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--></style></head><body lang=EN-US link=blue vlink="#954F72" style='word-wrap:break-word'><div class=WordSection1><p class=MsoNormal>Thanks for looking into this Tom,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Glad to have the race condition confirmed. In my view the “real” fix here would be to offer an advisory locking function that accepts a timeout. FWIW, similar functionality on other RDBMS systems (SQL Server, MySQL, Oracle) offers this. That way, users wouldn’t have to rely in SET LOCAL lock_timeout to get this functionality. Thoughts? Is there a backlog item for this or has the idea been considered previously?<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>-Mike</p><p class=MsoNormal><o:p> </o:p></p><div style='mso-element:para-border-div;border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal style='border:none;padding:0in'><b>From: </b><a href="mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us">Tom Lane</a><br><b>Sent: </b>Saturday, November 12, 2022 11:47 AM<br><b>To: </b><a href="mailto:mike(dot)adelson314(at)gmail(dot)com">mike(dot)adelson314(at)gmail(dot)com</a><br><b>Cc: </b><a href="mailto:pgsql-bugs(at)lists(dot)postgresql(dot)org">pgsql-bugs(at)lists(dot)postgresql(dot)org</a><br><b>Subject: </b>Re: BUG #17686: SELECT pg_advisory_lock(...) with low lock_timeout sometimes times out but the lock is acquired</p></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>PG Bug reporting form <noreply(at)postgresql(dot)org> writes:</p><p class=MsoNormal>> I'm finding that with relatively small values of lock_timeout and when the</p><p class=MsoNormal>> system is under load (e.g. 8 connections acquiring concurrently), I will</p><p class=MsoNormal>> encounter a case where the query exits with state 55P03 (lock_not_available)</p><p class=MsoNormal>> and yet the lock was actually acquired (I can tell it has been acquired by</p><p class=MsoNormal>> querying pg_locks and because other connections' calls to pg_advisory_lock</p><p class=MsoNormal>> block).</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Yeah, there's a fairly basic race condition there, which is that the</p><p class=MsoNormal>lock might get granted to us immediately after the timeout fires.</p><p class=MsoNormal>In ordinary usage this isn't very problematic because the lock would</p><p class=MsoNormal>get released anyway during the transaction abort resulting from the</p><p class=MsoNormal>timeout error. However, when you're asking for a session-level</p><p class=MsoNormal>advisory lock, that doesn't happen.</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>I spent a little time studying whether there's a way to close the race,</p><p class=MsoNormal>but if it's possible at all it'd take major restructuring of what's</p><p class=MsoNormal>already quite complex and delicate code. I can't get excited about</p><p class=MsoNormal>putting such effort into it, because related problems will exist</p><p class=MsoNormal>no matter what: if you get an error from the pg_advisory_lock command,</p><p class=MsoNormal>was the lock granted before that error occurred? I don't think there's</p><p class=MsoNormal>any bulletproof way to deal with that except to check the lock status</p><p class=MsoNormal>afterwards (e.g., via pg_locks). It's very much like the inherent issue</p><p class=MsoNormal>with errors late in COMMIT --- there's an atomic point where the commit</p><p class=MsoNormal>is logged, but the error report isn't going to be very clear about</p><p class=MsoNormal>whether we got past that, if indeed the error report gets to the client</p><p class=MsoNormal>at all.</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal> regards, tom lane</p><p class=MsoNormal><o:p> </o:p></p></div></body></html>