_LINE 1024
aofManifest *aofLoadManifestFromFile(sds am_filepath) {
    const char *err = NULL;
    long long maxseq = 0;

    aofManifest *am = aofManifestCreate();
    FILE *fp = fopen(am_filepath, "r");
    if (fp == NULL) {
        serverLog(LL_WARNING, "Fatal error: can't open the AOF manifest "
            "file %s for reading: %s", am_filepath, strerror(errno));
        exit(1);
    }

    char buf[MANIFEST_MAX_LINE+1];
    sds *argv = NULL;
    int argc;
    aofInfo *ai = NULL;

    sds line = NULL;
    int linenum = 0;

    while (1) {
        if (fgets(buf, MANIFEST_MAX_LINE+1, fp) == NULL) {
            if (feof(fp)) {
                if (linenum == 0) {
                    err = "Found an empty AOF manifest";
                    goto loaderr;
                } else {
                    break;
                }
            } else {
                err = "Read AOF manifest failed";
                goto loaderr;
            }
        }

        linenum++;

        /* Skip comments lines */
        if (buf[0] == '#') continue;

        if (strchr(buf, '\n') == NULL) {
            err = "The AOF manifest file contains too long line";
            goto loaderr;
        }

        line = sdstrim(sdsnew(buf), " \t\r\n");
        if (!sdslen(line)) {
            err = "Invalid AOF manifest file format";
            goto loaderr;
        }

        argv = sdssplitargs(line, &argc);
        /* 'argc < 6' was done for forward compatibility. */
        if (argv == NULL || argc < 6 || (argc % 2)) {
            err = "Invalid AOF manifest file format";
            goto loaderr;
        }

        ai = aofInfoCreate();
        for (int i = 0; i < argc; i += 2) {
            if (!strcasecmp(argv[i], AOF_MANIFEST_KEY_FILE_NAME)) {
                ai->file_name = sdsnew(argv[i+1]);
                if (!pathIsBaseName(ai->file_name)) {
                    err = "File can't be a path, just a filename";
                    goto loaderr;
                }
            } else if (!strcasecmp(argv[i], AOF_MANIFEST_KEY_FILE_SEQ)) {
                ai->file_seq = atoll(argv[i+1]);
            } else if (!strcasecmp(argv[i], AOF_MANIFEST_KEY_FILE_TYPE)) {
                ai->file_type = (argv[i+1])[0];
            }
            /* else if (!strcasecmp(argv[i], AOF_MANIFEST_KEY_OTHER)) {} */
        }

        /* We have to make sure we load all the information. */
        if (!ai->file_name || !ai->file_seq || !ai->file_type) {
            err = "Invalid AOF manifest file format";
            goto loaderr;
        }

        sdsfreesplitres(argv, argc);
        argv = NULL;

        if (ai->file_type == AOF_FILE_TYPE_BASE) {
            if (am->base_aof_info) {
                err = "Found duplicate base file information";
                goto loaderr;
            }
            am->base_aof_info = ai;
            am->curr_base_file_seq = ai->file_seq;
        } else if (ai->file_type == AOF_FILE_TYPE_HIST) {
            listAddNodeTail(am->history_aof_list, ai);
        } else if (ai->file_type == AOF_FILE_TYPE_INCR) {
            if (ai->file_seq <= maxseq) {
                err = "Found a non-monotonic sequence number";
                goto loaderr;
            }
            listAddNodeTail(am->incr_aof_list, ai);
            am->curr_incr_file_seq = ai->file_seq;
            maxseq = ai->file_seq;
        } else {
            err = "Unknown AOF file type";
            goto loaderr;
        }

        sdsfree(line);
        line = NULL;
        ai = NULL;
    }

    fclose(fp);
    return am;

loaderr:
    /* Sanitizer suppression: may report a false positive if we goto loaderr
     * and exit(1) without freeing these allocations. */
    if (argv) sdsfreesplitres(argv, argc);
    if (ai) aofInfoFree(ai);

    serverLog(LL_WARNING, "\n*** FATAL AOF MANIFEST FILE ERROR ***\n");
    if (line) {
        serverLog(LL_WARNING, "Reading the manifest file, at line %d\n", linenum);
        serverLog(LL_WARNING, ">>> '%s'\n", line);
    }
    serverLog(LL_WARNING, "%s\n", err);
    exit(1);
}

/* Deep copy an aofManifest from orig.
 *
 * In `backgroundRewriteDoneHandler` and `openNewIncrAofForAppend`, we will
 * first deep copy a temporary AOF manifest from the `server.aof_manifest` and
 * try to modify it. Once everything is modified, we will atomically make the
 * `server.aof_manifest` point to this temporary aof_manifest.
 */
/* 复制给定的 aofManifest 实例，生成一个新的 aofManifest 实例 */
aofManifest *aofManifestDup(aofManifest *orig) {
    serverAssert(orig != NULL);
    aofManifest *am = zcalloc(sizeof(aofManifest));

    am->curr_base_file_seq = orig->curr_base_file_seq;
    am->curr_incr_file_seq = orig->curr_incr_file_seq;
    am->dirty = orig->dirty;

    if (orig->base_aof_info) {
        am->base_aof_info = aofInfoDup(orig->base_aof_info);
    }

    am->incr_aof_list = listDup(orig->incr_aof_list);
    am->history_aof_list = listDup(orig->history_aof_list);
    serverAssert(am->incr_aof_list != NULL);
    serverAssert(am->history_aof_list != NULL);
    return am;
}

/* Change the `server.aof_manifest` pointer to 'am' and free the previous
 * one if we have. */
/* 释放之前旧的 aofManifest 实例，使用新的 am 设置 server.aof_manifest 属性 */
void aofManifestFreeAndUpdate(aofManifest *am) {
    serverAssert(am != NULL);
    if (server.aof_manifest) aofManifestFree(server.aof_manifest);
    server.aof_manifest = am;
}

/* Called in `backgroundRewriteDoneHandler` to get a new BASE file
 * name, and mark the previous (if we have) BASE file as HISTORY type.
 *
 * BASE file naming rules: `server.aof_filename`.seq.base.format
 *
 * for example:
 *  appendonly.aof.1.base.aof  (server.aof_use_rdb_preamble is no)
 *  appendonly.aof.1.base.rdb  (server.aof_use_rdb_preamble is yes)
 */
/* 获取新的 base 文件名，并标记之前的 base 文件为历史文件
 * 新的 base 文件名会根据 server.aof_use_rdb_preamble 来进行判断，该属性默认值是 1，认为是 rdb 这种就行了 */
sds getNewBaseFileNameAndMarkPreAsHistory(aofManifest *am) {
    serverAssert(am != NULL);
    if (am->base_aof_info) {
        serverAssert(am->base_aof_info->file_type == AOF_FILE_TYPE_BASE);
        /* 标记之前的 base 文件为历史类型 */
        am->base_aof_info->file_type = AOF_FILE_TYPE_HIST;
        listAddNodeHead(am->history_aof_list, am->base_aof_info);
    }

    /* 选择后缀是 .aof 还是 .rdb */
    char *format_suffix = server.aof_use_rdb_preamble ?
        RDB_FORMAT_SUFFIX:AOF_FORMAT_SUFFIX;

    aofInfo *ai = aofInfoCreate();
    /* 设置文件名 */
    ai->file_name = sdscatprintf(sdsempty(), "%s.%lld%s%s", server.aof_filename,
                        ++am->curr_base_file_seq, BASE_FILE_SUFFIX, format_suffix);
    /* 设置序列号 */
    ai->file_seq = am->curr_base_file_seq;
    /* 设置文件类型 */
    ai->file_type = AOF_FILE_TYPE_BASE;
    am->base_aof_info = ai;
    am->dirty = 1;
    return am->base_aof_info->file_name;
}

/* Get a new INCR type AOF name.
 *
 * INCR AOF naming rules: `server.aof_filename`.seq.incr.aof
 *
 * for example:
 *  appendonly.aof.1.incr.aof
 */
/* 获取新的 incr aof 文件信息，添加进清单中，返回新的 incr aof 文件名 */
sds getNewIncrAofName(aofManifest *am) {
    aofInfo *ai = aofInfoCreate();
    ai->file_type = AOF_FILE_TYPE_INCR;
    ai->file_name = sdscatprintf(sdsempty(), "%s.%lld%s%s", server.aof_filename,
                        ++am->curr_incr_file_seq, INCR_FILE_SUFFIX, AOF_FORMAT_SUFFIX);
    ai->file_seq = am->curr_incr_file_seq;
    listAddNodeTail(am->incr_aof_list, ai);
    am->dirty = 1;
    return ai->file_name;
}

/* Get temp INCR type AOF name. */
/* 获取临时 incr 文件名 temp-xxx.incr */
sds getTempIncrAofName() {
    return sdscatprintf(sdsempty(), "%s%s%s", TEMP_FILE_NAME_PREFIX, server.aof_filename,
        INCR_FILE_SUFFIX);
}

/* Get the last INCR AOF name or create a new one. */
/* 获取最后的 incr 文件名，若不存在 incr 文件则创建一个 */
sds getLastIncrAofName(aofManifest *am) {
    serverAssert(am != NULL);

    /* If 'incr_aof_list' is empty, just create a new one. */
    if (!listLength(am->incr_aof_list)) {
        return getNewIncrAofName(am);
    }

    /* Or return the last one. */
    listNode *lastnode = listIndex(am->incr_aof_list, -1);
    aofInfo *ai = listNodeValue(lastnode);
    return ai->file_name;
}

/* Called in `backgroundRewriteDoneHandler`. when AOFRW success, This
 * function will change the AOF file type in 'incr_aof_list' from
 * AOF_FILE_TYPE_INCR to AOF_FILE_TYPE_HIST, and move them to the
 * 'history_aof_list'.
 */
/* 标记 incr aof 文件为历史文件 */
void markRewrittenIncrAofAsHistory(aofManifest *am) {
    serverAssert(am != NULL);
    if (!listLength(am->incr_aof_list)) {
        return;
    }

    listNode *ln;
    listIter li;

    /* 根据清单信息中的 incr aof 文件信息列表从链表尾部向头部生成迭代器 */
    listRewindTail(am->incr_aof_list, &li);

    /* "server.aof_fd != -1" means AOF enabled, then we must skip the
     * last AOF, because this file is our currently writing. */
    /* 如果之前存在 incr aof 文件，需要忽略最后一个，因为它是当前写入的 incr aof */
    if (server.aof_fd != -1) {
        /* 获取尾结点的前一个结点 */
        ln = listNext(&li);
        serverAssert(ln != NULL);
    }

    /* Move aofInfo from 'incr_aof_list' to 'history_aof_list'. */
    /* 从尾结点开始，如果上一个节点不为空 */
    while ((ln = listNext(&li)) != NULL) {
        /* 获取 incr aof 文件信息 */
        aofInfo *ai = (aofInfo*)ln->value;
        serverAssert(ai->file_type == AOF_FILE_TYPE_INCR);

        /* 复制一份 incr aof 信息 */
        aofInfo *hai = aofInfoDup(ai);
        /* 设置文件类型为历史文件 */
        hai->file_type = AOF_FILE_TYPE_HIST;
        /* 将新复制的 aofInfo 实例加入历史文件列表 */
        listAddNodeHead(am->history_aof_list, hai);
        /* 从 incr aof 信息列表移除当前节点 */
        listDelNode(am->incr_aof_list, ln);
    }

    /* 标记清单信息修改了，需要持久化 */
    am->dirty = 1;
}

/* Write the formatted manifest string to disk. */
/* 把清单信息的格式化字符串写入清单文件中 */
int writeAofManifestFile(sds buf) {
    int ret = C_OK;
    ssize_t nwritten;
    int len;

    sds am_name = getAofManifestFileName();
    sds am_filepath = makePath(server.aof_dirname, am_name);
    sds tmp_am_name = getTempAofManifestFileName();
    sds tmp_am_filepath = makePath(server.aof_dirname, tmp_am_name);

    /* 打开临时文件 */
    int fd = open(tmp_am_filepath, O_WRONLY|O_TRUNC|O_CREAT, 0644);
    if (fd == -1) {
        serverLog(LL_WARNING, "Can't open the AOF manifest file %s: %s",
            tmp_am_name, strerror(errno));

        ret = C_ERR;
        goto cleanup;
    }

    /* 循环将清单字符串写入临时清单文件中 */
    len = sdslen(buf);
    while(len) {
        nwritten = write(fd, buf, len);

        if (nwritten < 0) {
            if (errno == EINTR) continue;

            serverLog(LL_WARNING, "Error trying to write the temporary AOF manifest file %s: %s",
                tmp_am_name, strerror(errno));

            ret = C_ERR;
            goto cleanup;
        }

        len -= nwritten;
        buf += nwritten;
    }

    /* 刷盘操作 */
    if (redis_fsync(fd) == -1) {
        serverLog(LL_WARNING, "Fail to fsync the temp AOF file %s: %s.",
            tmp_am_name, strerror(errno));

        ret = C_ERR;
        goto cleanup;
    }

    /* 将临时文件重命名为新的清单文件 */
    if (rename(tmp_am_filepath, am_filepath) != 0) {
        serverLog(LL_WARNING,
            "Error trying to rename the temporary AOF manifest file %s into %s: %s",
            tmp_am_name, am_name, strerror(errno));

        ret = C_ERR;
        goto cleanup;
    }

    /* Also sync the AOF directory as new AOF files may be added in the directory */
    /* 同时也对 AOF 目录执行刷盘操作 */
    if (fsyncFileDir(am_filepath) == -1) {
        serverLog(LL_WARNING, "Fail to fsync AOF directory %s: %s.",
            am_filepath, strerror(errno));

        ret = C_ERR;
        goto cleanup;
    }

cleanup:
    if (fd != -1) close(fd);
    sdsfree(am_name);
    sdsfree(am_filepath);
    sdsfree(tmp_am_name);
    sdsfree(tmp_am_filepath);
    return ret;
}

