分库分表：merger 分页实现

.CHANGELOG.md

@@ -16,6 +16,7 @@
 - [eorm: 分库分表: Merger抽象与批量查询实现](https://github.com/ecodeclub/eorm/pull/160)
 - [eorm: 增强的 ShardingAlgorithm 设计与实现](https://github.com/ecodeclub/eorm/pull/161)
 - [eorm: 分库分表: Merger排序实现](https://github.com/ecodeclub/eorm/pull/166)
+- [eorm: 分库分表: Merger分页实现](https://github.com/ecodeclub/eorm/pull/175)
 - [eorm: BasicTypeValue重命名](https://github.com/ecodeclub/eorm/pull/177)
 - [eorm: 分库分表: hash、shadow_hash算法不符合预期](https://github.com/ecodeclub/eorm/pull/174)
 

internal/merger/internal/errs/error.go

@@ -20,12 +20,13 @@ import (
 )
 
 var (
-	ErrEmptySortColumns  = errors.New("merger: 排序列为空")
-	ErrMergerEmptyRows   = errors.New("merger: sql.Rows列表为空")
-	ErrMergerRowsIsNull  = errors.New("merger: sql.Rows列表中有元素为nil")
-	ErrMergerScanNotNext = errors.New("merger: Scan之前没有调用Next方法")
-	ErrMergerRowsClosed  = errors.New("merger: Rows已经关闭")
-	ErrMergerRowsDiff    = errors.New("merger: sql.Rows列表中的字段不同")
+	ErrEmptySortColumns           = errors.New("merger: 排序列为空")
+	ErrMergerEmptyRows            = errors.New("merger: sql.Rows列表为空")
+	ErrMergerRowsIsNull           = errors.New("merger: sql.Rows列表中有元素为nil")
+	ErrMergerScanNotNext          = errors.New("merger: Scan之前没有调用Next方法")
+	ErrMergerRowsClosed           = errors.New("merger: Rows已经关闭")
+	ErrMergerRowsDiff             = errors.New("merger: sql.Rows列表中的字段不同")
+	ErrMergerInvalidLimitOrOffset = errors.New("merger: offset或limit小于0")
 )
 
 func NewRepeatSortColumn(column string) error {

internal/merger/pagedmerger/merger.go

@@ -0,0 +1,148 @@
+// Copyright 2021 ecodeclub
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package pagedmerger
+
+import (
+	"context"
+	"database/sql"
+	"sync"
+
+	"github.com/ecodeclub/eorm/internal/merger"
+	"github.com/ecodeclub/eorm/internal/merger/internal/errs"
+)
+
+type Merger struct {
+	m      merger.Merger
+	limit  int
+	offset int
+}
+
+func NewMerger(m merger.Merger, offset int, limit int) (*Merger, error) {
+	if offset < 0 || limit <= 0 {
+		return nil, errs.ErrMergerInvalidLimitOrOffset
+	}
+
+	return &Merger{
+		m:      m,
+		limit:  limit,
+		offset: offset,
+	}, nil
+}
+
+func (m *Merger) Merge(ctx context.Context, results []*sql.Rows) (merger.Rows, error) {
+	rows, err := m.m.Merge(ctx, results)
+	if err != nil {
+		return nil, err
+	}
+	err = m.nextOffset(ctx, rows)
+	if err != nil {
+		return nil, err
+	}
+	return &Rows{
+		rows:  rows,
+		mu:    &sync.RWMutex{},
+		limit: m.limit,
+	}, nil
+}
+
+// nextOffset 会把游标挪到 offset 所指定的位置。
+func (m *Merger) nextOffset(ctx context.Context, rows merger.Rows) error {
+	offset := m.offset
+	for i := 0; i < offset; i++ {
+		if ctx.Err() != nil {
+			return ctx.Err()
+		}
+		// 如果偏移量超过rows结果集返回的行数，不会报错。用户最终查到0行
+		if !rows.Next() {
+			return rows.Err()
+		}
+	}
+	return nil
+}
+
+type Rows struct {
+	rows    merger.Rows
+	limit   int
+	cnt     int
+	lastErr error
+	closed  bool
+	mu      *sync.RWMutex
+}
+
+func (r *Rows) Next() bool {
+	r.mu.Lock()
+	if r.closed {
+		r.mu.Unlock()
+		return false
+	}
+	if r.cnt >= r.limit || r.lastErr != nil {
+		r.mu.Unlock()
+		_ = r.Close()
+		return false
+	}
+	canNext, err := r.nextVal()
+	if err != nil {
+		r.lastErr = err
+		r.mu.Unlock()
+		_ = r.Close()
+		return false
+	}
+	if !canNext {
+		r.mu.Unlock()
+		_ = r.Close()
+		return canNext
+	}
+	r.cnt++
+	r.mu.Unlock()
+	return canNext
+}
+func (r *Rows) nextVal() (bool, error) {
+	if r.rows.Next() {
+		return true, nil
+	}
+	if r.rows.Err() != nil {
+		return false, r.rows.Err()
+	}
+	return false, nil
+}
+
+func (r *Rows) Scan(dest ...any) error {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	if r.lastErr != nil {
+		return r.lastErr
+	}
+	if r.closed {
+		return errs.ErrMergerRowsClosed
+	}
+	return r.rows.Scan(dest...)
+}
+
+func (r *Rows) Close() error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.closed = true
+	return r.rows.Close()
+}
+
+func (r *Rows) Columns() ([]string, error) {
+	return r.rows.Columns()
+}
+
+func (r *Rows) Err() error {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	return r.lastErr
+}