Fix replica unable trigger migration when it received CLUSTER SETSLOT in advance

src/cluster_legacy.c

@@ -6390,7 +6390,9 @@ void clusterCommandSetSlot(client *c) {
             server.cluster->migrating_slots_to[slot] = NULL;
         }
 
+        clusterNode *my_primary = clusterNodeGetPrimary(myself);
         int slot_was_mine = server.cluster->slots[slot] == myself;
+        int slot_was_my_primary = server.cluster->slots[slot] == my_primary;
         clusterDelSlot(slot);
         clusterAddSlot(n, slot);
 
@@ -6407,6 +6409,25 @@ void clusterCommandSetSlot(client *c) {
             clusterDoBeforeSleep(CLUSTER_TODO_SAVE_CONFIG | CLUSTER_TODO_UPDATE_STATE | CLUSTER_TODO_FSYNC_CONFIG);
         }
 
+        /* If we are a replica and executing the CLUSTER SETSLOT command from
+         * my primary, and my primary left without slots, we should turn into a
+         * replica of the new primary. */
+        if (nodeIsReplica(myself) && slot_was_my_primary && my_primary->numslots == 0 &&
+            server.cluster_allow_replica_migration) {
+            serverLog(LL_NOTICE,
+                      "Lost my last slot during slot migration. Reconfiguring myself "
+                      "as a replica of %.40s (%s) in shard %.40s",
+                      n->name, n->human_nodename, n->shard_id);
+            /* We are a replica and the client is actually my primary, the following
+             * clusterSetPrimary will free the client so we need to protect it. */
+            protectClient(c);
+            /* We are migrating to a different shard that has a completely different
+             * replication history, so a full sync is required. */
+            clusterSetPrimary(n, 1, 1);
+            unprotectClient(c);
+            clusterDoBeforeSleep(CLUSTER_TODO_SAVE_CONFIG | CLUSTER_TODO_UPDATE_STATE | CLUSTER_TODO_FSYNC_CONFIG);
+        }
+
         /* If this node or this node's primary was importing this slot,
          * assigning the slot to itself also clears the importing status. */
         if ((n == myself || n == myself->replicaof) && server.cluster->importing_slots_from[slot]) {

src/networking.c

@@ -2560,6 +2560,7 @@ void freeSharedQueryBuf(void) {
  *
  * * DEBUG RELOAD and similar.
  * * When a Lua script is in -BUSY state.
+ * * A cluster replica doing CLUSTER SETSLOT and doing migration.
  *
  * So the function will protect the client by doing two things:
  *

tests/unit/cluster/replica-migration.tcl

@@ -334,11 +334,17 @@ start_cluster 4 4 {tags {external:skip cluster} overrides {cluster-node-timeout
     test_sub_replica "sigstop"
 } my_slot_allocation cluster_allocate_replicas ;# start_cluster
 
-start_cluster 4 4 {tags {external:skip cluster} overrides {cluster-node-timeout 1000 cluster-migration-barrier 999}} {
-    test "valkey-cli make source node ignores NOREPLICAS error when doing the last CLUSTER SETSLOT" {
+proc test_cluster_setslot {type} {
+    test "valkey-cli make source node ignores NOREPLICAS error when doing the last CLUSTER SETSLOT - $type" {
         R 3 config set cluster-allow-replica-migration no
         R 7 config set cluster-allow-replica-migration yes
 
+        if {$type == "setslot"} {
+            # Make R 7 drop the PING message so that we have a higher
+            # chance to trigger the migration from CLUSTER SETSLOT.
+            R 7 DEBUG DROP-CLUSTER-PACKET-FILTER 1
+        }
+
         # Move slot 0 from primary 3 to primary 0.
         set addr "[srv 0 host]:[srv 0 port]"
         set myid [R 3 CLUSTER MYID]
@@ -349,6 +355,13 @@ start_cluster 4 4 {tags {external:skip cluster} overrides {cluster-node-timeout
             fail "valkey-cli --cluster rebalance returns non-zero exit code, output below:\n$result"
         }
 
+        # Wait for R 3 to report that it is an empty replica (cluster-allow-replica-migration no)
+        wait_for_log_messages -3 {"*I am now an empty primary*"} 0 1000 50
+
+        if {$type == "setslot"} {
+            R 7 DEBUG DROP-CLUSTER-PACKET-FILTER -1
+        }
+
         # Make sure server 3 lost its replica (server 7) and server 7 becomes a replica of primary 0.
         wait_for_condition 1000 50 {
             [s -3 role] eq {master} &&
@@ -361,4 +374,12 @@ start_cluster 4 4 {tags {external:skip cluster} overrides {cluster-node-timeout
             fail "Server 3 and 7 role response has not changed"
         }
     }
+}
+
+start_cluster 4 4 {tags {external:skip cluster} overrides {cluster-node-timeout 1000 cluster-migration-barrier 999}} {
+    test_cluster_setslot "gossip"
+} my_slot_allocation cluster_allocate_replicas ;# start_cluster
+
+start_cluster 4 4 {tags {external:skip cluster} overrides {cluster-node-timeout 1000 cluster-migration-barrier 999}} {
+    test_cluster_setslot "setslot"
 } my_slot_allocation cluster_allocate_replicas ;# start_cluster