Reset repl_down_since to zero only on state change

We should reset repl_down_since only on state change, in the
current code, if the rdb channel in the dual channel is normal,
that is, rdb is loaded normally, but the psync channel is
abnormal, we will set repl_down_since 0 here. If the primary
is down at this time, the replica may be abnormal when calculating
data_age in cluster failover, since repl_state != REPL_STATE_CONNECTED,
this causes the replica to be unable to initiate an election due
to the old data_age.

In dualChannelSyncHandleRdbLoadCompletion, if the psync channel
is not established, the function will return. We will set repl_state
to REPL_STATE_CONNECTED and set repl_down_since to 0 in
dualChannelSyncSuccess, that is, in establishPrimaryConnection.

See also 677d10b for more details.

Signed-off-by: Binbin <[email protected]>

src/replication.c

@@ -2389,10 +2389,10 @@ void readSyncBulkPayload(connection *conn) {
     } else {
         replicationCreatePrimaryClient(server.repl_transfer_s, rsi.repl_stream_db);
         server.repl_state = REPL_STATE_CONNECTED;
+        server.repl_down_since = 0;
         /* Send the initial ACK immediately to put this replica in online state. */
         replicationSendAck();
     }
-    server.repl_down_since = 0;
 
     /* Fire the primary link modules event. */
     moduleFireServerEvent(VALKEYMODULE_EVENT_PRIMARY_LINK_CHANGE, VALKEYMODULE_SUBEVENT_PRIMARY_LINK_UP, NULL);