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Commitaf3855c

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Improve TransactionIdDidAbort() documentation.
Document that TransactionIdDidAbort() won't indicate that transactionsthat were in-progress during a crash have aborted. Tie this to existingdiscussion of the TransactionIdDidCommit() and TransactionIdDidCommit()protocol that code in heapam_visibility.c (and a few other places) mustobserve.Follow-up to bugfix commiteb5ad4f.Author: Peter Geoghegan <pg@bowt.ie>Reviewed-By: Andres Freund <andres@anarazel.de>Discussion:https://postgr.es/m/CAH2-Wzn4bEEqgmaUQL3aJ73yM9gAeK-wE4ngi7kjRjLztb+P0w@mail.gmail.com
1 parent8bf6ec3 commitaf3855c

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2 files changed

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‎src/backend/access/heap/heapam_visibility.c‎

Lines changed: 18 additions & 15 deletions
Original file line numberDiff line numberDiff line change
@@ -11,21 +11,24 @@
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* shared buffer content lock on the buffer containing the tuple.
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*
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* NOTE: When using a non-MVCC snapshot, we must check
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* TransactionIdIsInProgress (which looks in the PGPROC array)
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* before TransactionIdDidCommit/TransactionIdDidAbort (which look in
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* pg_xact). Otherwise we have a race condition: we might decide that a
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* just-committed transaction crashed, because none of the tests succeed.
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* xact.c is careful to record commit/abort in pg_xact before it unsets
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* MyProc->xid in the PGPROC array. That fixes that problem, but it
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* also means there is a window where TransactionIdIsInProgress and
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* TransactionIdDidCommit will both return true. If we check only
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* TransactionIdDidCommit, we could consider a tuple committed when a
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* later GetSnapshotData call will still think the originating transaction
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* is in progress, which leads to application-level inconsistency. The
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* upshot is that we gotta check TransactionIdIsInProgress first in all
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* code paths, except for a few cases where we are looking at
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* subtransactions of our own main transaction and so there can't be any
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* race condition.
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* TransactionIdIsInProgress (which looks in the PGPROC array) before
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* TransactionIdDidCommit (which look in pg_xact). Otherwise we have a race
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* condition: we might decide that a just-committed transaction crashed,
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* because none of the tests succeed. xact.c is careful to record
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* commit/abort in pg_xact before it unsets MyProc->xid in the PGPROC array.
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* That fixes that problem, but it also means there is a window where
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* TransactionIdIsInProgress and TransactionIdDidCommit will both return true.
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* If we check only TransactionIdDidCommit, we could consider a tuple
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* committed when a later GetSnapshotData call will still think the
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* originating transaction is in progress, which leads to application-level
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* inconsistency. The upshot is that we gotta check TransactionIdIsInProgress
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* first in all code paths, except for a few cases where we are looking at
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* subtransactions of our own main transaction and so there can't be any race
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* condition.
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*
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* We can't use TransactionIdDidAbort here because it won't treat transactions
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* that were in progress during a crash as aborted. We determine that
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* transactions aborted/crashed through process of elimination instead.
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*
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* When using an MVCC snapshot, we rely on XidInMVCCSnapshot rather than
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* TransactionIdIsInProgress, but the logic is otherwise the same: do not

‎src/backend/access/transam/transam.c‎

Lines changed: 8 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -110,7 +110,8 @@ TransactionLogFetch(TransactionId transactionId)
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* transaction tree.
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*
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* See also TransactionIdIsInProgress, which once was in this module
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* but now lives in procarray.c.
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* but now lives in procarray.c, as well as comments at the top of
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* heapam_visibility.c that explain how everything fits together.
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* ----------------------------------------------------------------
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*/
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@@ -176,6 +177,12 @@ TransactionIdDidCommit(TransactionId transactionId)
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*
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* Note:
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*Assumes transaction identifier is valid and exists in clog.
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*
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*Returns true only for explicitly aborted transactions, as transactions
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*implicitly aborted due to a crash will commonly still appear to be
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*in-progress in the clog. Most of the time TransactionIdDidCommit(),
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*with a preceding TransactionIdIsInProgress() check, should be used
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*instead of TransactionIdDidAbort().
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*/
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bool/* true if given transaction aborted */
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TransactionIdDidAbort(TransactionIdtransactionId)

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