Fix crash where command duration is not reset when client is blocked … (#526)

In #11012, we changed the way command durations were computed to handle
the same command being executed multiple times. In #11970, we added an
assert if the duration is not properly reset, potentially indicating
that a call to report statistics was missed.

I found an edge case where this happens - easily reproduced by blocking
a client on `XGROUPREAD` and migrating the stream's slot. This causes
the engine to process the `XGROUPREAD` command twice:

1. First time, we are blocked on the stream, so we wait for unblock to
come back to it a second time. In most cases, when we come back to
process the command second time after unblock, we process the command
normally, which includes recording the duration and then resetting it.
2. After unblocking we come back to process the command, and this is
where we hit the edge case - at this point, we had already migrated the
slot to another node, so we return a `MOVED` response. But when we do
that, we don’t reset the duration field.

Fix: also reset the duration when returning a `MOVED` response. I think
this is right, because the client should redirect the command to the
right node, which in turn will calculate the execution duration.

Also wrote a test which reproduces this, it fails without the fix and
passes with it.

---------

Signed-off-by: Nitai Caro <caronita@amazon.com>
Co-authored-by: Nitai Caro <caronita@amazon.com>

src/server.c

@@ -3883,6 +3883,7 @@ int processCommand(client *c) {
                 flagTransaction(c);
             }
             clusterRedirectClient(c, n, c->slot, error_code);
+            c->duration = 0;
             c->cmd->rejected_calls++;
             return C_OK;
         }

tests/unit/cluster/slot-migration.tcl

@@ -422,3 +422,24 @@ start_cluster 3 3 {tags {external:skip cluster} overrides {cluster-allow-replica
         resume_process [srv -3 pid]
     }
 }
+
+start_cluster 2 0 {tags {external:skip cluster regression} overrides {cluster-allow-replica-migration no cluster-node-timeout 1000} } {
+    # Issue #563 regression test
+    test "Client blocked on XREADGROUP while stream's slot is migrated" {
+        set stream_name aga
+        set slot 609
+
+        # Start a deferring client to simulate a blocked client on XREADGROUP
+        R 0 XGROUP CREATE $stream_name mygroup $ MKSTREAM
+        set rd [valkey_deferring_client]
+        $rd xreadgroup GROUP mygroup consumer BLOCK 0 streams $stream_name >
+        wait_for_blocked_client
+
+        # Migrate the slot to the target node
+        R 0 CLUSTER SETSLOT $slot MIGRATING [dict get [cluster_get_myself 1] id]
+        R 1 CLUSTER SETSLOT $slot IMPORTING [dict get [cluster_get_myself 0] id]
+
+        # This line should cause the crash
+        R 0 MIGRATE 127.0.0.1 [lindex [R 1 CONFIG GET port] 1] $stream_name 0 5000
+    }
+}