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Commitfdcee9f

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Avoid testing tuple visibility without buffer lock.
INSERT ... ON CONFLICT (specifically ExecCheckHeapTupleVisible) containsanother example of this unsafe coding practice. It is much harder to geta failure out of it than the case fixed in commit6292c23, because inmost scenarios any hint bits that could be set would have already been setearlier in the command. However, Konstantin Knizhnik reported a failurewith a custom transaction manager, and it's clearly possible to get afailure via a race condition in async-commit mode.For lack of a reproducible example, no regression test case in thiscommit.I did some testing with Asserts added to tqual.c's functions, and can saythat running "make check-world" exposed these two bugs and no others.The Asserts are messy enough that I've not added them to the code for now.Report: <57EE93C8.8080504@postgrespro.ru>Related-Discussion: <CAO3NbwOycQjt2Oqy2VW-eLTq2M5uGMyHnGm=RNga4mjqcYD7gQ@mail.gmail.com>
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‎src/backend/executor/nodeModifyTable.c

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@@ -194,6 +194,11 @@ ExecCheckHeapTupleVisible(EState *estate,
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if (!IsolationUsesXactSnapshot())
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return;
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/*
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* We need buffer pin and lock to call HeapTupleSatisfiesVisibility.
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* Caller should be holding pin, but not lock.
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*/
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LockBuffer(buffer,BUFFER_LOCK_SHARE);
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if (!HeapTupleSatisfiesVisibility(tuple,estate->es_snapshot,buffer))
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{
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/*
@@ -207,6 +212,7 @@ ExecCheckHeapTupleVisible(EState *estate,
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(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
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errmsg("could not serialize access due to concurrent update")));
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}
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LockBuffer(buffer,BUFFER_LOCK_UNLOCK);
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}
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/*

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