chore: Forbid replicating a replica (#3779)

* chore: Forbid replicating a replica

We do not support connecting a replica to a replica, but before this PR
we allowed doing so. This PR disables that behavior.

Fixes #3679

* `replicaof_mu_`

src/server/server_family.cc

@@ -2839,6 +2839,13 @@ void ServerFamily::ReplTakeOver(CmdArgList args, ConnectionContext* cntx) {
 }
 
 void ServerFamily::ReplConf(CmdArgList args, ConnectionContext* cntx) {
+  {
+    util::fb2::LockGuard lk(replicaof_mu_);
+    if (!ServerState::tlocal()->is_master) {
+      return cntx->SendError("Replicating a replica is unsupported");
+    }
+  }
+
   if (args.size() % 2 == 1)
     goto err;
   for (unsigned i = 0; i < args.size(); i += 2) {

tests/dragonfly/replication_test.py

@@ -2663,3 +2663,25 @@ async def check_master_status():
     assert await seeder.compare(capture, port=master.port)
 
     await disconnect_clients(c_master, c_replica)
+
+
+@pytest.mark.asyncio
+async def test_replica_of_replica(df_factory):
+    # Can't connect a replica to a replica, but OK to connect 2 replicas to the same master
+    master = df_factory.create(proactor_threads=2)
+    replica = df_factory.create(proactor_threads=2)
+    replica2 = df_factory.create(proactor_threads=2)
+
+    df_factory.start_all([master, replica, replica2])
+
+    c_replica = replica.client()
+    c_replica2 = replica2.client()
+
+    assert await c_replica.execute_command(f"REPLICAOF localhost {master.port}") == "OK"
+
+    with pytest.raises(redis.exceptions.ResponseError):
+        await c_replica2.execute_command(f"REPLICAOF localhost {replica.port}")
+
+    assert await c_replica2.execute_command(f"REPLICAOF localhost {master.port}") == "OK"
+
+    await disconnect_clients(c_replica, c_replica2)