/* Persist the aofManifest information pointed to by am to disk. */
/* 将 aofManifest 实例持久化到磁盘中 */
int persistAofManifest(aofManifest *am) {
    if (am->dirty == 0) {
        return C_OK;
    }

    /* 将 aofManifest 清单实例格式化成字符串 */
    sds amstr = getAofManifestAsString(am);
    /* 将格式化后的字符串写入清单文件 */
    int ret = writeAofManifestFile(amstr);
    sdsfree(amstr);
    if (ret == C_OK) am->dirty = 0;
    return ret;
}

/* Called in `loadAppendOnlyFiles` when we upgrade from a old version redis.
 *
 * 1) Create AOF directory use 'server.aof_dirname' as the name.
 * 2) Use 'server.aof_filename' to construct a BASE type aofInfo and add it to
 *    aofManifest, then persist the manifest file to AOF directory.
 * 3) Move the old AOF file (server.aof_filename) to AOF directory.
 *
 * If any of the above steps fails or crash occurs, this will not cause any
 * problems, and redis will retry the upgrade process when it restarts.
 */
/* 将旧 redis 版本的 aof 文件进行升级 */
void aofUpgradePrepare(aofManifest *am) {
    serverAssert(!aofFileExist(server.aof_filename));

    /* Create AOF directory use 'server.aof_dirname' as the name. */
    /* 使用 'server.aof_dirname' 变量（默认值 appendonlydir）作为目录名创建 AOF 目录 */
    if (dirCreateIfMissing(server.aof_dirname) == -1) {
        serverLog(LL_WARNING, "Can't open or create append-only dir %s: %s",
            server.aof_dirname, strerror(errno));
        exit(1);
    }

    /* Manually construct a BASE type aofInfo and add it to aofManifest. */
    /* 构造 Base 类型 aofInfo 并将其添加到 aofManifest */
    if (am->base_aof_info) aofInfoFree(am->base_aof_info);
    aofInfo *ai = aofInfoCreate();
    ai->file_name = sdsnew(server.aof_filename);
    ai->file_seq = 1;
    ai->file_type = AOF_FILE_TYPE_BASE;
    am->base_aof_info = ai;
    am->curr_base_file_seq = 1;
    am->dirty = 1;

    /* Persist the manifest file to AOF directory. */
    /* 持久化 AOF 清单文件 */
    if (persistAofManifest(am) != C_OK) {
        exit(1);
    }

    /* Move the old AOF file to AOF directory. */
    /* 将旧 AOF 文件移动至 AOF 目录 */
    sds aof_filepath = makePath(server.aof_dirname, server.aof_filename);
    if (rename(server.aof_filename, aof_filepath) == -1) {
        serverLog(LL_WARNING,
            "Error trying to move the old AOF file %s into dir %s: %s",
            server.aof_filename,
            server.aof_dirname,
            strerror(errno));
        sdsfree(aof_filepath);
        exit(1);
    }
    sdsfree(aof_filepath);

    serverLog(LL_NOTICE, "Successfully migrated an old-style AOF file (%s) into the AOF directory (%s).",
        server.aof_filename, server.aof_dirname);
}

/* When AOFRW success, the previous BASE and INCR AOFs will
 * become HISTORY type and be moved into 'history_aof_list'.
 *
 * The function will traverse the 'history_aof_list' and submit
 * the delete task to the bio thread.
 */
/* 该方法是删除内存中历史文件信息，然后提交一个关闭历史文件的任务给 bio 线程 */
int aofDelHistoryFiles(void) {
    if (server.aof_manifest == NULL ||
        server.aof_disable_auto_gc == 1 ||
        !listLength(server.aof_manifest->history_aof_list))
    {
        return C_OK;
    }

    listNode *ln;
    listIter li;

    /* 遍历清单中所有历史文件信息 */
    listRewind(server.aof_manifest->history_aof_list, &li);
    while ((ln = listNext(&li)) != NULL) {
        aofInfo *ai = (aofInfo*)ln->value;
        serverAssert(ai->file_type == AOF_FILE_TYPE_HIST);
        serverLog(LL_NOTICE, "Removing the history file %s in the background", ai->file_name);
        sds aof_filepath = makePath(server.aof_dirname, ai->file_name);
        /* 提交关闭文件的任务 */
        bg_unlink(aof_filepath);
        sdsfree(aof_filepath);
        /* 删除列表中的节点 */
        listDelNode(server.aof_manifest->history_aof_list, ln);
    }

    /* 标记清单改变了 */
    server.aof_manifest->dirty = 1;
    /* 持久化 AOF 清单文件 */
    return persistAofManifest(server.aof_manifest);
}

/* Used to clean up temp INCR AOF when AOFRW fails. */
/* 关闭临时文件 */
void aofDelTempIncrAofFile() {
    sds aof_filename = getTempIncrAofName();
    sds aof_filepath = makePath(server.aof_dirname, aof_filename);
    serverLog(LL_NOTICE, "Removing the temp incr aof file %s in the background", aof_filename);
    bg_unlink(aof_filepath);
    sdsfree(aof_filepath);
    sdsfree(aof_filename);
    return;
}

/* Called after `loadDataFromDisk` when redis start. If `server.aof_state` is
 * 'AOF_ON', It will do three things:
 * 1. Force create a BASE file when redis starts with an empty dataset
 * 2. Open the last opened INCR type AOF for writing, If not, create a new one
 * 3. Synchronously update the manifest file to the disk
 *
 * If any of the above steps fails, the redis process will exit.
 */
/* 开启 AOF 模式时打开 AOF 文件，不存在则创建，在服务器启动时调用 */
void aofOpenIfNeededOnServerStart(void) {
    if (server.aof_state != AOF_ON) {
        return;
    }

    serverAssert(server.aof_manifest != NULL);
    serverAssert(server.aof_fd == -1);

    if (dirCreateIfMissing(server.aof_dirname) == -1) {
        serverLog(LL_WARNING, "Can't open or create append-only dir %s: %s",
            server.aof_dirname, strerror(errno));
        exit(1);
    }

    /* If we start with an empty dataset, we will force create a BASE file. */
    size_t incr_aof_len = listLength(server.aof_manifest->incr_aof_list);
    if (!server.aof_manifest->base_aof_info && !incr_aof_len) {
        sds base_name = getNewBaseFileNameAndMarkPreAsHistory(server.aof_manifest);
        sds base_filepath = makePath(server.aof_dirname, base_name);
        if (rewriteAppendOnlyFile(base_filepath) != C_OK) {
            exit(1);
        }
        sdsfree(base_filepath);
        serverLog(LL_NOTICE, "Creating AOF base file %s on server start",
            base_name);
    }

    /* Because we will 'exit(1)' if open AOF or persistent manifest fails, so
     * we don't need atomic modification here. */
    sds aof_name = getLastIncrAofName(server.aof_manifest);

    /* Here we should use 'O_APPEND' flag. */
    sds aof_filepath = makePath(server.aof_dirname, aof_name);
    server.aof_fd = open(aof_filepath, O_WRONLY|O_APPEND|O_CREAT, 0644);
    sdsfree(aof_filepath);
    if (server.aof_fd == -1) {
        serverLog(LL_WARNING, "Can't open the append-only file %s: %s",
            aof_name, strerror(errno));
        exit(1);
    }

    /* Persist our changes. */
    int ret = persistAofManifest(server.aof_manifest);
    if (ret != C_OK) {
        exit(1);
    }

    server.aof_last_incr_size = getAppendOnlyFileSize(aof_name, NULL);

    if (incr_aof_len) {
        serverLog(LL_NOTICE, "Opening AOF incr file %s on server start", aof_name);
    } else {
        serverLog(LL_NOTICE, "Creating AOF incr file %s on server start", aof_name);
    }
}

int aofFileExist(char *filename) {
    sds file_path = makePath(server.aof_dirname, filename);
    int ret = fileExist(file_path);
    sdsfree(file_path);
    return ret;
}

/* Called in `rewriteAppendOnlyFileBackground`. If `server.aof_state`
 * is 'AOF_ON', It will do two things:
 * 1. Open a new INCR type AOF for writing
 * 2. Synchronously update the manifest file to the disk
 *
 * The above two steps of modification are atomic, that is, if
 * any step fails, the entire operation will rollback and returns
 * C_ERR, and if all succeeds, it returns C_OK.
 * 
 * If `server.aof_state` is 'AOF_WAIT_REWRITE', It will open a temporary INCR AOF 
 * file to accumulate data during AOF_WAIT_REWRITE, and it will eventually be 
 * renamed in the `backgroundRewriteDoneHandler` and written to the manifest file.
 * */
/* 打开一个新的 aof 文件 */
/* 若执行时 aof 正在进行重写，则会先创建临时 incr 文件并用于在 aof 重写期间记录数据 */
/* 最后临时文件会在 'backgroundRewriteDoneHandler' 函数中重命名并写入清单文件 */
int openNewIncrAofForAppend(void) {
    serverAssert(server.aof_manifest != NULL);
    int newfd = -1;
    aofManifest *temp_am = NULL;
    sds new_aof_name = NULL;

    /* Only open new INCR AOF when AOF enabled. */
    if (server.aof_state == AOF_OFF) return C_OK;

    /* Open new AOF. */
    if (server.aof_state == AOF_WAIT_REWRITE) {
        /* Use a temporary INCR AOF file to accumulate data during AOF_WAIT_REWRITE. */
        new_aof_name = getTempIncrAofName();
    } else {
        /* Dup a temp aof_manifest to modify. */
        temp_am = aofManifestDup(server.aof_manifest);
        new_aof_name = sdsdup(getNewIncrAofName(temp_am));
    }
    sds new_aof_filepath = makePath(server.aof_dirname, new_aof_name);
    newfd = open(new_aof_filepath, O_WRONLY|O_TRUNC|O_CREAT, 0644);
    sdsfree(new_aof_filepath);
    if (newfd == -1) {
        serverLog(LL_WARNING, "Can't open the append-only file %s: %s",
            new_aof_name, strerror(errno));
        goto cleanup;
    }

    if (temp_am) {
        /* Persist AOF Manifest. */
        if (persistAofManifest(temp_am) == C_ERR) {
            goto cleanup;
        }
    }

    serverLog(LL_NOTICE, "Creating AOF incr file %s on background rewrite",
            new_aof_name);
    sdsfree(new_aof_name);

    /* If reaches here, we can safely modify the `server.aof_manifest`
     * and `server.aof_fd`. */

    /* fsync and close old aof_fd if needed. In fsync everysec it's ok to delay
     * the fsync as long as we grantee it happens, and in fsync always the file
     * is already synced at this point so fsync doesn't matter. */
    if (server.aof_fd != -1) {
        aof_background_fsync_and_close(server.aof_fd);
        server.aof_fsync_offset = server.aof_current_size;
        server.aof_last_fsync = server.unixtime;
    }
    server.aof_fd = newfd;

    /* Reset the aof_last_incr_size. */
    server.aof_last_incr_size = 0;
    /* Update `server.aof_manifest`. */
    if (temp_am) aofManifestFreeAndUpdate(temp_am);
    return C_OK;

cleanup:
    if (new_aof_name) sdsfree(new_aof_name);
    if (newfd != -1) close(newfd);
    if (temp_am) aofManifestFree(temp_am);
    return C_ERR;
}

/* Whether to limit the execution of Background AOF rewrite.
 *
 * At present, if AOFRW fails, redis will automatically retry. If it continues
 * to fail, we may get a lot of very small INCR files. so we need an AOFRW
 * limiting measure.
 *
 * We can't directly use `server.aof_current_size` and `server.aof_last_incr_size`,
 * because there may be no new writes after AOFRW fails.
 *
 * So, we use time delay to achieve our goal. When AOFRW fails, we delay the execution
 * of the next AOFRW by 1 minute. If the next AOFRW also fails, it will be delayed by 2
 * minutes. The next is 4, 8, 16, the maximum delay is 60 minutes (1 hour).
 *
 * During the limit period, we can still use the 'bgrewriteaof' command to execute AOFRW
 * immediately.
 *
 * Return 1 means that AOFRW is limited and cannot be executed. 0 means that we can execute
 * AOFRW, which may be that we have reached the 'next_rewrite_time' or the number of INCR
 * AOFs has not reached the limit threshold.
 * */
/* 是否限制了执行 AOFRW
 *
 * 当前，如果 AOFRW 失败，redis 将会自动重试 AOFRW，如果重试后还是一直失败，将会存在大量的非常小的
 * incr 文件，因此需要采取措施限制 AOFRW 执行
 *
 * 为了解决上面的问题，使用了时间限制。当 AOFRW 失败，将会延迟 1 分钟再执行下一次 AOFRW，如果之后还
 * 失败，就延迟 2 分钟，接着是 4,8,16,.... 最大是 60 分钟
 *
 * 在 AOFRW 限制期间，我们仍然可以使用 bgrewriteaof 命令主动并且立即触发 AOFRW
 *
 * 该函数返回 1 表示 AOFRW 被限制了，不能执行，如果是 0 则可以执行 AOFRW
 */
/* 开始限制的阈值，即 AOFRW 连续失败 3 次，就开始进行限制 */
#define AOF_REWRITE_LIMITE_THRESHOLD    3
/* AOFRW 限制的最大延迟时间是 60 分钟 */
#define AOF_REWRITE_LIMITE_MAX_MINUTES  60 /* 1 hour */
int aofRewriteLimited(void) {
    static int next_delay_minutes = 0;
    static time_t next_rewrite_time = 0;

