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Commit033eb15

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Fix LISTEN/NOTIFY race condition reported by Laurent Birtz, by postponing
pg_listener modifications commanded by LISTEN and UNLISTEN until the endof the current transaction. This allows us to hold the ExclusiveLock onpg_listener until after commit, with no greater risk of deadlock than therewas before. Aside from fixing the race condition, this gets rid of atruly ugly kludge that was there before, namely having to ignoreHeapTupleBeingUpdated failures during NOTIFY. There is a small potentialincompatibility, which is that if a transaction issues LISTEN or UNLISTENand then looks into pg_listener before committing, it won't see any resultingrow insertion or deletion, where before it would have. It seems unlikelythat anyone would be depending on that, though.This patch also disallows LISTEN and UNLISTEN inside a prepared transaction.That case had some pretty undesirable properties already, such as possiblyallowing pg_listener entries to be made for PIDs no longer present, sodisallowing it seems like a better idea than trying to maintain the behavior.
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‎doc/src/sgml/ref/prepare_transaction.sgml

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<!--
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$PostgreSQL: pgsql/doc/src/sgml/ref/prepare_transaction.sgml,v 1.6 2007/09/11 00:06:41 tgl Exp $
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$PostgreSQL: pgsql/doc/src/sgml/ref/prepare_transaction.sgml,v 1.7 2008/03/12 20:11:45 tgl Exp $
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PostgreSQL documentation
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-->
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@@ -88,8 +88,10 @@ PREPARE TRANSACTION <replaceable class="PARAMETER">transaction_id</replaceable>
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<para>
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It is not currently allowed to <command>PREPARE</> a transaction that
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has executed any operations involving temporary tables or
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created any cursors <literal>WITH HOLD</>. Those features are too tightly
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has executed any operations involving temporary tables,
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created any cursors <literal>WITH HOLD</>, or executed
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<command>LISTEN</> or <command>UNLISTEN</>.
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Those features are too tightly
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tied to the current session to be useful in a transaction to be prepared.
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</para>
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