Authenticate slot migration client on source node to internal user (#2785)

Just setting the authenticated flag actually authenticates to the
default user in this case. The default user may be granted no permission
to use CLUSTER SYNCSLOTS.

Instaed, we now authenticate to the NULL/internal user, which grants
access to all commands. This is the same as what we do for replication:


https://github.com/valkey-io/valkey/blob/864de555ced5354976ae4f97f44977041556115f/src/replication.c#L4717

Add a test for this case as well.

Closes #2783

Signed-off-by: Jacob Murphy <[email protected]>

Author: murphyjacob4

src/cluster_migrateslots.c

@@ -1385,7 +1385,7 @@ void initSlotExportJobClient(slotMigrationJob *job) {
     serverAssert(job->type == SLOT_MIGRATION_EXPORT);
     job->client = createClient(job->conn);
     job->conn = NULL;
-    job->client->flag.authenticated = 1;
+    clientSetUser(job->client, NULL, 1);
     job->client->slot_migration_job = job;
     initClientReplicationData(job->client);
 }

tests/unit/cluster/cluster-migrateslots.tcl

@@ -2116,6 +2116,42 @@ start_cluster 3 3 {tags {logreqres:skip external:skip cluster} overrides {cluste
     }
 }
 
+start_cluster 3 0 {tags {logreqres:skip external:skip cluster}} {
+    set 16383_slot_tag "{6ZJ}"
+    set node0_id [R 0 CLUSTER MYID]
+
+    test "Import with default user having no permission" {
+        # Configure ACLs on both source and target nodes
+        R 2 ACL SETUSER admin on >S3cureAdmin! ~* &* +@all
+        R 2 ACL SETUSER default off -@all
+        R 2 AUTH admin S3cureAdmin!
+
+        R 0 ACL SETUSER admin on >S3cureAdmin! ~* &* +@all
+        R 0 ACL SETUSER default off -@all
+        R 0 AUTH admin S3cureAdmin!
+
+        # Set primaryuser and primaryauth on the source node
+        R 2 CONFIG SET primaryuser admin
+        R 2 CONFIG SET primaryauth S3cureAdmin!
+
+        # Populate data before migration
+        populate 1000 "$16383_slot_tag:" 1000 -2
+
+        # Perform one-shot import
+        assert_match "OK" [R 2 CLUSTER MIGRATESLOTS SLOTSRANGE 16383 16383 NODE $node0_id]
+        set jobname [get_job_name 2 16383]
+        wait_for_migration 0 16383
+
+        # Keys successfully migrated
+        assert_match "1000" [R 0 CLUSTER COUNTKEYSINSLOT 16383]
+        assert_match "0" [R 2 CLUSTER COUNTKEYSINSLOT 16383]
+
+        # Migration log shows success on both ends
+        assert {[dict get [get_migration_by_name 0 $jobname] state] eq "success"}
+        assert {[dict get [get_migration_by_name 2 $jobname] state] eq "success"}
+    }
+}
+
 start_cluster 3 6 {tags {logreqres:skip external:skip cluster}} {
     set node0_id [R 0 CLUSTER MYID]
     set node1_id [R 1 CLUSTER MYID]