Free backlog only if rsi is invalid when master reboot (#12088)

When master reboot from RDB, if rsi in RDB is valid we should not free replication backlog, even if master_repl_offset or repl-offset is 0.

Since if master doesn't send any data to replicas master_repl_offset is 0, it's a valid number.

A clear example:

1. start a master and apply some write commands, the master's master_repl_offset is 0 since it has no replicas.
2. stop write commands on master, and start another instance and replicaof the master, trigger an FULLRESYNC
3. the master's master_repl_offset is still 0 (set a large number for repl-ping-replica-period), do BGSAVE and restart the master
4. master load master_repl_offset from RDB's rsi and it's still 0, and we should make sure replica can partially resync with master.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/server.c b/src/server.c
index 381edabc6..b6ed1d9c5 100644
--- a/src/server.c
+++ b/src/server.c
@@ -6700,6 +6700,7 @@ void loadDataFromDisk(void) {
             serverLog(LL_NOTICE, "DB loaded from append only file: %.3f seconds", (float)(ustime()-start)/1000000);
     } else {
         rdbSaveInfo rsi = RDB_SAVE_INFO_INIT;
+        int rsi_is_valid = 0;
         errno = 0; /* Prevent a stale value from affecting error checking */
         int rdb_flags = RDBFLAGS_NONE;
         if (iAmMaster()) {
@@ -6721,6 +6722,7 @@ void loadDataFromDisk(void) {
                  * information in function rdbPopulateSaveInfo. */
                 rsi.repl_stream_db != -1)
             {
+                rsi_is_valid = 1;
                 if (!iAmMaster()) {
                     memcpy(server.replid,rsi.repl_id,sizeof(server.replid));
                     server.master_repl_offset = rsi.repl_offset;
@@ -6754,7 +6756,7 @@ void loadDataFromDisk(void) {
          * if RDB doesn't have replication info or there is no rdb, it is not
          * possible to support partial resynchronization, to avoid extra memory
          * of replication backlog, we drop it. */
-        if (server.master_repl_offset == 0 && server.repl_backlog)
+        if (!rsi_is_valid && server.repl_backlog)
             freeReplicationBacklog();
     }
 }