    /* 如果 AOFRW 连续失败次数小于 3 次，可以执行 AOFRW */
    if (server.stat_aofrw_consecutive_failures < AOF_REWRITE_LIMITE_THRESHOLD) {
        /* We may be recovering from limited state, so reset all states. */
        next_delay_minutes = 0;
        next_rewrite_time = 0;
        return 0;
    }

    /* if it is in the limiting state, then check if the next_rewrite_time is reached */
    if (next_rewrite_time != 0) {
        /* 如果当前时间还没超过指定的限制时间，不能执行 AOFRW */
        if (server.unixtime < next_rewrite_time) {
            return 1;
        } else {
            /* 超过了，将 next_rewrite_time 置 0，并且返回 0 */
            next_rewrite_time = 0;
            return 0;
        }
    }

    /* 随着连续失败次数的增加，需要调整限制时间，每次增长之前的两倍时间，知道达到最大时间限制 */
    next_delay_minutes = (next_delay_minutes == 0) ? 1 : (next_delay_minutes * 2);
    if (next_delay_minutes > AOF_REWRITE_LIMITE_MAX_MINUTES) {
        next_delay_minutes = AOF_REWRITE_LIMITE_MAX_MINUTES;
    }

    /* 下一次执行 AOFRW 的时间戳 */
    next_rewrite_time = server.unixtime + next_delay_minutes * 60;
    serverLog(LL_WARNING,
        "Background AOF rewrite has repeatedly failed and triggered the limit, will retry in %d minutes", next_delay_minutes);
    return 1;
}

/* ----------------------------------------------------------------------------
 * AOF file implementation
 * ------------------------------------------------------------------------- */

/* Return true if an AOf fsync is currently already in progress in a
 * BIO thread. */
/* bio 线程是否有 aof fysnc 任务要执行 */
int aofFsyncInProgress(void) {
    /* Note that we don't care about aof_background_fsync_and_close because
     * server.aof_fd has been replaced by the new INCR AOF file fd,
     * see openNewIncrAofForAppend. */
    return bioPendingJobsOfType(BIO_AOF_FSYNC) != 0;
}

/* Starts a background task that performs fsync() against the specified
 * file descriptor (the one of the AOF file) in another thread. */
/* 使用 bio 线程异步刷盘 */
void aof_background_fsync(int fd) {
    bioCreateFsyncJob(fd);
}

/* Close the fd on the basis of aof_background_fsync. */
/* 使用 bio 线程异步刷盘后关闭文件 */
void aof_background_fsync_and_close(int fd) {
    bioCreateCloseJob(fd, 1);
}

/* Kills an AOFRW child process if exists */
/* 关闭 aof rewrite 子进程 */
void killAppendOnlyChild(void) {
    int statloc;
    /* No AOFRW child? return. */
    if (server.child_type != CHILD_TYPE_AOF) return;
    /* Kill AOFRW child, wait for child exit. */
    serverLog(LL_NOTICE,"Killing running AOF rewrite child: %ld",
        (long) server.child_pid);
    if (kill(server.child_pid,SIGUSR1) != -1) {
        while(waitpid(-1, &statloc, 0) != server.child_pid);
    }
    /* 子进程退出需要移除掉自己产生的临时 base 文件 */
    aofRemoveTempFile(server.child_pid);
    resetChildState();
    server.aof_rewrite_time_start = -1;
}

/* Called when the user switches from "appendonly yes" to "appendonly no"
 * at runtime using the CONFIG command. */
/* 运行时停止 aof （用户在运行时使用 CONFIG 命令关闭 aof 时调用）*/
void stopAppendOnly(void) {
    serverAssert(server.aof_state != AOF_OFF);
    flushAppendOnlyFile(1);
    if (redis_fsync(server.aof_fd) == -1) {
        serverLog(LL_WARNING,"Fail to fsync the AOF file: %s",strerror(errno));
    } else {
        server.aof_fsync_offset = server.aof_current_size;
        server.aof_last_fsync = server.unixtime;
    }
    close(server.aof_fd);

    server.aof_fd = -1;
    server.aof_selected_db = -1;
    server.aof_state = AOF_OFF;
    server.aof_rewrite_scheduled = 0;
    server.aof_last_incr_size = 0;
    killAppendOnlyChild();
    sdsfree(server.aof_buf);
    server.aof_buf = sdsempty();
}

/* Called when the user switches from "appendonly no" to "appendonly yes"
 * at runtime using the CONFIG command. */
/* 用户运行时启用 aof 的时候，会调用该函数 */
int startAppendOnly(void) {
    serverAssert(server.aof_state == AOF_OFF);

    server.aof_state = AOF_WAIT_REWRITE;
    if (hasActiveChildProcess() && server.child_type != CHILD_TYPE_AOF) {
        /* 如果有非 AOF 的子进程正在执行，这里不能直接执行 rewrite aof，会设置 aof_rewrite_scheduled 为 1，表示之后需要执行
         * 一次 rewrite aof 操作 */
        server.aof_rewrite_scheduled = 1;
        serverLog(LL_WARNING,"AOF was enabled but there is already another background operation. An AOF background was scheduled to start when possible.");
    } else if (server.in_exec){
        /* 这里是正在进行事务的情况下，也不能直接执行 rewrite aof */
        server.aof_rewrite_scheduled = 1;
        serverLog(LL_WARNING,"AOF was enabled during a transaction. An AOF background was scheduled to start when possible.");
    } else {
        /* If there is a pending AOF rewrite, we need to switch it off and
         * start a new one: the old one cannot be reused because it is not
         * accumulating the AOF buffer. */
        /* 如果当前子进程类型是 aof 子进程，直接干掉当前子进程，开始一个新的 aof 子进程 */
        if (server.child_type == CHILD_TYPE_AOF) {
            serverLog(LL_WARNING,"AOF was enabled but there is already an AOF rewriting in background. Stopping background AOF and starting a rewrite now.");
            killAppendOnlyChild();
        }

        /* 开始执行 bgrewrite */
        if (rewriteAppendOnlyFileBackground() == C_ERR) {
            server.aof_state = AOF_OFF;
            serverLog(LL_WARNING,"Redis needs to enable the AOF but can't trigger a background AOF rewrite operation. Check the above logs for more info about the error.");
            return C_ERR;
        }
    }
    server.aof_last_fsync = server.unixtime;
    /* If AOF fsync error in bio job, we just ignore it and log the event. */
    int aof_bio_fsync_status;
    atomicGet(server.aof_bio_fsync_status, aof_bio_fsync_status);
    if (aof_bio_fsync_status == C_ERR) {
        serverLog(LL_WARNING,
            "AOF reopen, just ignore the AOF fsync error in bio job");
        atomicSet(server.aof_bio_fsync_status,C_OK);
    }

    /* If AOF was in error state, we just ignore it and log the event. */
    if (server.aof_last_write_status == C_ERR) {
        serverLog(LL_WARNING,"AOF reopen, just ignore the last error.");
        server.aof_last_write_status = C_OK;
    }
    return C_OK;
}

/* This is a wrapper to the write syscall in order to retry on short writes
 * or if the syscall gets interrupted. It could look strange that we retry
 * on short writes given that we are writing to a block device: normally if
 * the first call is short, there is a end-of-space condition, so the next
 * is likely to fail. However apparently in modern systems this is no longer
 * true, and in general it looks just more resilient to retry the write. If
 * there is an actual error condition we'll get it at the next try. */
/* 将 aof_buf 中的数据写到 incr 增量命令文件中 */
ssize_t aofWrite(int fd, const char *buf, size_t len) {
    ssize_t nwritten = 0, totwritten = 0;

    while(len) {
        /* 调用 write 写数据到文件 */
        nwritten = write(fd, buf, len);

        if (nwritten < 0) {
            if (errno == EINTR) continue;
            return totwritten ? totwritten : -1;
        }

        /* 更新 buf 剩余待写的数据的大小 */
        len -= nwritten;
        /* 更新待写数据的指针 */
        buf += nwritten;
        totwritten += nwritten;
    }

    /* 返回本次写入的数据量 */
    return totwritten;
}

/* Write the append only file buffer on disk.
 *
 * Since we are required to write the AOF before replying to the client,
 * and the only way the client socket can get a write is entering when
 * the event loop, we accumulate all the AOF writes in a memory
 * buffer and write it on disk using this function just before entering
 * the event loop again.
 *
 * About the 'force' argument:
 *
 * When the fsync policy is set to 'everysec' we may delay the flush if there
 * is still an fsync() going on in the background thread, since for instance
 * on Linux write(2) will be blocked by the background fsync anyway.
 * When this happens we remember that there is some aof buffer to be
 * flushed ASAP, and will try to do that in the serverCron() function.
 *
 * However if force is set to 1 we'll write regardless of the background
 * fsync. */
#define AOF_WRITE_LOG_ERROR_RATE 30 /* Seconds between errors logging. */
/* 将 aof_buf 中的数据写入磁盘中.
 * force 参数表示要执行刷盘，redis 有三种情况下会强制刷盘
 * 1) redis 服务关闭的时候
 * 2) 停止 aof 的时候
 * 3) 做 bgrewrite 的时候
 *
 * 这里还要注意两种 aof fsync 刷盘策略：
 * always 表示调用 write 将数据写入 page cache 之后就执行刷盘
 * everysec 会使用 bio 大概每秒做一次异步刷盘，且该策略下存在非强制刷盘的功能
 */
void flushAppendOnlyFile(int force) {
    ssize_t nwritten;
    int sync_in_progress = 0;
    mstime_t latency;

    if (sdslen(server.aof_buf) == 0) {
        /* Check if we need to do fsync even the aof buffer is empty,
         * because previously in AOF_FSYNC_EVERYSEC mode, fsync is
         * called only when aof buffer is not empty, so if users
         * stop write commands before fsync called in one second,
         * the data in page cache cannot be flushed in time. */
        /* 每秒刷盘一次 */
        if (server.aof_fsync == AOF_FSYNC_EVERYSEC &&
            server.aof_fsync_offset != server.aof_current_size &&
            /* unixtime 是 redis 的秒级时间戳 */
            server.unixtime > server.aof_last_fsync &&
            /* 如果 bio 线程的所有 aof fsync 任务都执行完了 */
            !(sync_in_progress = aofFsyncInProgress())) {
            goto try_fsync;
        } else {
            return;
        }
    }

    /* sync_in_progress 表示是否有正在执行的 bg aof fsync 任务 */
    if (server.aof_fsync == AOF_FSYNC_EVERYSEC)
        sync_in_progress = aofFsyncInProgress();

    /* 如果没有强制要将 aof_buf 缓冲区中的数据写入 aof incr 文件中的情况 */
    if (server.aof_fsync == AOF_FSYNC_EVERYSEC && !force) {
        /* With this append fsync policy we do background fsyncing.
         * If the fsync is still in progress we can try to delay
         * the write for a couple of seconds. */
        /* 如果有正在执行的 bg aof fsync 任务，这里会尝试延迟 2 秒将内存数据写入文件 */
        if (sync_in_progress) {
            /* 这里每次调用完 aofWrite 函数后，会将 server.aof_flush_postponed_start 置 0
             * 该属性为 0 表示现在开始新一次的 aof_buf 写磁盘操作 */
            if (server.aof_flush_postponed_start == 0) {
                /* No previous write postponing, remember that we are
                 * postponing the flush and return. */
                /* 记录当前的秒级时间戳 */
                server.aof_flush_postponed_start = server.unixtime;
                return;
            } else if (server.unixtime - server.aof_flush_postponed_start < 2) {
                /* We were already waiting for fsync to finish, but for less
                 * than two seconds this is still ok. Postpone again. */
                /* 如果还没超过 2s 直接返回，不执行写磁盘操作 */
                return;
            }
            /* Otherwise fall through, and go write since we can't wait
             * over two seconds. */
            /* 如果上面的逻辑都没有走，没有返回，下面就要开始执行写磁盘操作了，这里将 fsync 计数器 +1 */
            server.aof_delayed_fsync++;
            serverLog(LL_NOTICE,"Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.");
        }
    }
    /* We want to perform a single write. This should be guaranteed atomic
     * at least if the filesystem we are writing is a real physical one.
     * While this will save us against the server being killed I don't think
     * there is much to do about the whole server stopping for power problems
     * or alike */

    if (server.aof_flush_sleep && sdslen(server.aof_buf)) {
        usleep(server.aof_flush_sleep);
    }

    latencyStartMonitor(latency);
    /* 将 aof_buf 中的数据写入文件 */
    nwritten = aofWrite(server.aof_fd,server.aof_buf,sdslen(server.aof_buf));
    latencyEndMonitor(latency);
    /* We want to capture different events for delayed writes:
     * when the delay happens with a pending fsync, or with a saving child
     * active, and when the above two conditions are missing.
     * We also use an additional event name to save all samples which is
     * useful for graphing / monitoring purposes. */
    /* 记录需要延迟写入的事件，日后分析可能用到 */
    if (sync_in_progress) {
        latencyAddSampleIfNeeded("aof-write-pending-fsync",latency);
    } else if (hasActiveChildProcess()) {
        latencyAddSampleIfNeeded("aof-write-active-child",latency);
    } else {
        latencyAddSampleIfNeeded("aof-write-alone",latency);
    }
    latencyAddSampleIfNeeded("aof-write",latency);

    /* We performed the write so reset the postponed flush sentinel to zero. */
    /* 这里将 aof_flush_postponed_start 属性置 0 */
    server.aof_flush_postponed_start = 0;

    if (nwritten != (ssize_t)sdslen(server.aof_buf)) {
        static time_t last_write_error_log = 0;
        int can_log = 0;

