Converge divergent shard-id persisted in nodes.conf to primary's shard id

src/cluster_legacy.c

@@ -316,10 +316,16 @@ int auxShardIdSetter(clusterNode *n, void *value, size_t length) {
     }
     memcpy(n->shard_id, value, CLUSTER_NAMELEN);
     /* if n already has replicas, make sure they all agree
-     * on the shard id */
+     * on the shard id. If not, update them. */
     for (int i = 0; i < n->num_replicas; i++) {
         if (memcmp(n->replicas[i]->shard_id, n->shard_id, CLUSTER_NAMELEN) != 0) {
-            return C_ERR;
+            serverLog(LL_NOTICE,
+                      "Node %.40s has a different shard id (%.40s) than its primary's shard id %.40s (%.40s). "
+                      "Updating replica's shard id to match primary's shard id.",
+                      n->replicas[i]->name, n->replicas[i]->shard_id, n->name, n->shard_id);
+            clusterRemoveNodeFromShard(n->replicas[i]);
+            memcpy(n->replicas[i]->shard_id, n->shard_id, CLUSTER_NAMELEN);
+            clusterAddNodeToShard(n->shard_id, n->replicas[i]);
         }
     }
     clusterAddNodeToShard(value, n);
@@ -720,10 +726,16 @@ int clusterLoadConfig(char *filename) {
                 clusterAddNodeToShard(primary->shard_id, n);
             } else if (clusterGetNodesInMyShard(primary) != NULL &&
                        memcmp(primary->shard_id, n->shard_id, CLUSTER_NAMELEN) != 0) {
-                /* If the primary has been added to a shard, make sure this
-                 * node has the same persisted shard id as the primary. */
-                sdsfreesplitres(argv, argc);
-                goto fmterr;
+                /* If the primary has been added to a shard and this replica has
+                 * a different shard id stored in nodes.conf, update it to match
+                 * the primary instead of aborting the startup. */
+                serverLog(LL_NOTICE,
+                          "Node %.40s has a different shard id (%.40s) than its primary %.40s (%.40s). "
+                          "Updating replica's shard id to match primary's shard id.",
+                          n->name, n->shard_id, primary->name, primary->shard_id);
+                clusterRemoveNodeFromShard(n);
+                memcpy(n->shard_id, primary->shard_id, CLUSTER_NAMELEN);
+                clusterAddNodeToShard(primary->shard_id, n);
             }
             n->replicaof = primary;
             clusterNodeAddReplica(primary, n);

tests/assets/divergent-shard-1.conf

@@ -0,0 +1,3 @@
+43ee1cacd6948ee96bb367eb8795e62e8d153f05 127.0.0.1:0@6379,,tls-port=0,shard-id=f91532eb722943e035f34292cf586f3f750d65bd myself,master - 0 1749488968682 13 connected 0-16383
+8d89f4d4e7c57a2819277732f86213241c3ec0d3 127.0.0.1:0@6380,,tls-port=0,shard-id=a91532eb722943e035f34292cf586f3f750d65bd slave 43ee1cacd6948ee96bb367eb8795e62e8d153f05 0 1749488968682 13 connected
+vars currentEpoch 13 lastVoteEpoch 9

tests/assets/divergent-shard-2.conf

@@ -0,0 +1,3 @@
+8d89f4d4e7c57a2819277732f86213241c3ec0d3 127.0.0.1:0@6380,,tls-port=0,shard-id=a91532eb722943e035f34292cf586f3f750d65bd slave 43ee1cacd6948ee96bb367eb8795e62e8d153f05 0 1749488968682 13 connected
+43ee1cacd6948ee96bb367eb8795e62e8d153f05 127.0.0.1:0@6379,,tls-port=0,shard-id=f91532eb722943e035f34292cf586f3f750d65bd myself,master - 0 1749488968682 13 connected 0-16383
+vars currentEpoch 13 lastVoteEpoch 9

tests/assets/divergent-shard-3.conf

@@ -0,0 +1,3 @@
+43ee1cacd6948ee96bb367eb8795e62e8d153f05 127.0.0.1:0@6379,,tls-port=0,shard-id=f91532eb722943e035f34292cf586f3f750d65bd master - 0 1749488968682 13 connected 0-16383
+8d89f4d4e7c57a2819277732f86213241c3ec0d3 127.0.0.1:0@6380,,tls-port=0,shard-id=a91532eb722943e035f34292cf586f3f750d65bd myself,slave 43ee1cacd6948ee96bb367eb8795e62e8d153f05 0 1749488968682 13 connected
+vars currentEpoch 13 lastVoteEpoch 9

tests/assets/divergent-shard-4.conf

@@ -0,0 +1,3 @@
+8d89f4d4e7c57a2819277732f86213241c3ec0d3 127.0.0.1:0@6380,,tls-port=0,shard-id=a91532eb722943e035f34292cf586f3f750d65bd myself,slave 43ee1cacd6948ee96bb367eb8795e62e8d153f05 0 1749488968682 13 connected
+43ee1cacd6948ee96bb367eb8795e62e8d153f05 127.0.0.1:0@6379,,tls-port=0,shard-id=f91532eb722943e035f34292cf586f3f750d65bd master - 0 1749488968682 13 connected 0-16383
+vars currentEpoch 13 lastVoteEpoch 9

tests/test_helper.tcl

@@ -281,6 +281,7 @@ proc cleanup {} {
     if {!$::quiet} {puts -nonewline "Cleanup: may take some time... "}
     flush stdout
     catch {exec rm -rf {*}[glob tests/tmp/valkey.conf.*]}
+    catch {exec rm -rf {*}[glob tests/tmp/nodes.conf.*]}
     catch {exec rm -rf {*}[glob tests/tmp/server*.*]}
     catch {exec rm -rf {*}[glob tests/tmp/*.acl.*]}
     if {!$::quiet} {puts "OK"}

tests/unit/cluster/divergent-cluster-shardid-conf.tcl

@@ -0,0 +1,14 @@
+tags {external:skip cluster singledb} {
+    # Start a cluster with a divergent shard ID configuration
+    test "divergent cluster shardid conflict" {
+        for {set i 1} {$i <= 4} {incr i} {
+            if {$::verbose} { puts "Testing for tests/assets/divergent-shard-$i.conf"; flush stdout;}
+            exec cp -f tests/assets/divergent-shard-$i.conf tests/tmp/nodes.conf.divergent
+            start_server {overrides {"cluster-enabled" "yes" "cluster-config-file" "../nodes.conf.divergent"}} {
+                set shardid [r CLUSTER MYSHARDID]
+                set count [exec grep -c $shardid tests/tmp/nodes.conf.divergent];
+                assert_equal $count 2 "Expect shard ID to be present twice in the configuration file"
+            }
+        }
+    }
+}