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Commitbc0550f

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Clean up after erroneous SELECT FOR UPDATE/SHARE on a sequence.
My previous commit disallowed this operation, but did nothing aboutcleaning up the damage if one had already been done. With the operationdisallowed, it's okay to just forcibly clear xmax in a sequence's tuple,since any value seen there could not represent a live transaction's lock.So, any sequence-specific operation will repair the problem automatically,whether or not the user has already seen "could not access status oftransaction" failures.
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‎src/backend/commands/sequence.c

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@@ -982,6 +982,22 @@ read_info(SeqTable elm, Relation rel, Buffer *buf)
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Assert(ItemIdIsNormal(lp));
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tuple.t_data= (HeapTupleHeader)PageGetItem(page,lp);
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/*
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* Previous releases of Postgres neglected to prevent SELECT FOR UPDATE
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* on a sequence, which would leave a non-frozen XID in the sequence
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* tuple's xmax, which eventually leads to clog access failures or worse.
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* If we see this has happened, clean up after it. We treat this like a
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* hint bit update, ie, don't bother to WAL-log it, since we can certainly
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* do this again if the update gets lost.
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*/
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if (HeapTupleHeaderGetXmax(tuple.t_data)!=InvalidTransactionId)
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{
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HeapTupleHeaderSetXmax(tuple.t_data,InvalidTransactionId);
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tuple.t_data->t_infomask &= ~HEAP_XMAX_COMMITTED;
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tuple.t_data->t_infomask |=HEAP_XMAX_INVALID;
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SetBufferCommitInfoNeedsSave(*buf);
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}
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seq= (Form_pg_sequence)GETSTRUCT(&tuple);
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/* this is a handy place to update our copy of the increment */

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