forbid-LISTEN-in-background.patch

diff --git a/src/backend/tcop/utility.c b/src/backend/tcop/utility.c
index 27fbf1f3aa..bf085aa93b 100644
--- 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;