Replication: start BGSAVE for replication always in replicationCron().

This makes the replication code conceptually simpler by removing the
synchronous BGSAVE trigger in syncCommand(). This also means that
socket and disk BGSAVE targets are handled by the same code.

src/replication.c

@@ -647,7 +647,7 @@ void syncCommand(client *c) {
         } else {
             /* No way, we need to wait for the next BGSAVE in order to
              * register differences. */
-            serverLog(LL_NOTICE,"Waiting for next BGSAVE for SYNC");
+            serverLog(LL_NOTICE,"Can't attach the slave to the current BGSAVE. Waiting for next BGSAVE for SYNC");
         }
 
     /* CASE 2: BGSAVE is in progress, with socket target. */
@@ -657,7 +657,7 @@ void syncCommand(client *c) {
         /* There is an RDB child process but it is writing directly to
          * children sockets. We need to wait for the next BGSAVE
          * in order to synchronize. */
-        serverLog(LL_NOTICE,"Waiting for next BGSAVE for SYNC");
+        serverLog(LL_NOTICE,"Current BGSAVE has socket target. Waiting for next BGSAVE for SYNC");
 
     /* CASE 3: There is no BGSAVE is progress. */
     } else {
@@ -671,7 +671,7 @@ void syncCommand(client *c) {
             /* Target is disk (or the slave is not capable of supporting
              * diskless replication) and we don't have a BGSAVE in progress,
              * let's start one. */
-            if (startBgsaveForReplication(c->slave_capa) != C_OK) return;
+            serverLog(LL_NOTICE,"No BGSAVE in progress. Starting one ASAP");
         }
     }
 
@@ -2307,13 +2307,12 @@ void replicationCron(void) {
         replicationScriptCacheFlush();
     }
 
-    /* If we are using diskless replication and there are slaves waiting
-     * in WAIT_BGSAVE_START state, check if enough seconds elapsed and
-     * start a BGSAVE.
+    /* Start a BGSAVE good for replication if we have slaves in
+     * WAIT_BGSAVE_START state.
      *
-     * This code is also useful to trigger a BGSAVE if the diskless
-     * replication was turned off with CONFIG SET, while there were already
-     * slaves in WAIT_BGSAVE_START state. */
+     * In case of diskless replication, we make sure to wait the specified
+     * number of seconds (according to configuration) so that other slaves
+     * have the time to arrive before we start streaming. */
     if (server.rdb_child_pid == -1 && server.aof_child_pid == -1) {
         time_t idle, max_idle = 0;
         int slaves_waiting = 0;
@@ -2333,9 +2332,13 @@ void replicationCron(void) {
             }
         }
 
-        if (slaves_waiting && max_idle > server.repl_diskless_sync_delay) {
-            /* Start a BGSAVE. Usually with socket target, or with disk target
-             * if there was a recent socket -> disk config change. */
+        if (slaves_waiting &&
+            (!server.repl_diskless_sync ||
+             max_idle > server.repl_diskless_sync_delay))
+        {
+            /* Start the BGSAVE. The called function may start a
+             * BGSAVE with socket target or disk target depending on the
+             * configuration and slaves capabilities. */
             startBgsaveForReplication(mincapa);
         }
     }