        /* Limit logging rate to 1 line per AOF_WRITE_LOG_ERROR_RATE seconds. */
        /* 限制打印日志速度 （默认每 30s 能打印 1 行）*/
        if ((server.unixtime - last_write_error_log) > AOF_WRITE_LOG_ERROR_RATE) {
            can_log = 1;
            last_write_error_log = server.unixtime;
        }

        /* Log the AOF write error and record the error code. */
        if (nwritten == -1) {
            if (can_log) {
                serverLog(LL_WARNING,"Error writing to the AOF file: %s",
                    strerror(errno));
            }
            server.aof_last_write_errno = errno;
        } else {
            if (can_log) {
                serverLog(LL_WARNING,"Short write while writing to "
                                       "the AOF file: (nwritten=%lld, "
                                       "expected=%lld)",
                                       (long long)nwritten,
                                       (long long)sdslen(server.aof_buf));
            }

            if (ftruncate(server.aof_fd, server.aof_last_incr_size) == -1) {
                if (can_log) {
                    serverLog(LL_WARNING, "Could not remove short write "
                             "from the append-only file.  Redis may refuse "
                             "to load the AOF the next time it starts.  "
                             "ftruncate: %s", strerror(errno));
                }
            } else {
                /* If the ftruncate() succeeded we can set nwritten to
                 * -1 since there is no longer partial data into the AOF. */
                nwritten = -1;
            }
            server.aof_last_write_errno = ENOSPC;
        }

        /* Handle the AOF write error. */
        /* 处理 AOF 写入错误 */
        if (server.aof_fsync == AOF_FSYNC_ALWAYS) {
            /* We can't recover when the fsync policy is ALWAYS since the reply
             * for the client is already in the output buffers (both writes and
             * reads), and the changes to the db can't be rolled back. Since we
             * have a contract with the user that on acknowledged or observed
             * writes are is synced on disk, we must exit. */
            /* 当 fsync 策略为 always 时，发生写入错误必须退出，
             * 这是因为该策略相当于和客户端约定了确认写入时必须能够保证落盘 */
            serverLog(LL_WARNING,"Can't recover from AOF write error when the AOF fsync policy is 'always'. Exiting...");
            exit(1);
        } else {
            /* Recover from failed write leaving data into the buffer. However
             * set an error to stop accepting writes as long as the error
             * condition is not cleared. */
            server.aof_last_write_status = C_ERR;

            /* Trim the sds buffer if there was a partial write, and there
             * was no way to undo it with ftruncate(2). */
            /* 这里是每次只同步部分 aof_buf 中的数据到磁盘的情况 */
            if (nwritten > 0) {
                server.aof_current_size += nwritten;
                server.aof_last_incr_size += nwritten;
                /* 会将 aof_buf 中已经同步到磁盘的部分空间释放掉 */
                sdsrange(server.aof_buf,nwritten,-1);
            }
            return; /* We'll try again on the next call... */
        }
    } else {
        /* Successful write(2). If AOF was in error state, restore the
         * OK state and log the event. */
        if (server.aof_last_write_status == C_ERR) {
            serverLog(LL_WARNING,
                "AOF write error looks solved, Redis can write again.");
            server.aof_last_write_status = C_OK;
        }
    }
    server.aof_current_size += nwritten;
    server.aof_last_incr_size += nwritten;

    /* Re-use AOF buffer when it is small enough. The maximum comes from the
     * arena size of 4k minus some overhead (but is otherwise arbitrary). */
    /* 当 AOF 缓冲区足够小时复用 */
    if ((sdslen(server.aof_buf)+sdsavail(server.aof_buf)) < 4000) {
        sdsclear(server.aof_buf);
    } else {
        sdsfree(server.aof_buf);
        server.aof_buf = sdsempty();
    }

try_fsync:
    /* Don't fsync if no-appendfsync-on-rewrite is set to yes and there are
     * children doing I/O in the background. */
    if (server.aof_no_fsync_on_rewrite && hasActiveChildProcess())
        return;

    /* Perform the fsync if needed. */
    if (server.aof_fsync == AOF_FSYNC_ALWAYS) {
        /* redis_fsync is defined as fdatasync() for Linux in order to avoid
         * flushing metadata. */
        latencyStartMonitor(latency);
        /* Let's try to get this data on the disk. To guarantee data safe when
         * the AOF fsync policy is 'always', we should exit if failed to fsync
         * AOF (see comment next to the exit(1) after write error above). */
        /* aof fsync 策略为 always 的情况下，直接刷盘 */
        if (redis_fsync(server.aof_fd) == -1) {
            serverLog(LL_WARNING,"Can't persist AOF for fsync error when the "
              "AOF fsync policy is 'always': %s. Exiting...", strerror(errno));
            exit(1);
        }
        latencyEndMonitor(latency);
        latencyAddSampleIfNeeded("aof-fsync-always",latency);
        server.aof_fsync_offset = server.aof_current_size;
        server.aof_last_fsync = server.unixtime;
    } else if ((server.aof_fsync == AOF_FSYNC_EVERYSEC &&
                server.unixtime > server.aof_last_fsync)) {
        if (!sync_in_progress) {
            /* 这里是使用 bio 做 aof fsync 刷盘操作 */
            aof_background_fsync(server.aof_fd);
            server.aof_fsync_offset = server.aof_current_size;
        }
        server.aof_last_fsync = server.unixtime;
    }
}

/* 将命令格式化 */
sds catAppendOnlyGenericCommand(sds dst, int argc, robj **argv) {
    char buf[32];
    int len, j;
    robj *o;

    /* 第一个字符为 * */
    buf[0] = '*';
    /* 写入参数数量 */
    len = 1+ll2string(buf+1,sizeof(buf)-1,argc);
    buf[len++] = '\r';
    buf[len++] = '\n';
    dst = sdscatlen(dst,buf,len);

    /* 每个参数格式化 */
    for (j = 0; j < argc; j++) {
        o = getDecodedObject(argv[j]);
        /* 写入参数字节大小 */
        buf[0] = '$';
        len = 1+ll2string(buf+1,sizeof(buf)-1,sdslen(o->ptr));
        buf[len++] = '\r';
        buf[len++] = '\n';
        dst = sdscatlen(dst,buf,len);
        /* 写入参数字符串 */
        dst = sdscatlen(dst,o->ptr,sdslen(o->ptr));
        dst = sdscatlen(dst,"\r\n",2);
        decrRefCount(o);
    }
    return dst;
}

/* Generate a piece of timestamp annotation for AOF if current record timestamp
 * in AOF is not equal server unix time. If we specify 'force' argument to 1,
 * we would generate one without check, currently, it is useful in AOF rewriting
 * child process which always needs to record one timestamp at the beginning of
 * rewriting AOF.
 *
 * Timestamp annotation format is "#TS:${timestamp}\r\n". "TS" is short of
 * timestamp and this method could save extra bytes in AOF. */
sds genAofTimestampAnnotationIfNeeded(int force) {
    sds ts = NULL;

    if (force || server.aof_cur_timestamp < server.unixtime) {
        server.aof_cur_timestamp = force ? time(NULL) : server.unixtime;
        ts = sdscatfmt(sdsempty(), "#TS:%I\r\n", server.aof_cur_timestamp);
        serverAssert(sdslen(ts) <= AOF_ANNOTATION_LINE_MAX_LEN);
    }
    return ts;
}

/* 向 aof 缓冲区中追加一个命令 */
void feedAppendOnlyFile(int dictid, robj **argv, int argc) {
    sds buf = sdsempty();

    serverAssert(dictid >= 0 && dictid < server.dbnum);

    /* Feed timestamp if needed */
    if (server.aof_timestamp_enabled) {
        sds ts = genAofTimestampAnnotationIfNeeded(0);
        if (ts != NULL) {
            buf = sdscatsds(buf, ts);
            sdsfree(ts);
        }
    }

    /* The DB this command was targeting is not the same as the last command
     * we appended. To issue a SELECT command is needed. */
    /* 如果命令操作的数据库和上一次选择的数据库不同 */
    if (dictid != server.aof_selected_db) {
        char seldb[64];

        /* 向缓冲区添加一条 select dictid 命令 */
        snprintf(seldb,sizeof(seldb),"%d",dictid);
        buf = sdscatprintf(buf,"*2\r\n$6\r\nSELECT\r\n$%lu\r\n%s\r\n",
            (unsigned long)strlen(seldb),seldb);
        /* 更新操作的数据库号 */
        server.aof_selected_db = dictid;
    }

    /* All commands should be propagated the same way in AOF as in replication.
     * No need for AOF-specific translation. */
    /* 将命令和参数格式化 */
    buf = catAppendOnlyGenericCommand(buf,argc,argv);

    /* Append to the AOF buffer. This will be flushed on disk just before
     * of re-entering the event loop, so before the client will get a
     * positive reply about the operation performed. */
    if (server.aof_state == AOF_ON ||
        (server.aof_state == AOF_WAIT_REWRITE && server.child_type == CHILD_TYPE_AOF))
    {
        /* 存入 aof_buf 中 */
        server.aof_buf = sdscatlen(server.aof_buf, buf, sdslen(buf));
    }

    sdsfree(buf);
}

/* ----------------------------------------------------------------------------
 * AOF loading
 * ------------------------------------------------------------------------- */

/* In Redis commands are always executed in the context of a client, so in
 * order to load the append only file we need to create a fake client. */
struct client *createAOFClient(void) {
    struct client *c = createClient(NULL);

    c->id = CLIENT_ID_AOF; /* So modules can identify it's the AOF client. */

    /*
     * The AOF client should never be blocked (unlike master
     * replication connection).
     * This is because blocking the AOF client might cause
     * deadlock (because potentially no one will unblock it).
     * Also, if the AOF client will be blocked just for
     * background processing there is a chance that the
     * command execution order will be violated.
     */
    c->flags = CLIENT_DENY_BLOCKING;

    /* We set the fake client as a slave waiting for the synchronization
     * so that Redis will not try to send replies to this client. */
    c->replstate = SLAVE_STATE_WAIT_BGSAVE_START;
    return c;
}

/* Replay an append log file. On success AOF_OK or AOF_TRUNCATED is returned,
 * otherwise, one of the following is returned:
 * AOF_OPEN_ERR: Failed to open the AOF file.
 * AOF_NOT_EXIST: AOF file doesn't exist.
 * AOF_EMPTY: The AOF file is empty (nothing to load).
 * AOF_FAILED: Failed to load the AOF file. */
/* 加载单个 AOF 文件，
 * 也包括 RDB 格式的 base 文件 */
int loadSingleAppendOnlyFile(char *filename) {
    struct client *fakeClient;
    struct redis_stat sb;
    int old_aof_state = server.aof_state;
    long loops = 0;
    off_t valid_up_to = 0; /* Offset of latest well-formed command loaded. */
    off_t valid_before_multi = 0; /* Offset before MULTI command loaded. */
    off_t last_progress_report_size = 0;
    int ret = AOF_OK;

    sds aof_filepath = makePath(server.aof_dirname, filename);
    /* 打开 base 文件 */
    FILE *fp = fopen(aof_filepath, "r");
    if (fp == NULL) {
        int en = errno;
        if (redis_stat(aof_filepath, &sb) == 0 || errno != ENOENT) {
            serverLog(LL_WARNING,"Fatal error: can't open the append log file %s for reading: %s", filename, strerror(en));
            sdsfree(aof_filepath);
            return AOF_OPEN_ERR;
        } else {
            serverLog(LL_WARNING,"The append log file %s doesn't exist: %s", filename, strerror(errno));
            sdsfree(aof_filepath);
            return AOF_NOT_EXIST;
        }
    }

    if (fp && redis_fstat(fileno(fp),&sb) != -1 && sb.st_size == 0) {
        fclose(fp);
        sdsfree(aof_filepath);
        return AOF_EMPTY;
    }

    /* Temporarily disable AOF, to prevent EXEC from feeding a MULTI
     * to the same file we're about to read. */
    server.aof_state = AOF_OFF;

    client *old_client = server.current_client;
    /* 伪造一个 AOF 客户端 */
    fakeClient = server.current_client = createAOFClient();

    /* Check if the AOF file is in RDB format (it may be RDB encoded base AOF
     * or old style RDB-preamble AOF). In that case we need to load the RDB file 
     * and later continue loading the AOF tail if it is an old style RDB-preamble AOF. */
    char sig[5]; /* "REDIS" */
    /* 这里判断 base 文件是 rdb 的格式存储还是 aof 格式存储 */
    if (fread(sig,1,5,fp) != 5 || memcmp(sig,"REDIS",5) != 0) {
        /* Not in RDB format, seek back at 0 offset. */
        if (fseek(fp,0,SEEK_SET) == -1) goto readerr;
    } else {
        /* RDB format. Pass loading the RDB functions. */
        /* 这里是处理 RDB 格式的 base 文件的读取 */
        rio rdb;
        int old_style = !strcmp(filename, server.aof_filename);
        if (old_style)
            serverLog(LL_NOTICE, "Reading RDB preamble from AOF file...");
        else 
            serverLog(LL_NOTICE, "Reading RDB base file on AOF loading..."); 

        if (fseek(fp,0,SEEK_SET) == -1) goto readerr;
        /* 初始化一个 rio 结构 */
        rioInitWithFile(&rdb,fp);
        /* 这里调用 rdb 加载文件的接口加载 base 文件 */
        if (rdbLoadRio(&rdb,RDBFLAGS_AOF_PREAMBLE,NULL) != C_OK) {
            if (old_style)
                serverLog(LL_WARNING, "Error reading the RDB preamble of the AOF file %s, AOF loading aborted", filename);
            else
                serverLog(LL_WARNING, "Error reading the RDB base file %s, AOF loading aborted", filename);

            goto readerr;
        } else {
            loadingAbsProgress(ftello(fp));
            last_progress_report_size = ftello(fp);
            if (old_style) serverLog(LL_NOTICE, "Reading the remaining AOF tail...");
        }
    }

