To avoid bouncing -REDIRECT during FAILOVER (valkey-io#871)

Fix valkey-io#821

During the `FAILOVER` process, when conditions are met (such as when the
force time is reached or the primary and replica offsets are
consistent), the primary actively becomes the replica and transitions to
the `FAILOVER_IN_PROGRESS` state. After the primary becomes the replica,
and after handshaking and other operations, it will eventually send the
`PSYNC FAILOVER` command to the replica, after which the replica will
become the primary. This means that the upgrade of the replica to the
primary is an asynchronous operation, which implies that during the
`FAILOVER_IN_PROGRESS` state, there may be a period of time where both
nodes are replicas. In this scenario, if a `-REDIRECT` is returned, the
request will be redirected to the replica and then redirected back,
causing back and forth redirection. To avoid this situation, during the
`FAILOVER_IN_PROGRESS state`, we temporarily suspend the clients that
need to be redirected until the replica truly becomes the primary, and
then resume the execution.

---------

Signed-off-by: zhaozhao.zz <[email protected]>
Signed-off-by: Ping Xie <[email protected]>

src/server.c

@@ -3963,13 +3963,6 @@ int processCommand(client *c) {
              * and then resume the execution. */
             blockPostponeClient(c);
         } else {
-            if (c->cmd->proc == execCommand) {
-                discardTransaction(c);
-            } else {
-                flagTransaction(c);
-            }
-            c->duration = 0;
-            c->cmd->rejected_calls++;
             addReplyErrorSds(c, sdscatprintf(sdsempty(), "-REDIRECT %s:%d", server.primary_host, server.primary_port));
         }
         return C_OK;

tests/integration/replica-redirect.tcl

@@ -9,6 +9,10 @@ start_server {tags {needs:repl external:skip}} {
         set replica_port [srv 0 port]
         set replica_pid [srv 0 pid]
 
+        set replica_host [srv 0 host]
+        set replica_port [srv 0 port]
+        set replica_pid [srv 0 pid]
+
         test {write command inside MULTI is QUEUED, EXEC should be REDIRECT} {
             set rr [valkey_client]
             $rr client capa redirect