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Improve C comments in GetSnapshotData.

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Move discussion of why our algorithm for taking snapshots in recovery
to a more appropriate location in the function, and delete incorrect
mention of taking a lock.
This commit is contained in:
Robert Haas 2012-08-21 11:47:10 -04:00
parent 82ef3d3008
commit 4b373e42d1
1 changed files with 6 additions and 8 deletions
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14
src/backend/storage/ipc/procarray.c
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@ -1307,14 +1307,6 @@ GetSnapshotData(Snapshot snapshot)
/* initialize xmin calculation with xmax */
globalxmin = xmin = xmax;
/*
* If we're in recovery then snapshot data comes from a different place,
* so decide which route we take before grab the lock. It is possible for
* recovery to end before we finish taking snapshot, and for newly
* assigned transaction ids to be added to the procarray. Xmax cannot
* change while we hold ProcArrayLock, so those newly added transaction
* ids would be filtered away, so we need not be concerned about them.
*/
snapshot->takenDuringRecovery = RecoveryInProgress();
if (!snapshot->takenDuringRecovery)
@ -1429,6 +1421,12 @@ GetSnapshotData(Snapshot snapshot)
* Either way we need to change the way XidInMVCCSnapshot() works
* depending upon when the snapshot was taken, or change normal
* snapshot processing so it matches.
*
* Note: It is possible for recovery to end before we finish taking the
* snapshot, and for newly assigned transaction ids to be added to the
* ProcArray. xmax cannot change while we hold ProcArrayLock, so those
* newly added transaction ids would be filtered away, so we need not
* be concerned about them.
*/
subcount = KnownAssignedXidsGetAndSetXmin(snapshot->subxip, &xmin,
xmax);