From 46c333a9638b329a3c8076d978f27c5b05c0d5f0 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Tue, 8 Mar 2011 21:01:29 +0200
Subject: [PATCH] Fix overly strict assertion in
SummarizeOldestCommittedSxact(). There's a race condition where
SummarizeOldestCommittedSxact() is called even though another backend already
cleared out all finished sxact entries. That's OK,
RegisterSerializableTransactionInt() can just retry getting a news xact slot
from the available-list when that happens.
Reported by YAMAMOTO Takashi, bug #5918.
---
src/backend/storage/lmgr/predicate.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)
diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index 15f0a64d3c2..870cf4277cf 100644
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -1314,15 +1314,21 @@ SummarizeOldestCommittedSxact(void)
LWLockAcquire(SerializableFinishedListLock, LW_EXCLUSIVE);
-#ifdef TEST_OLDSERXID
+ /*
+ * This function is only called if there are no sxact slots available.
+ * Some of them must belong to old, already-finished transactions, so
+ * there should be something in FinishedSerializableTransactions list
+ * that we can summarize. However, there's a race condition: while we
+ * were not holding any locks, a transaction might have ended and cleaned
+ * up all the finished sxact entries already, freeing up their sxact
+ * slots. In that case, we have nothing to do here. The caller will find
+ * one of the slots released by the other backend when it retries.
+ */
if (SHMQueueEmpty(FinishedSerializableTransactions))
{
LWLockRelease(SerializableFinishedListLock);
return;
}
-#else
- Assert(!SHMQueueEmpty(FinishedSerializableTransactions));
-#endif
/*
* Grab the first sxact off the finished list -- this will be the earliest
--
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