Test rdb-connection race condition

Test the case in which the replica establish psync connection after the
RDB was already loaded.
Added debug command in order to froce main process to be slower then the
bg child.

src/debug.c

@@ -494,6 +494,8 @@ void debugCommand(client *c) {
 "    Enable or disable the reply buffer resize cron job",
 "CLUSTERLINK KILL <to|from|all> <node-id>",
 "    Kills the link based on the direction to/from (both) with the provided node." ,
+"SLEEP-AFTER-FORK <seconds>",
+"    Stop the server's main process for <seconds> after forking.",
 NULL
         };
         addReplyHelp(c, help);
@@ -1031,6 +1033,10 @@ NULL
             addReplyErrorFormat(c, "Unknown direction %s", (char*) c->argv[3]->ptr);
         }
         addReply(c,shared.ok);
+    } else if(!strcasecmp(c->argv[1]->ptr,"SLEEP-AFTER-FORK") &&
+        c->argc == 3) {
+        server.debug_sleep_after_fork = atoi(c->argv[2]->ptr);
+        addReply(c,shared.ok);
     } else {
         addReplySubcommandSyntaxError(c);
         return;

src/replication.c

@@ -900,6 +900,7 @@ int startBgsaveForReplication(int mincapa, int req) {
             /* Keep the page cache since it'll get used soon */
             retval = rdbSaveBackground(req,server.rdb_filename,rsiptr,RDBFLAGS_KEEP_CACHE);
         }
+        if (server.debug_sleep_after_fork) sleep(server.debug_sleep_after_fork);
     } else {
         serverLog(LL_WARNING,"BGSAVE for replication: replication information not available, can't generate the RDB file right now. Try later.");
         retval = C_ERR;

src/server.h

@@ -1887,6 +1887,8 @@ struct redisServer {
                                          * delay (start sooner if they all connect). */
     int rdb_channel_enabled;        /* Config used to determine if the replica should 
                                      * use rdb channel for full syncs. */
+    int debug_sleep_after_fork;     /* Debug param that force the main connection to 
+                                     * sleep for N seconds after fork() in repl. */
     size_t repl_buffer_mem;         /* The memory of replication buffer. */
     list *repl_buffer_blocks;       /* Replication buffers blocks list
                                      * (serving replica clients and repl backlog) */

tests/integration/replication.tcl

@@ -1789,4 +1789,38 @@ start_server {tags {"repl rdb-channel external:skip"}} {
             }
         }
     }
+}
+
+start_server {tags {"repl rdb-channel external:skip"}} {
+    set replica [srv 0 client]
+    set replica_host [srv 0 host]
+    set replica_port [srv 0 port]
+    set replica_log [srv 0 stdout]
+    start_server {} {
+        set master [srv 0 client]
+        set master_host [srv 0 host]
+        set master_port [srv 0 port]
+        set loglines [count_log_lines -1]
+        # Create small enough db to be loaded before replica establish psync connection
+        $master set key1 val1
+
+        $master config set repl-diskless-sync yes
+        $master debug sleep-after-fork 5;# Stop master after fork
+
+        $replica config set repl-rdb-channel yes
+        $replica config set loglevel debug
+
+        test "Test rdb-channel psync established after rdb load" {
+            $replica slaveof $master_host $master_port
+
+            wait_for_value_to_propegate_to_replica $master $replica "key1"
+
+            verify_replica_online $master 0 500
+            set res [wait_for_log_messages -1 {"*MASTER <-> REPLICA sync: Finished with success*"} $loglines 2000 1]
+            # Confirm the occurrence of a race condition.
+            set res [wait_for_log_messages -1 {"*RDB channel sync - psync established after rdb load*"} $loglines 2000 1]
+            set loglines [lindex $res 1]
+            incr $loglines                
+        }
+    }
 }