Disallow LISTEN in background workers.

tglsfdc · tglsfdc · commit 1316be28664f · 2021-09-15T12:31:56.000-04:00
It's possible to execute user-defined SQL in some background processes; for example, logical replication workers can fire triggers. This opens the possibility that someone would try to execute LISTEN in such a context. But since only regular backends ever call ProcessNotifyInterrupt, no messages would actually be received, and thus the registered listener would simply prevent the message queue from being cleaned. Eventually NOTIFY would stop working, which is bad. Perhaps someday somebody will invent infrastructure to make listening in a background worker actually useful. In the meantime, forbid it. Back-patch to v13, which is where we introduced the MyBackendType variable. It'd be a lot harder to implement the check without that, and it doesn't seem worth the trouble. Discussion: https://postgr.es/m/153243441449.1404.2274116228506175596@wrigleys.postgresql.org
diff --git a/src/backend/tcop/utility.c b/src/backend/tcop/utility.c
@@ -803,6 +803,23 @@ standard_ProcessUtility(PlannedStmt *pstmt,
 				ListenStmt *stmt = (ListenStmt *) parsetree;
 
 				CheckRestrictedOperation("LISTEN");
+
+				/*
+				 * We don't allow LISTEN in background processes, as there is
+				 * no mechanism for them to collect NOTIFY messages, so they'd
+				 * just block cleanout of the async SLRU indefinitely.
+				 * (Authors of custom background workers could bypass this
+				 * restriction by calling Async_Listen directly, but then it's
+				 * on them to provide some mechanism to process the message
+				 * queue.)  Note there seems no reason to forbid UNLISTEN.
+				 */
+				if (MyBackendType != B_BACKEND)
+					ereport(ERROR,
+							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					/* translator: %s is name of a SQL command, eg LISTEN */
+							 errmsg("cannot execute %s within a background process",
+									"LISTEN")));
+
 				Async_Listen(stmt->conditionname);
 			}
 			break;
