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Commit1316be2

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Disallow LISTEN in background workers.
It's possible to execute user-defined SQL in some background processes;for example, logical replication workers can fire triggers. This opensthe possibility that someone would try to execute LISTEN in such acontext. But since only regular backends ever callProcessNotifyInterrupt, no messages would actually be received, andthus the registered listener would simply prevent the message queuefrom being cleaned. Eventually NOTIFY would stop working, which is bad.Perhaps someday somebody will invent infrastructure to make listeningin a background worker actually useful. In the meantime, forbid it.Back-patch to v13, which is where we introduced the MyBackendTypevariable. 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
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‎src/backend/tcop/utility.c

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@@ -803,6 +803,23 @@ standard_ProcessUtility(PlannedStmt *pstmt,
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ListenStmt*stmt= (ListenStmt*)parsetree;
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CheckRestrictedOperation("LISTEN");
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/*
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* We don't allow LISTEN in background processes, as there is
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* no mechanism for them to collect NOTIFY messages, so they'd
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* just block cleanout of the async SLRU indefinitely.
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* (Authors of custom background workers could bypass this
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* restriction by calling Async_Listen directly, but then it's
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* on them to provide some mechanism to process the message
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* queue.) Note there seems no reason to forbid UNLISTEN.
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*/
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if (MyBackendType!=B_BACKEND)
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ereport(ERROR,
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(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
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/* translator: %s is name of a SQL command, eg LISTEN */
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errmsg("cannot execute %s within a background process",
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"LISTEN")));
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Async_Listen(stmt->conditionname);
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}
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break;

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