    /* Read the actual AOF file, in REPL format, command by command. */
    /* 这里是读取实际的 aof 文件 */
    while(1) {
        int argc, j;
        unsigned long len;
        robj **argv;
        char buf[AOF_ANNOTATION_LINE_MAX_LEN];
        sds argsds;
        struct redisCommand *cmd;

        /* Serve the clients from time to time */
        if (!(loops++ % 1024)) {
            off_t progress_delta = ftello(fp) - last_progress_report_size;
            loadingIncrProgress(progress_delta);
            last_progress_report_size += progress_delta;
            processEventsWhileBlocked();
            processModuleLoadingProgressEvent(1);
        }
        /* 从 base 文件中读取一行数据 */
        if (fgets(buf,sizeof(buf),fp) == NULL) {
            if (feof(fp)) {
                break;
            } else {
                goto readerr;
            }
        }
        if (buf[0] == '#') continue; /* Skip annotations */
        if (buf[0] != '*') goto fmterr;
        if (buf[1] == '\0') goto readerr;
        /* 读取参数数量 */
        argc = atoi(buf+1);
        if (argc < 1) goto fmterr;
        if ((size_t)argc > SIZE_MAX / sizeof(robj*)) goto fmterr;

        /* Load the next command in the AOF as our fake client
         * argv. */
        argv = zmalloc(sizeof(robj*)*argc);
        fakeClient->argc = argc;
        fakeClient->argv = argv;
        fakeClient->argv_len = argc;

        /* 根据参数数量从 aof 文件中读取参数 */
        for (j = 0; j < argc; j++) {
            /* Parse the argument len. */
            char *readres = fgets(buf,sizeof(buf),fp);
            if (readres == NULL || buf[0] != '$') {
                fakeClient->argc = j; /* Free up to j-1. */
                freeClientArgv(fakeClient);
                if (readres == NULL)
                    goto readerr;
                else
                    goto fmterr;
            }
            /* 获取当前参数的字节数 */
            len = strtol(buf+1,NULL,10);

            /* Read it into a string object. */
            /* 读取出参数 */
            argsds = sdsnewlen(SDS_NOINIT,len);
            if (len && fread(argsds,len,1,fp) == 0) {
                sdsfree(argsds);
                fakeClient->argc = j; /* Free up to j-1. */
                freeClientArgv(fakeClient);
                goto readerr;
            }
            /* 将参数封装成 redis 对象 */
            argv[j] = createObject(OBJ_STRING,argsds);

            /* Discard CRLF. */
            /* 丢弃换行符 */
            if (fread(buf,2,1,fp) == 0) {
                fakeClient->argc = j+1; /* Free up to j. */
                freeClientArgv(fakeClient);
                goto readerr;
            }
        }

        /* Command lookup */
        /* 根据参数和参数数量从命令表中获取对应的命令 */
        cmd = lookupCommand(argv,argc);
        if (!cmd) {
            serverLog(LL_WARNING,
                "Unknown command '%s' reading the append only file %s",
                (char*)argv[0]->ptr, filename);
            freeClientArgv(fakeClient);
            ret = AOF_FAILED;
            goto cleanup;
        }

        if (cmd->proc == multiCommand) valid_before_multi = valid_up_to;

        /* Run the command in the context of a fake client */
        fakeClient->cmd = fakeClient->lastcmd = cmd;
        /* 如果不是 exec 命令，就将命令入队到事务队列 */
        if (fakeClient->flags & CLIENT_MULTI &&
            fakeClient->cmd->proc != execCommand)
        {
            /* Note: we don't have to attempt calling evalGetCommandFlags,
             * since this is AOF, the checks in processCommand are not made
             * anyway.*/
            queueMultiCommand(fakeClient, cmd->flags);
        } else {
            /* 是 exec 命令或不处于事务中，调用命令的处理函数 */
            cmd->proc(fakeClient);
        }

        /* The fake client should not have a reply */
        serverAssert(fakeClient->bufpos == 0 &&
                     listLength(fakeClient->reply) == 0);

        /* The fake client should never get blocked */
        serverAssert((fakeClient->flags & CLIENT_BLOCKED) == 0);

        /* Clean up. Command code may have changed argv/argc so we use the
         * argv/argc of the client instead of the local variables. */
        freeClientArgv(fakeClient);
        if (server.aof_load_truncated) valid_up_to = ftello(fp);
        if (server.key_load_delay)
            debugDelay(server.key_load_delay);
    }

    /* This point can only be reached when EOF is reached without errors.
     * If the client is in the middle of a MULTI/EXEC, handle it as it was
     * a short read, even if technically the protocol is correct: we want
     * to remove the unprocessed tail and continue. */
    /* 事务未执行 exec 但文件已到达末尾 */
    if (fakeClient->flags & CLIENT_MULTI) {
        serverLog(LL_WARNING,
            "Revert incomplete MULTI/EXEC transaction in AOF file %s", filename);
        valid_up_to = valid_before_multi;
        goto uxeof;
    }

loaded_ok: /* DB loaded, cleanup and return success (AOF_OK or AOF_TRUNCATED). */
    loadingIncrProgress(ftello(fp) - last_progress_report_size);
    server.aof_state = old_aof_state;
    goto cleanup;

readerr: /* Read error. If feof(fp) is true, fall through to unexpected EOF. */
    if (!feof(fp)) {
        serverLog(LL_WARNING,"Unrecoverable error reading the append only file %s: %s", filename, strerror(errno));
        ret = AOF_FAILED;
        goto cleanup;
    }

uxeof: /* Unexpected AOF end of file. */
    if (server.aof_load_truncated) {
        serverLog(LL_WARNING,"!!! Warning: short read while loading the AOF file %s!!!", filename);
        serverLog(LL_WARNING,"!!! Truncating the AOF %s at offset %llu !!!",
            filename, (unsigned long long) valid_up_to);
        if (valid_up_to == -1 || truncate(aof_filepath,valid_up_to) == -1) {
            if (valid_up_to == -1) {
                serverLog(LL_WARNING,"Last valid command offset is invalid");
            } else {
                serverLog(LL_WARNING,"Error truncating the AOF file %s: %s",
                    filename, strerror(errno));
            }
        } else {
            /* Make sure the AOF file descriptor points to the end of the
             * file after the truncate call. */
            if (server.aof_fd != -1 && lseek(server.aof_fd,0,SEEK_END) == -1) {
                serverLog(LL_WARNING,"Can't seek the end of the AOF file %s: %s",
                    filename, strerror(errno));
            } else {
                serverLog(LL_WARNING,
                    "AOF %s loaded anyway because aof-load-truncated is enabled", filename);
                ret = AOF_TRUNCATED;
                goto loaded_ok;
            }
        }
    }
    serverLog(LL_WARNING, "Unexpected end of file reading the append only file %s. You can: "
        "1) Make a backup of your AOF file, then use ./redis-check-aof --fix <filename.manifest>. "
        "2) Alternatively you can set the 'aof-load-truncated' configuration option to yes and restart the server.", filename);
    ret = AOF_FAILED;
    goto cleanup;

fmterr: /* Format error. */
    serverLog(LL_WARNING, "Bad file format reading the append only file %s: "
        "make a backup of your AOF file, then use ./redis-check-aof --fix <filename.manifest>", filename);
    ret = AOF_FAILED;
    /* fall through to cleanup. */

cleanup:
    if (fakeClient) freeClient(fakeClient);
    server.current_client = old_client;
    fclose(fp);
    sdsfree(aof_filepath);
    return ret;
}

/* Load the AOF files according the aofManifest pointed by am. */
/* 加载清单文件中包含的 AOF 文件 */
int loadAppendOnlyFiles(aofManifest *am) {
    serverAssert(am != NULL);
    int status, ret = AOF_OK;
    long long start;
    off_t total_size = 0, base_size = 0;
    sds aof_name;
    int total_num, aof_num = 0, last_file;

    /* If the 'server.aof_filename' file exists in dir, we may be starting
     * from an old redis version. We will use enter upgrade mode in three situations.
     *
     * 1. If the 'server.aof_dirname' directory not exist
     * 2. If the 'server.aof_dirname' directory exists but the manifest file is missing
     * 3. If the 'server.aof_dirname' directory exists and the manifest file it contains
     *    has only one base AOF record, and the file name of this base AOF is 'server.aof_filename',
     *    and the 'server.aof_filename' file not exist in 'server.aof_dirname' directory
     * */
    /* 检测 AOF 文件是否为旧版本，并进行升级 */
    if (fileExist(server.aof_filename)) {
        if (!dirExists(server.aof_dirname) ||
            (am->base_aof_info == NULL && listLength(am->incr_aof_list) == 0) ||
            (am->base_aof_info != NULL && listLength(am->incr_aof_list) == 0 &&
             !strcmp(am->base_aof_info->file_name, server.aof_filename) && !aofFileExist(server.aof_filename)))
        {
            aofUpgradePrepare(am);
        }
    }

    if (am->base_aof_info == NULL && listLength(am->incr_aof_list) == 0) {
        return AOF_NOT_EXIST;
    }

    total_num = getBaseAndIncrAppendOnlyFilesNum(am);
    serverAssert(total_num > 0);

    /* Here we calculate the total size of all BASE and INCR files in
     * advance, it will be set to `server.loading_total_bytes`. */
    total_size = getBaseAndIncrAppendOnlyFilesSize(am, &status);
    if (status != AOF_OK) {
        /* If an AOF exists in the manifest but not on the disk, we consider this to be a fatal error. */
        if (status == AOF_NOT_EXIST) status = AOF_FAILED;

        return status;
    } else if (total_size == 0) {
        return AOF_EMPTY;
    }

    startLoading(total_size, RDBFLAGS_AOF_PREAMBLE, 0);

    /* Load BASE AOF if needed. */
    /* 加载 base 文件 */
    if (am->base_aof_info) {
        serverAssert(am->base_aof_info->file_type == AOF_FILE_TYPE_BASE);
        aof_name = (char*)am->base_aof_info->file_name;
        updateLoadingFileName(aof_name);
        base_size = getAppendOnlyFileSize(aof_name, NULL);
        last_file = ++aof_num == total_num;
        start = ustime();
        ret = loadSingleAppendOnlyFile(aof_name);
        if (ret == AOF_OK || (ret == AOF_TRUNCATED && last_file)) {
            serverLog(LL_NOTICE, "DB loaded from base file %s: %.3f seconds",
                aof_name, (float)(ustime()-start)/1000000);
        }

        /* If the truncated file is not the last file, we consider this to be a fatal error. */
        if (ret == AOF_TRUNCATED && !last_file) {
            ret = AOF_FAILED;
        }

        if (ret == AOF_OPEN_ERR || ret == AOF_FAILED) {
            goto cleanup;
        }
    }

    /* Load INCR AOFs if needed. */
    /* 加载 incr 文件（如果存在） */
    if (listLength(am->incr_aof_list)) {
        listNode *ln;
        listIter li;

        listRewind(am->incr_aof_list, &li);
        while ((ln = listNext(&li)) != NULL) {
            aofInfo *ai = (aofInfo*)ln->value;
            serverAssert(ai->file_type == AOF_FILE_TYPE_INCR);
            aof_name = (char*)ai->file_name;
            updateLoadingFileName(aof_name);
            last_file = ++aof_num == total_num;
            start = ustime();
            ret = loadSingleAppendOnlyFile(aof_name);
            if (ret == AOF_OK || (ret == AOF_TRUNCATED && last_file)) {
                serverLog(LL_NOTICE, "DB loaded from incr file %s: %.3f seconds",
                    aof_name, (float)(ustime()-start)/1000000);
            }

            /* We know that (at least) one of the AOF files has data (total_size > 0),
             * so empty incr AOF file doesn't count as a AOF_EMPTY result */
            if (ret == AOF_EMPTY) ret = AOF_OK;

            if (ret == AOF_TRUNCATED && !last_file) {
                ret = AOF_FAILED;
            }

            if (ret == AOF_OPEN_ERR || ret == AOF_FAILED) {
                goto cleanup;
            }
        }
    }

    server.aof_current_size = total_size;
    /* Ideally, the aof_rewrite_base_size variable should hold the size of the
     * AOF when the last rewrite ended, this should include the size of the
     * incremental file that was created during the rewrite since otherwise we
     * risk the next automatic rewrite to happen too soon (or immediately if
     * auto-aof-rewrite-percentage is low). However, since we do not persist
     * aof_rewrite_base_size information anywhere, we initialize it on restart
     * to the size of BASE AOF file. This might cause the first AOFRW to be
     * executed early, but that shouldn't be a problem since everything will be
     * fine after the first AOFRW. */
    /* 理想情况下 'aof_rewrite_base_size' 应该包括写入的 base + incr 文件的大小，
     * 而 redis 启动加载 AOF 文件时只令 'aof_rewrite_base_size' 等于 base 文件大小，
     * 这可能会导致第一次 AOF 自动重写提前发生，但一切都会在第一次 AOF 重写后变得正常，
     * 而如果启动时令 'aof_rewrite_base_size' = base + incr 文件大小，第一次重写可能会比预期晚
     * 详见 https://github.com/redis/redis/issues/10549 */
    server.aof_rewrite_base_size = base_size;
    server.aof_fsync_offset = server.aof_current_size;

cleanup:
    stopLoading(ret == AOF_OK || ret == AOF_TRUNCATED);
    return ret;
}

/* ----------------------------------------------------------------------------
 * AOF rewrite
 * ------------------------------------------------------------------------- */

