improve comments

src/backend/distributed/shared_library_init.c

@@ -2900,11 +2900,16 @@ ApplicationNameAssignHook(const char *newval, void *extra)
 	 * indicate when this responsibility handoff has happened.
 	 *
 	 * On the other hand, even if now it's this hook's responsibility to update
-	 * the global pid, we cannot do so if the cached local node id is invalidated
-	 * and we're not allowed to access the catalog tables. Within a transaction
-	 * block, we can access the catalog tables. For this reason, in addition to
-	 * checking FinishedStartupCitusBackend, we also require either being in a
-	 * transaction block or the cached local node id to be valid.
+	 * the global pid, we cannot do so if we might need to read from catalog
+	 * but it's unsafe to do so. For Citus internal backends, this cannot be the
+	 * case because in that case AssignGlobalPID() just extracts its global pid
+	 * from the application_name. But for external backends, we either need to
+	 * guarantee that we won't read from catalog tables or that it's safe to do
+	 * so. The only case where AssignGlobalPID() could read from catalog tables
+	 * is when the cached local node id is invalidated. So for this reason, for
+	 * external client backends, we require that either the cached local node id
+	 * is valid or that we are in a transaction block -because in that case it's
+	 * safe to read from catalog.
 	 *
 	 * Another solution to the catalog table acccess problem would be to update
 	 * global pid lazily, like we do for HideShards. But that's not possible
@@ -2915,7 +2920,8 @@ ApplicationNameAssignHook(const char *newval, void *extra)
 	 * AuthHook already (extracting doesn't require catalog access).
 	 */
 	if (FinishedStartupCitusBackend &&
-		(IsTransactionState() || CachedLocalNodeIdIsValid()))
+		(!IsExternalClientBackend() || CachedLocalNodeIdIsValid() ||
+		 IsTransactionState()))
 	{
 		AssignGlobalPID(newval);
 	}