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Improve documentation about reasoning behind the order of operations

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in GetSnapshotData, GetNewTransactionId, CommitTransaction, AbortTransaction,
etc.  Correct race condition in transaction status testing in
HeapTupleSatisfiesVacuum --- this wasn't important for old VACUUM with
exclusive lock on its table, but it sure is important now.  All per
pghackers discussion 7/11/01 and 7/12/01.
This commit is contained in:
Tom Lane
2001-07-16 22:43:34 +00:00
parent ffbd97c8ac
commit ed5c4e4a14
4 changed files with 110 additions and 49 deletions
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31
src/backend/utils/time/tqual.c
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@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/utils/time/tqual.c,v 1.38 2001/07/12 04:11:13 tgl Exp $
* $Header: /cvsroot/pgsql/src/backend/utils/time/tqual.c,v 1.39 2001/07/16 22:43:34 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -610,6 +610,13 @@ HeapTupleSatisfiesVacuum(HeapTupleHeader tuple, TransactionId XmaxRecent)
*
* If the inserting transaction aborted, then the tuple was never visible
* to any other transaction, so we can delete it immediately.
*
* NOTE: must check TransactionIdIsInProgress (which looks in shared mem)
* before TransactionIdDidCommit/TransactionIdDidAbort (which look in
* pg_log). Otherwise we have a race condition where we might decide
* that a just-committed transaction crashed, because none of the tests
* succeed. xact.c is careful to record commit/abort in pg_log before
* it unsets MyProc->xid in shared memory.
*/
if (!(tuple->t_infomask & HEAP_XMIN_COMMITTED))
{
@@ -636,19 +643,19 @@ HeapTupleSatisfiesVacuum(HeapTupleHeader tuple, TransactionId XmaxRecent)
}
tuple->t_infomask |= HEAP_XMIN_COMMITTED;
}
else if (TransactionIdIsInProgress(tuple->t_xmin))
return HEAPTUPLE_INSERT_IN_PROGRESS;
else if (TransactionIdDidCommit(tuple->t_xmin))
tuple->t_infomask |= HEAP_XMIN_COMMITTED;
else if (TransactionIdDidAbort(tuple->t_xmin))
{
tuple->t_infomask |= HEAP_XMIN_INVALID;
return HEAPTUPLE_DEAD;
}
else if (TransactionIdDidCommit(tuple->t_xmin))
tuple->t_infomask |= HEAP_XMIN_COMMITTED;
else if (TransactionIdIsInProgress(tuple->t_xmin))
return HEAPTUPLE_INSERT_IN_PROGRESS;
else
{
/*
* Not Aborted, Not Committed, Not in Progress -
* Not in Progress, Not Committed, Not Aborted -
* so it's from crashed process. - vadim 11/26/96
*/
tuple->t_infomask |= HEAP_XMIN_INVALID;
@@ -667,19 +674,19 @@ HeapTupleSatisfiesVacuum(HeapTupleHeader tuple, TransactionId XmaxRecent)
if (!(tuple->t_infomask & HEAP_XMAX_COMMITTED))
{
if (TransactionIdDidAbort(tuple->t_xmax))
if (TransactionIdIsInProgress(tuple->t_xmax))
return HEAPTUPLE_DELETE_IN_PROGRESS;
else if (TransactionIdDidCommit(tuple->t_xmax))
tuple->t_infomask |= HEAP_XMAX_COMMITTED;
else if (TransactionIdDidAbort(tuple->t_xmax))
{
tuple->t_infomask |= HEAP_XMAX_INVALID;
return HEAPTUPLE_LIVE;
}
else if (TransactionIdDidCommit(tuple->t_xmax))
tuple->t_infomask |= HEAP_XMAX_COMMITTED;
else if (TransactionIdIsInProgress(tuple->t_xmax))
return HEAPTUPLE_DELETE_IN_PROGRESS;
else
{
/*
* Not Aborted, Not Committed, Not in Progress -
* Not in Progress, Not Committed, Not Aborted -
* so it's from crashed process. - vadim 06/02/97
*/
tuple->t_infomask |= HEAP_XMAX_INVALID;