/* Delegate writing an object to writing a bulk string or bulk long long.
 * This is not placed in rio.c since that adds the server.h dependency. */
int rioWriteBulkObject(rio *r, robj *obj) {
    /* Avoid using getDecodedObject to help copy-on-write (we are often
     * in a child process when this function is called). */
    if (obj->encoding == OBJ_ENCODING_INT) {
        return rioWriteBulkLongLong(r,(long)obj->ptr);
    } else if (sdsEncodedObject(obj)) {
        return rioWriteBulkString(r,obj->ptr,sdslen(obj->ptr));
    } else {
        serverPanic("Unknown string encoding");
    }
}

/* Emit the commands needed to rebuild a list object.
 * The function returns 0 on error, 1 on success. */
int rewriteListObject(rio *r, robj *key, robj *o) {
    long long count = 0, items = listTypeLength(o);

    if (o->encoding == OBJ_ENCODING_QUICKLIST) {
        quicklist *list = o->ptr;
        quicklistIter *li = quicklistGetIterator(list, AL_START_HEAD);
        quicklistEntry entry;

        while (quicklistNext(li,&entry)) {
            if (count == 0) {
                int cmd_items = (items > AOF_REWRITE_ITEMS_PER_CMD) ?
                    AOF_REWRITE_ITEMS_PER_CMD : items;
                if (!rioWriteBulkCount(r,'*',2+cmd_items) ||
                    !rioWriteBulkString(r,"RPUSH",5) ||
                    !rioWriteBulkObject(r,key)) 
                {
                    quicklistReleaseIterator(li);
                    return 0;
                }
            }

            if (entry.value) {
                if (!rioWriteBulkString(r,(char*)entry.value,entry.sz)) {
                    quicklistReleaseIterator(li);
                    return 0;
                }
            } else {
                if (!rioWriteBulkLongLong(r,entry.longval)) {
                    quicklistReleaseIterator(li);
                    return 0;
                }
            }
            if (++count == AOF_REWRITE_ITEMS_PER_CMD) count = 0;
            items--;
        }
        quicklistReleaseIterator(li);
    } else {
        serverPanic("Unknown list encoding");
    }
    return 1;
}

/* Emit the commands needed to rebuild a set object.
 * The function returns 0 on error, 1 on success. */
int rewriteSetObject(rio *r, robj *key, robj *o) {
    long long count = 0, items = setTypeSize(o);

    if (o->encoding == OBJ_ENCODING_INTSET) {
        int ii = 0;
        int64_t llval;

        while(intsetGet(o->ptr,ii++,&llval)) {
            if (count == 0) {
                int cmd_items = (items > AOF_REWRITE_ITEMS_PER_CMD) ?
                    AOF_REWRITE_ITEMS_PER_CMD : items;

                if (!rioWriteBulkCount(r,'*',2+cmd_items) ||
                    !rioWriteBulkString(r,"SADD",4) ||
                    !rioWriteBulkObject(r,key)) 
                {
                    return 0;
                }
            }
            if (!rioWriteBulkLongLong(r,llval)) return 0;
            if (++count == AOF_REWRITE_ITEMS_PER_CMD) count = 0;
            items--;
        }
    } else if (o->encoding == OBJ_ENCODING_HT) {
        dictIterator *di = dictGetIterator(o->ptr);
        dictEntry *de;

        while((de = dictNext(di)) != NULL) {
            sds ele = dictGetKey(de);
            if (count == 0) {
                int cmd_items = (items > AOF_REWRITE_ITEMS_PER_CMD) ?
                    AOF_REWRITE_ITEMS_PER_CMD : items;

                if (!rioWriteBulkCount(r,'*',2+cmd_items) ||
                    !rioWriteBulkString(r,"SADD",4) ||
                    !rioWriteBulkObject(r,key)) 
                {
                    dictReleaseIterator(di);
                    return 0;
                }
            }
            if (!rioWriteBulkString(r,ele,sdslen(ele))) {
                dictReleaseIterator(di);
                return 0;          
            }
            if (++count == AOF_REWRITE_ITEMS_PER_CMD) count = 0;
            items--;
        }
        dictReleaseIterator(di);
    } else {
        serverPanic("Unknown set encoding");
    }
    return 1;
}

/* Emit the commands needed to rebuild a sorted set object.
 * The function returns 0 on error, 1 on success. */
int rewriteSortedSetObject(rio *r, robj *key, robj *o) {
    long long count = 0, items = zsetLength(o);

    if (o->encoding == OBJ_ENCODING_LISTPACK) {
        unsigned char *zl = o->ptr;
        unsigned char *eptr, *sptr;
        unsigned char *vstr;
        unsigned int vlen;
        long long vll;
        double score;

        eptr = lpSeek(zl,0);
        serverAssert(eptr != NULL);
        sptr = lpNext(zl,eptr);
        serverAssert(sptr != NULL);

        while (eptr != NULL) {
            vstr = lpGetValue(eptr,&vlen,&vll);
            score = zzlGetScore(sptr);

            if (count == 0) {
                int cmd_items = (items > AOF_REWRITE_ITEMS_PER_CMD) ?
                    AOF_REWRITE_ITEMS_PER_CMD : items;

                if (!rioWriteBulkCount(r,'*',2+cmd_items*2) ||
                    !rioWriteBulkString(r,"ZADD",4) ||
                    !rioWriteBulkObject(r,key)) 
                {
                    return 0;
                }
            }
            if (!rioWriteBulkDouble(r,score)) return 0;
            if (vstr != NULL) {
                if (!rioWriteBulkString(r,(char*)vstr,vlen)) return 0;
            } else {
                if (!rioWriteBulkLongLong(r,vll)) return 0;
            }
            zzlNext(zl,&eptr,&sptr);
            if (++count == AOF_REWRITE_ITEMS_PER_CMD) count = 0;
            items--;
        }
    } else if (o->encoding == OBJ_ENCODING_SKIPLIST) {
        zset *zs = o->ptr;
        dictIterator *di = dictGetIterator(zs->dict);
        dictEntry *de;

        while((de = dictNext(di)) != NULL) {
            sds ele = dictGetKey(de);
            double *score = dictGetVal(de);

            if (count == 0) {
                int cmd_items = (items > AOF_REWRITE_ITEMS_PER_CMD) ?
                    AOF_REWRITE_ITEMS_PER_CMD : items;

                if (!rioWriteBulkCount(r,'*',2+cmd_items*2) ||
                    !rioWriteBulkString(r,"ZADD",4) ||
                    !rioWriteBulkObject(r,key)) 
                {
                    dictReleaseIterator(di);
                    return 0;
                }
            }
            if (!rioWriteBulkDouble(r,*score) ||
                !rioWriteBulkString(r,ele,sdslen(ele)))
            {
                dictReleaseIterator(di);
                return 0;
            }
            if (++count == AOF_REWRITE_ITEMS_PER_CMD) count = 0;
            items--;
        }
        dictReleaseIterator(di);
    } else {
        serverPanic("Unknown sorted zset encoding");
    }
    return 1;
}

/* Write either the key or the value of the currently selected item of a hash.
 * The 'hi' argument passes a valid Redis hash iterator.
 * The 'what' filed specifies if to write a key or a value and can be
 * either OBJ_HASH_KEY or OBJ_HASH_VALUE.
 *
 * The function returns 0 on error, non-zero on success. */
static int rioWriteHashIteratorCursor(rio *r, hashTypeIterator *hi, int what) {
    if (hi->encoding == OBJ_ENCODING_LISTPACK) {
        unsigned char *vstr = NULL;
        unsigned int vlen = UINT_MAX;
        long long vll = LLONG_MAX;

        hashTypeCurrentFromListpack(hi, what, &vstr, &vlen, &vll);
        if (vstr)
            return rioWriteBulkString(r, (char*)vstr, vlen);
        else
            return rioWriteBulkLongLong(r, vll);
    } else if (hi->encoding == OBJ_ENCODING_HT) {
        sds value = hashTypeCurrentFromHashTable(hi, what);
        return rioWriteBulkString(r, value, sdslen(value));
    }

    serverPanic("Unknown hash encoding");
    return 0;
}

/* Emit the commands needed to rebuild a hash object.
 * The function returns 0 on error, 1 on success. */
int rewriteHashObject(rio *r, robj *key, robj *o) {
    hashTypeIterator *hi;
    long long count = 0, items = hashTypeLength(o);

    hi = hashTypeInitIterator(o);
    while (hashTypeNext(hi) != C_ERR) {
        if (count == 0) {
            int cmd_items = (items > AOF_REWRITE_ITEMS_PER_CMD) ?
                AOF_REWRITE_ITEMS_PER_CMD : items;

            if (!rioWriteBulkCount(r,'*',2+cmd_items*2) ||
                !rioWriteBulkString(r,"HMSET",5) ||
                !rioWriteBulkObject(r,key)) 
            {
                hashTypeReleaseIterator(hi);
                return 0;
            }
        }

        if (!rioWriteHashIteratorCursor(r, hi, OBJ_HASH_KEY) ||
            !rioWriteHashIteratorCursor(r, hi, OBJ_HASH_VALUE))
        {
            hashTypeReleaseIterator(hi);
            return 0;           
        }
        if (++count == AOF_REWRITE_ITEMS_PER_CMD) count = 0;
        items--;
    }

    hashTypeReleaseIterator(hi);

    return 1;
}

/* Helper for rewriteStreamObject() that generates a bulk string into the
 * AOF representing the ID 'id'. */
int rioWriteBulkStreamID(rio *r,streamID *id) {
    int retval;

    sds replyid = sdscatfmt(sdsempty(),"%U-%U",id->ms,id->seq);
    retval = rioWriteBulkString(r,replyid,sdslen(replyid));
    sdsfree(replyid);
    return retval;
}

/* Helper for rewriteStreamObject(): emit the XCLAIM needed in order to
 * add the message described by 'nack' having the id 'rawid', into the pending
 * list of the specified consumer. All this in the context of the specified
 * key and group. */
int rioWriteStreamPendingEntry(rio *r, robj *key, const char *groupname, size_t groupname_len, streamConsumer *consumer, unsigned char *rawid, streamNACK *nack) {
     /* XCLAIM <key> <group> <consumer> 0 <id> TIME <milliseconds-unix-time>
               RETRYCOUNT <count> JUSTID FORCE. */
    streamID id;
    streamDecodeID(rawid,&id);
    if (rioWriteBulkCount(r,'*',12) == 0) return 0;
    if (rioWriteBulkString(r,"XCLAIM",6) == 0) return 0;
    if (rioWriteBulkObject(r,key) == 0) return 0;
    if (rioWriteBulkString(r,groupname,groupname_len) == 0) return 0;
    if (rioWriteBulkString(r,consumer->name,sdslen(consumer->name)) == 0) return 0;
    if (rioWriteBulkString(r,"0",1) == 0) return 0;
    if (rioWriteBulkStreamID(r,&id) == 0) return 0;
    if (rioWriteBulkString(r,"TIME",4) == 0) return 0;
    if (rioWriteBulkLongLong(r,nack->delivery_time) == 0) return 0;
    if (rioWriteBulkString(r,"RETRYCOUNT",10) == 0) return 0;
    if (rioWriteBulkLongLong(r,nack->delivery_count) == 0) return 0;
    if (rioWriteBulkString(r,"JUSTID",6) == 0) return 0;
    if (rioWriteBulkString(r,"FORCE",5) == 0) return 0;
    return 1;
}

/* Helper for rewriteStreamObject(): emit the XGROUP CREATECONSUMER is
 * needed in order to create consumers that do not have any pending entries.
 * All this in the context of the specified key and group. */
int rioWriteStreamEmptyConsumer(rio *r, robj *key, const char *groupname, size_t groupname_len, streamConsumer *consumer) {
    /* XGROUP CREATECONSUMER <key> <group> <consumer> */
    if (rioWriteBulkCount(r,'*',5) == 0) return 0;
    if (rioWriteBulkString(r,"XGROUP",6) == 0) return 0;
    if (rioWriteBulkString(r,"CREATECONSUMER",14) == 0) return 0;
    if (rioWriteBulkObject(r,key) == 0) return 0;
    if (rioWriteBulkString(r,groupname,groupname_len) == 0) return 0;
    if (rioWriteBulkString(r,consumer->name,sdslen(consumer->name)) == 0) return 0;
    return 1;
}

/* Emit the commands needed to rebuild a stream object.
 * The function returns 0 on error, 1 on success. */
int rewriteStreamObject(rio *r, robj *key, robj *o) {
    stream *s = o->ptr;
    streamIterator si;
    streamIteratorStart(&si,s,NULL,NULL,0);
    streamID id;
    int64_t numfields;

    if (s->length) {
        /* Reconstruct the stream data using XADD commands. */
        while(streamIteratorGetID(&si,&id,&numfields)) {
            /* Emit a two elements array for each item. The first is
             * the ID, the second is an array of field-value pairs. */

            /* Emit the XADD <key> <id> ...fields... command. */
            if (!rioWriteBulkCount(r,'*',3+numfields*2) || 
                !rioWriteBulkString(r,"XADD",4) ||
                !rioWriteBulkObject(r,key) ||
                !rioWriteBulkStreamID(r,&id)) 
            {
                streamIteratorStop(&si);
                return 0;
            }
            while(numfields--) {
                unsigned char *field, *value;
                int64_t field_len, value_len;
                streamIteratorGetField(&si,&field,&value,&field_len,&value_len);
                if (!rioWriteBulkString(r,(char*)field,field_len) ||
                    !rioWriteBulkString(r,(char*)value,value_len)) 
                {
                    streamIteratorStop(&si);
                    return 0;                  
                }
            }
        }
    } else {
        /* Use the XADD MAXLEN 0 trick to generate an empty stream if
         * the key we are serializing is an empty string, which is possible
         * for the Stream type. */
        id.ms = 0; id.seq = 1; 
        if (!rioWriteBulkCount(r,'*',7) ||
            !rioWriteBulkString(r,"XADD",4) ||
            !rioWriteBulkObject(r,key) ||
            !rioWriteBulkString(r,"MAXLEN",6) ||
            !rioWriteBulkString(r,"0",1) ||
            !rioWriteBulkStreamID(r,&id) ||
            !rioWriteBulkString(r,"x",1) ||
            !rioWriteBulkString(r,"y",1))
        {
            streamIteratorStop(&si);
            return 0;     
        }
    }

    /* Append XSETID after XADD, make sure lastid is correct,
     * in case of XDEL lastid. */
    if (!rioWriteBulkCount(r,'*',7) ||
        !rioWriteBulkString(r,"XSETID",6) ||
        !rioWriteBulkObject(r,key) ||
        !rioWriteBulkStreamID(r,&s->last_id) ||
        !rioWriteBulkString(r,"ENTRIESADDED",12) ||
        !rioWriteBulkLongLong(r,s->entries_added) ||
        !rioWriteBulkString(r,"MAXDELETEDID",12) ||
        !rioWriteBulkStreamID(r,&s->max_deleted_entry_id)) 
    {
        streamIteratorStop(&si);
        return 0; 
    }


    /* Create all the stream consumer groups. */
    if (s->cgroups) {
        raxIterator ri;
        raxStart(&ri,s->cgroups);
        raxSeek(&ri,"^",NULL,0);
        while(raxNext(&ri)) {
            streamCG *group = ri.data;
            /* Emit the XGROUP CREATE in order to create the group. */
            if (!rioWriteBulkCount(r,'*',7) ||
                !rioWriteBulkString(r,"XGROUP",6) ||
                !rioWriteBulkString(r,"CREATE",6) ||
                !rioWriteBulkObject(r,key) ||
                !rioWriteBulkString(r,(char*)ri.key,ri.key_len) ||
                !rioWriteBulkStreamID(r,&group->last_id) ||
                !rioWriteBulkString(r,"ENTRIESREAD",11) ||
                !rioWriteBulkLongLong(r,group->entries_read))
            {
                raxStop(&ri);
                streamIteratorStop(&si);
                return 0;
            }

            /* Generate XCLAIMs for each consumer that happens to
             * have pending entries. Empty consumers would be generated with
             * XGROUP CREATECONSUMER. */
            raxIterator ri_cons;
            raxStart(&ri_cons,group->consumers);
            raxSeek(&ri_cons,"^",NULL,0);
            while(raxNext(&ri_cons)) {
                streamConsumer *consumer = ri_cons.data;
                /* If there are no pending entries, just emit XGROUP CREATECONSUMER */
                if (raxSize(consumer->pel) == 0) {
                    if (rioWriteStreamEmptyConsumer(r,key,(char*)ri.key,
                                                    ri.key_len,consumer) == 0)
                    {
                        raxStop(&ri_cons);
                        raxStop(&ri);
                        streamIteratorStop(&si);
                        return 0;
                    }
                    continue;
                }
                /* For the current consumer, iterate all the PEL entries
                 * to emit the XCLAIM protocol. */
                raxIterator ri_pel;
                raxStart(&ri_pel,consumer->pel);
                raxSeek(&ri_pel,"^",NULL,0);
                while(raxNext(&ri_pel)) {
                    streamNACK *nack = ri_pel.data;
                    if (rioWriteStreamPendingEntry(r,key,(char*)ri.key,
                                                   ri.key_len,consumer,
                                                   ri_pel.key,nack) == 0)
                    {
                        raxStop(&ri_pel);
                        raxStop(&ri_cons);
                        raxStop(&ri);
                        streamIteratorStop(&si);
                        return 0;
                    }
                }
                raxStop(&ri_pel);
            }
            raxStop(&ri_cons);
        }
        raxStop(&ri);
    }

    streamIteratorStop(&si);
    return 1;
}

/* Call the module type callback in order to rewrite a data type
 * that is exported by a module and is not handled by Redis itself.
 * The function returns 0 on error, 1 on success. */
int rewriteModuleObject(rio *r, robj *key, robj *o, int dbid) {
    RedisModuleIO io;
    moduleValue *mv = o->ptr;
    moduleType *mt = mv->type;
    moduleInitIOContext(io,mt,r,key,dbid);
    mt->aof_rewrite(&io,key,mv->value);
    if (io.ctx) {
        moduleFreeContext(io.ctx);
        zfree(io.ctx);
    }
    return io.error ? 0 : 1;
}

static int rewriteFunctions(rio *aof) {
    dict *functions = functionsLibGet();
    dictIterator *iter = dictGetIterator(functions);
    dictEntry *entry = NULL;
    while ((entry = dictNext(iter))) {
        functionLibInfo *li = dictGetVal(entry);
        if (rioWrite(aof, "*3\r\n", 4) == 0) goto werr;
        char function_load[] = "$8\r\nFUNCTION\r\n$4\r\nLOAD\r\n";
        if (rioWrite(aof, function_load, sizeof(function_load) - 1) == 0) goto werr;
        if (rioWriteBulkString(aof, li->code, sdslen(li->code)) == 0) goto werr;
    }
    dictReleaseIterator(iter);
    return 1;

werr:
    dictReleaseIterator(iter);
    return 0;
}

int rewriteAppendOnlyFileRio(rio *aof) {
    dictIterator *di = NULL;
    dictEntry *de;
    int j;
    long key_count = 0;
    long long updated_time = 0;

    /* Record timestamp at the beginning of rewriting AOF. */
    if (server.aof_timestamp_enabled) {
        sds ts = genAofTimestampAnnotationIfNeeded(1);
        if (rioWrite(aof,ts,sdslen(ts)) == 0) { sdsfree(ts); goto werr; }
        sdsfree(ts);
    }

    if (rewriteFunctions(aof) == 0) goto werr;

    for (j = 0; j < server.dbnum; j++) {
        char selectcmd[] = "*2\r\n$6\r\nSELECT\r\n";
        redisDb *db = server.db+j;
        dict *d = db->dict;
        if (dictSize(d) == 0) continue;
        di = dictGetSafeIterator(d);

        /* SELECT the new DB */
        if (rioWrite(aof,selectcmd,sizeof(selectcmd)-1) == 0) goto werr;
        if (rioWriteBulkLongLong(aof,j) == 0) goto werr;

        /* Iterate this DB writing every entry */
        while((de = dictNext(di)) != NULL) {
            sds keystr;
            robj key, *o;
            long long expiretime;
            size_t aof_bytes_before_key = aof->processed_bytes;

            keystr = dictGetKey(de);
            o = dictGetVal(de);
            initStaticStringObject(key,keystr);

            expiretime = getExpire(db,&key);

            /* Save the key and associated value */
            if (o->type == OBJ_STRING) {
                /* Emit a SET command */
                char cmd[]="*3\r\n$3\r\nSET\r\n";
                if (rioWrite(aof,cmd,sizeof(cmd)-1) == 0) goto werr;
                /* Key and value */
                if (rioWriteBulkObject(aof,&key) == 0) goto werr;
                if (rioWriteBulkObject(aof,o) == 0) goto werr;
            } else if (o->type == OBJ_LIST) {
                if (rewriteListObject(aof,&key,o) == 0) goto werr;
            } else if (o->type == OBJ_SET) {
                if (rewriteSetObject(aof,&key,o) == 0) goto werr;
            } else if (o->type == OBJ_ZSET) {
                if (rewriteSortedSetObject(aof,&key,o) == 0) goto werr;
            } else if (o->type == OBJ_HASH) {
                if (rewriteHashObject(aof,&key,o) == 0) goto werr;
            } else if (o->type == OBJ_STREAM) {
                if (rewriteStreamObject(aof,&key,o) == 0) goto werr;
            } else if (o->type == OBJ_MODULE) {
                if (rewriteModuleObject(aof,&key,o,j) == 0) goto werr;
            } else {
                serverPanic("Unknown object type");
            }

            /* In fork child process, we can try to release memory back to the
             * OS and possibly avoid or decrease COW. We give the dismiss
             * mechanism a hint about an estimated size of the object we stored. */
            size_t dump_size = aof->processed_bytes - aof_bytes_before_key;
            if (server.in_fork_child) dismissObject(o, dump_size);

            /* Save the expire time */
            if (expiretime != -1) {
                char cmd[]="*3\r\n$9\r\nPEXPIREAT\r\n";
                if (rioWrite(aof,cmd,sizeof(cmd)-1) == 0) goto werr;
                if (rioWriteBulkObject(aof,&key) == 0) goto werr;
                if (rioWriteBulkLongLong(aof,expiretime) == 0) goto werr;
            }

            /* Update info every 1 second (approximately).
             * in order to avoid calling mstime() on each iteration, we will
             * check the diff every 1024 keys */
            if ((key_count++ & 1023) == 0) {
                long long now = mstime();
                if (now - updated_time >= 1000) {
                    sendChildInfo(CHILD_INFO_TYPE_CURRENT_INFO, key_count, "AOF rewrite");
                    updated_time = now;
                }
            }

            /* Delay before next key if required (for testing) */
            if (server.rdb_key_save_delay)
                debugDelay(server.rdb_key_save_delay);
        }
        dictReleaseIterator(di);
        di = NULL;
    }
    return C_OK;

werr:
    if (di) dictReleaseIterator(di);
    return C_ERR;
}

/* Write a sequence of commands able to fully rebuild the dataset into
 * "filename". Used both by REWRITEAOF and BGREWRITEAOF.
 *
 * In order to minimize the number of commands needed in the rewritten
 * log Redis uses variadic commands when possible, such as RPUSH, SADD
 * and ZADD. However at max AOF_REWRITE_ITEMS_PER_CMD items per time
 * are inserted using a single command. */
/* 子进程重写 aof 文件 */
int rewriteAppendOnlyFile(char *filename) {
    rio aof;
    FILE *fp = NULL;
    char tmpfile[256];

    /* Note that we have to use a different temp name here compared to the
     * one used by rewriteAppendOnlyFileBackground() function. */
    /* 打开一个临时文件 */
    snprintf(tmpfile,256,"temp-rewriteaof-%d.aof", (int) getpid());
    fp = fopen(tmpfile,"w");
    if (!fp) {
        serverLog(LL_WARNING, "Opening the temp file for AOF rewrite in rewriteAppendOnlyFile(): %s", strerror(errno));
        return C_ERR;
    }

    /* 使用 FILE 类型的 RIO 来将数据写入临时文件 */

    /* 初始化一个 file 类型的 rio */
    rioInitWithFile(&aof,fp);

    /* 如果配置了 aof 增量刷盘 */
    if (server.aof_rewrite_incremental_fsync)
        /* 设置 rio 的 autoSync （自动刷盘的阈值） */
        rioSetAutoSync(&aof,REDIS_AUTOSYNC_BYTES);

    startSaving(RDBFLAGS_AOF_PREAMBLE);

    /* 使用 rdb 编码格式生成 base 文件内容，redis 默认使用这种方式 */
    if (server.aof_use_rdb_preamble) {
        int error;
        /* 使用 rdb 编码格式保存 redis 的数据快照 */
        if (rdbSaveRio(SLAVE_REQ_NONE,&aof,&error,RDBFLAGS_AOF_PREAMBLE,NULL) == C_ERR) {
            errno = error;
            goto werr;
        }
    } else {
        /* 使用 aof 自己的格式生成 base 文件内容 */
        if (rewriteAppendOnlyFileRio(&aof) == C_ERR) goto werr;
    }

    /* Make sure data will not remain on the OS's output buffers */
    /* page cache 刷盘 */
    if (fflush(fp)) goto werr;
    if (fsync(fileno(fp))) goto werr;
    if (fclose(fp)) { fp = NULL; goto werr; }
    fp = NULL;

    /* Use RENAME to make sure the DB file is changed atomically only
     * if the generate DB file is ok. */
    /* 重命名文件 */
    if (rename(tmpfile,filename) == -1) {
        serverLog(LL_WARNING,"Error moving temp append only file on the final destination: %s", strerror(errno));
        unlink(tmpfile);
        stopSaving(0);
        return C_ERR;
    }
    stopSaving(1);

    return C_OK;

werr:
    serverLog(LL_WARNING,"Write error writing append only file on disk: %s", strerror(errno));
    if (fp) fclose(fp);
    unlink(tmpfile);
    stopSaving(0);
    return C_ERR;
}
/* ----------------------------------------------------------------------------
 * AOF background rewrite
 * ------------------------------------------------------------------------- */

/* This is how rewriting of the append only file in background works:
 *
 * 1) The user calls BGREWRITEAOF
 * 2) Redis calls this function, that forks():
 *    2a) the child rewrite the append only file in a temp file.
 *    2b) the parent open a new INCR AOF file to continue writing.
 * 3) When the child finished '2a' exists.
 * 4) The parent will trap the exit code, if it's OK, it will:
 *    4a) get a new BASE file name and mark the previous (if we have) as the HISTORY type
 *    4b) rename(2) the temp file in new BASE file name
 *    4c) mark the rewritten INCR AOFs as history type
 *    4d) persist AOF manifest file
 *    4e) Delete the history files use bio
 */
/* 后台重写 aof 文件的步骤
 * 1) 用户调用 BGREWRITEAOF 命令
 * 2) redis 调用当前函数，fork 子进程
 *    2a) 子进程使用临时文件重写 aof
 *    2b) 父进程打开一个新的 incr 类型的 aof 文件继续写后续到来的写命令
 * 3) 当子进程完成 2a 步骤了
 * 4) 父进程会判断子进程的退出码，如果是 OK
 *    4a) 获取一个新的 base 文件名，标记之前的 base 文件为历史文件
 *    4b) 用新的 base 文件名重命名之前子进程的临时文件
 *    4c) 标记之前的 incr aof 文件为历史文件
 *    4d) 持久化 aof 清单文件
 *    4e) 使用 bio 线程删除历史文件
 *
 * 该函数其实并没有做 4) 的逻辑，4) 的逻辑是在 backgroundRewriteDoneHandler 函数中实现的
 * 看完该函数，可以直接跳到 backgroundRewriteDoneHandler 函数看剩余逻辑
 */
int rewriteAppendOnlyFileBackground(void) {
    pid_t childpid;

    /* 如果有正在运行的子进程，返回 */
    if (hasActiveChildProcess()) return C_ERR;

    if (dirCreateIfMissing(server.aof_dirname) == -1) {
        serverLog(LL_WARNING, "Can't open or create append-only dir %s: %s",
            server.aof_dirname, strerror(errno));
        server.aof_lastbgrewrite_status = C_ERR;
        return C_ERR;
    }

    /* We set aof_selected_db to -1 in order to force the next call to the
     * feedAppendOnlyFile() to issue a SELECT command. */
    /* 将 aof_selected_db 设置为 -1 */
    server.aof_selected_db = -1;
    /* 将 aof 缓冲区中的数据写入磁盘文件 */
    flushAppendOnlyFile(1);
    /* 打开一个新的 incr aof 文件 */
    if (openNewIncrAofForAppend() != C_OK) {
        server.aof_lastbgrewrite_status = C_ERR;
        return C_ERR;
    }
    server.stat_aof_rewrites++;
    /* 调用 fork 系统调用生成子进程 */
    if ((childpid = redisFork(CHILD_TYPE_AOF)) == 0) {
        char tmpfile[256];

        /* 子进程部分的逻辑 */
        /* Child */
        /* 设置进程名 */
        redisSetProcTitle("redis-aof-rewrite");
        /* 给子进程绑定 cpu 核 */
        redisSetCpuAffinity(server.aof_rewrite_cpulist);
        /* 获取临时文件名 */
        snprintf(tmpfile,256,"temp-rewriteaof-bg-%d.aof", (int) getpid());
        /* 使用临时文件写 redis 数据快照 */
        if (rewriteAppendOnlyFile(tmpfile) == C_OK) {
            /* rewrite 成功 */
            serverLog(LL_NOTICE,
                "Successfully created the temporary AOF base file %s", tmpfile);
            sendChildCowInfo(CHILD_INFO_TYPE_AOF_COW_SIZE, "AOF rewrite");
            exitFromChild(0);
        } else {
            exitFromChild(1);
        }
    } else {
        /* Parent */
        if (childpid == -1) {
            server.aof_lastbgrewrite_status = C_ERR;
            serverLog(LL_WARNING,
                "Can't rewrite append only file in background: fork: %s",
                strerror(errno));
            return C_ERR;
        }
        serverLog(LL_NOTICE,
            "Background append only file rewriting started by pid %ld",(long) childpid);
        server.aof_rewrite_scheduled = 0;
        server.aof_rewrite_time_start = time(NULL);
        return C_OK;
    }
    return C_OK; /* unreached */
}

void bgrewriteaofCommand(client *c) {
    if (server.child_type == CHILD_TYPE_AOF) {
        addReplyError(c,"Background append only file rewriting already in progress");
    } else if (hasActiveChildProcess() || server.in_exec) {
        server.aof_rewrite_scheduled = 1;
        /* When manually triggering AOFRW we reset the count 
         * so that it can be executed immediately. */
        server.stat_aofrw_consecutive_failures = 0;
        addReplyStatus(c,"Background append only file rewriting scheduled");
    } else if (rewriteAppendOnlyFileBackground() == C_OK) {
        addReplyStatus(c,"Background append only file rewriting started");
    } else {
        addReplyError(c,"Can't execute an AOF background rewriting. "
                        "Please check the server logs for more information.");
    }
}

/* 移除临时快照文件 */
void aofRemoveTempFile(pid_t childpid) {
    char tmpfile[256];

    snprintf(tmpfile,256,"temp-rewriteaof-bg-%d.aof", (int) childpid);
    /* 后台 bio 线程关闭文件 */
    bg_unlink(tmpfile);

    /* 这个是子进程的临时快照文件，该文件只有子进程会进行操作，子进程写完快照后就会将该文件修改为 temp-rewriteaof-bg-%d.aof 文件 */
    snprintf(tmpfile,256,"temp-rewriteaof-%d.aof", (int) childpid);
    bg_unlink(tmpfile);
}

/* Get size of an AOF file.
 * The status argument is an optional output argument to be filled with
 * one of the AOF_ status values. */
off_t getAppendOnlyFileSize(sds filename, int *status) {
    struct redis_stat sb;
    off_t size;
    mstime_t latency;

    sds aof_filepath = makePath(server.aof_dirname, filename);
    latencyStartMonitor(latency);
    if (redis_stat(aof_filepath, &sb) == -1) {
        if (status) *status = errno == ENOENT ? AOF_NOT_EXIST : AOF_OPEN_ERR;
        serverLog(LL_WARNING, "Unable to obtain the AOF file %s length. stat: %s",
            filename, strerror(errno));
        size = 0;
    } else {
        if (status) *status = AOF_OK;
        size = sb.st_size;
    }
    latencyEndMonitor(latency);
    latencyAddSampleIfNeeded("aof-fstat", latency);
    sdsfree(aof_filepath);
    return size;
}

/* Get size of all AOF files referred by the manifest (excluding history).
 * The status argument is an output argument to be filled with
 * one of the AOF_ status values. */
off_t getBaseAndIncrAppendOnlyFilesSize(aofManifest *am, int *status) {
    off_t size = 0;
    listNode *ln;
    listIter li;

    if (am->base_aof_info) {
        serverAssert(am->base_aof_info->file_type == AOF_FILE_TYPE_BASE);

        size += getAppendOnlyFileSize(am->base_aof_info->file_name, status);
        if (*status != AOF_OK) return 0;
    }

    listRewind(am->incr_aof_list, &li);
    while ((ln = listNext(&li)) != NULL) {
        aofInfo *ai = (aofInfo*)ln->value;
        serverAssert(ai->file_type == AOF_FILE_TYPE_INCR);
        size += getAppendOnlyFileSize(ai->file_name, status);
        if (*status != AOF_OK) return 0;
    }

    return size;
}

/* 获取 base 文件和 incr 文件的总数量 */
int getBaseAndIncrAppendOnlyFilesNum(aofManifest *am) {
    int num = 0;
    if (am->base_aof_info) num++;
    if (am->incr_aof_list) num += listLength(am->incr_aof_list);
    return num;
}

/* A background append only file rewriting (BGREWRITEAOF) terminated its work.
 * Handle this. */
/* 子进程执行完 rewrite aof 之后的工作 */
void backgroundRewriteDoneHandler(int exitcode, int bysignal) {
    if (!bysignal && exitcode == 0) {
        char tmpfile[256];
        long long now = ustime();
        sds new_base_filepath = NULL;
        sds new_incr_filepath = NULL;
        aofManifest *temp_am;
        mstime_t latency;

        serverLog(LL_NOTICE,
            "Background AOF rewrite terminated with success");

        snprintf(tmpfile, 256, "temp-rewriteaof-bg-%d.aof",
            (int)server.child_pid);

        serverAssert(server.aof_manifest != NULL);

        /* Dup a temporary aof_manifest for subsequent modifications. */
        /* 复制一份清单信息 */
        temp_am = aofManifestDup(server.aof_manifest);

        /* Get a new BASE file name and mark the previous (if we have)
         * as the HISTORY type. */
        /* 获取新的 base 文件名，标记之前的 base 文件为历史文件 */
        sds new_base_filename = getNewBaseFileNameAndMarkPreAsHistory(temp_am);
        serverAssert(new_base_filename != NULL);
        new_base_filepath = makePath(server.aof_dirname, new_base_filename);

        /* Rename the temporary aof file to 'new_base_filename'. */
        latencyStartMonitor(latency);
        /* 重命名临时文件为新的 base 文件 */
        if (rename(tmpfile, new_base_filepath) == -1) {
            serverLog(LL_WARNING,
                "Error trying to rename the temporary AOF base file %s into %s: %s",
                tmpfile,
                new_base_filepath,
                strerror(errno));
            aofManifestFree(temp_am);
            sdsfree(new_base_filepath);
            server.aof_lastbgrewrite_status = C_ERR;
            server.stat_aofrw_consecutive_failures++;
            goto cleanup;
        }
        latencyEndMonitor(latency);
        latencyAddSampleIfNeeded("aof-rename", latency);
        serverLog(LL_NOTICE,
            "Successfully renamed the temporary AOF base file %s into %s", tmpfile, new_base_filename);

        /* Rename the temporary incr aof file to 'new_incr_filename'. */
        if (server.aof_state == AOF_WAIT_REWRITE) {
            /* Get temporary incr aof name. */
            /* 获取临时 incr aof 文件名 */
            sds temp_incr_aof_name = getTempIncrAofName();
            sds temp_incr_filepath = makePath(server.aof_dirname, temp_incr_aof_name);
            /* Get next new incr aof name. */
            /* 获取新的 incr aof 文件名 */
            sds new_incr_filename = getNewIncrAofName(temp_am);
            new_incr_filepath = makePath(server.aof_dirname, new_incr_filename);
            latencyStartMonitor(latency);
            /* 将临时文件修改成新的 incr 文件 */
            if (rename(temp_incr_filepath, new_incr_filepath) == -1) {
                serverLog(LL_WARNING,
                    "Error trying to rename the temporary AOF incr file %s into %s: %s",
                    temp_incr_filepath,
                    new_incr_filepath,
                    strerror(errno));
                bg_unlink(new_base_filepath);
                sdsfree(new_base_filepath);
                aofManifestFree(temp_am);
                sdsfree(temp_incr_filepath);
                sdsfree(new_incr_filepath);
                sdsfree(temp_incr_aof_name);
                server.aof_lastbgrewrite_status = C_ERR;
                server.stat_aofrw_consecutive_failures++;
                goto cleanup;
            }
            latencyEndMonitor(latency);
            latencyAddSampleIfNeeded("aof-rename", latency);
            serverLog(LL_NOTICE,
                "Successfully renamed the temporary AOF incr file %s into %s", temp_incr_aof_name, new_incr_filename);
            sdsfree(temp_incr_filepath);
            sdsfree(temp_incr_aof_name);
        }

        /* Change the AOF file type in 'incr_aof_list' from AOF_FILE_TYPE_INCR
         * to AOF_FILE_TYPE_HIST, and move them to the 'history_aof_list'. */
        /* 标记之前的 incr aof 文件为历史文件，并修改清单信息 */
        markRewrittenIncrAofAsHistory(temp_am);

        /* Persist our modifications. */
        /* 持久化清单文件 */
        if (persistAofManifest(temp_am) == C_ERR) {
            bg_unlink(new_base_filepath);
            aofManifestFree(temp_am);
            sdsfree(new_base_filepath);
            if (new_incr_filepath) {
                bg_unlink(new_incr_filepath);
                sdsfree(new_incr_filepath);
            }
            server.aof_lastbgrewrite_status = C_ERR;
            server.stat_aofrw_consecutive_failures++;
            goto cleanup;
        }
        sdsfree(new_base_filepath);
        if (new_incr_filepath) sdsfree(new_incr_filepath);

        /* We can safely let `server.aof_manifest` point to 'temp_am' and free the previous one. */
        aofManifestFreeAndUpdate(temp_am);

        if (server.aof_fd != -1) {
            /* AOF enabled. */
            server.aof_selected_db = -1; /* Make sure SELECT is re-issued */
            server.aof_current_size = getAppendOnlyFileSize(new_base_filename, NULL) + server.aof_last_incr_size;
            server.aof_rewrite_base_size = server.aof_current_size;
            server.aof_fsync_offset = server.aof_current_size;
            server.aof_last_fsync = server.unixtime;
        }

        /* We don't care about the return value of `aofDelHistoryFiles`, because the history
         * deletion failure will not cause any problems. */
        aofDelHistoryFiles();

        server.aof_lastbgrewrite_status = C_OK;
        server.stat_aofrw_consecutive_failures = 0;

        serverLog(LL_NOTICE, "Background AOF rewrite finished successfully");
        /* Change state from WAIT_REWRITE to ON if needed */
        if (server.aof_state == AOF_WAIT_REWRITE)
            server.aof_state = AOF_ON;

        serverLog(LL_VERBOSE,
            "Background AOF rewrite signal handler took %lldus", ustime()-now);
    } else if (!bysignal && exitcode != 0) {
        server.aof_lastbgrewrite_status = C_ERR;
        server.stat_aofrw_consecutive_failures++;

        serverLog(LL_WARNING,
            "Background AOF rewrite terminated with error");
    } else {
        /* SIGUSR1 is whitelisted, so we have a way to kill a child without
         * triggering an error condition. */
        if (bysignal != SIGUSR1) {
            server.aof_lastbgrewrite_status = C_ERR;
            server.stat_aofrw_consecutive_failures++;
        }

        serverLog(LL_WARNING,
            "Background AOF rewrite terminated by signal %d", bysignal);
    }

cleanup:
    aofRemoveTempFile(server.child_pid);
    /* Clear AOF buffer and delete temp incr aof for next rewrite. */
    if (server.aof_state == AOF_WAIT_REWRITE) {
        sdsfree(server.aof_buf);
        server.aof_buf = sdsempty();
        aofDelTempIncrAofFile();
    }
    server.aof_rewrite_time_last = time(NULL)-server.aof_rewrite_time_start;
    server.aof_rewrite_time_start = -1;
    /* Schedule a new rewrite if we are waiting for it to switch the AOF ON. */
    if (server.aof_state == AOF_WAIT_REWRITE)
        server.aof_rewrite_scheduled = 1;
}