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Remove remants of "snapshot too old"

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Remove the 'whenTaken' and 'lsn' fields from SnapshotData. After the
removal of the "snapshot too old" feature, they were never set to a
non-zero value.

This largely reverts commit 3e2f3c2e42, which added the
OldestActiveSnapshot tracking, and the init_toast_snapshot()
function. That was only required for setting the 'whenTaken' and 'lsn'
fields. SnapshotToast is now a constant again, like SnapshotSelf and
SnapshotAny. I kept a thin get_toast_snapshot() wrapper around
SnapshotToast though, to check that you have a registered or active
snapshot. That's still a useful sanity check.

Reviewed-by: Nathan Bossart, Andres Freund, Tom Lane
Discussion: https://www.postgresql.org/message-id/cd4b4f8c-e63a-41c0-95f6-6e6cd9b83f6d@iki.fi
This commit is contained in:
Heikki Linnakangas
2024-12-09 18:13:03 +02:00
parent f64ec81a81
commit 4d8275046c
10 changed files with 28 additions and 114 deletions
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54
src/backend/utils/time/snapmgr.c
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@@ -83,6 +83,7 @@ static SnapshotData SecondarySnapshotData = {SNAPSHOT_MVCC};
SnapshotData CatalogSnapshotData = {SNAPSHOT_MVCC};
SnapshotData SnapshotSelfData = {SNAPSHOT_SELF};
SnapshotData SnapshotAnyData = {SNAPSHOT_ANY};
SnapshotData SnapshotToastData = {SNAPSHOT_TOAST};
/* Pointers to valid snapshots */
static Snapshot CurrentSnapshot = NULL;
@@ -119,9 +120,6 @@ typedef struct ActiveSnapshotElt
/* Top of the stack of active snapshots */
static ActiveSnapshotElt *ActiveSnapshot = NULL;
/* Bottom of the stack of active snapshots */
static ActiveSnapshotElt *OldestActiveSnapshot = NULL;
/*
* Currently registered Snapshots. Ordered in a heap by xmin, so that we can
* quickly find the one with lowest xmin, to advance our MyProc->xmin.
@@ -199,8 +197,6 @@ typedef struct SerializedSnapshotData
bool suboverflowed;
bool takenDuringRecovery;
CommandId curcid;
TimestampTz whenTaken;
XLogRecPtr lsn;
} SerializedSnapshotData;
/*
@@ -313,36 +309,6 @@ GetLatestSnapshot(void)
return SecondarySnapshot;
}
/*
* GetOldestSnapshot
*
* Get the transaction's oldest known snapshot, as judged by the LSN.
* Will return NULL if there are no active or registered snapshots.
*/
Snapshot
GetOldestSnapshot(void)
{
Snapshot OldestRegisteredSnapshot = NULL;
XLogRecPtr RegisteredLSN = InvalidXLogRecPtr;
if (!pairingheap_is_empty(&RegisteredSnapshots))
{
OldestRegisteredSnapshot = pairingheap_container(SnapshotData, ph_node,
pairingheap_first(&RegisteredSnapshots));
RegisteredLSN = OldestRegisteredSnapshot->lsn;
}
if (OldestActiveSnapshot != NULL)
{
XLogRecPtr ActiveLSN = OldestActiveSnapshot->as_snap->lsn;
if (XLogRecPtrIsInvalid(RegisteredLSN) || RegisteredLSN > ActiveLSN)
return OldestActiveSnapshot->as_snap;
}
return OldestRegisteredSnapshot;
}
/*
* GetCatalogSnapshot
* Get a snapshot that is sufficiently up-to-date for scan of the
@@ -684,8 +650,6 @@ PushActiveSnapshotWithLevel(Snapshot snapshot, int snap_level)
newactive->as_snap->active_count++;
ActiveSnapshot = newactive;
if (OldestActiveSnapshot == NULL)
OldestActiveSnapshot = ActiveSnapshot;
}
/*
@@ -756,8 +720,6 @@ PopActiveSnapshot(void)
pfree(ActiveSnapshot);
ActiveSnapshot = newstack;
if (ActiveSnapshot == NULL)
OldestActiveSnapshot = NULL;
SnapshotResetXmin();
}
@@ -902,13 +864,6 @@ xmin_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg)
* dropped. For efficiency, we only consider recomputing PGPROC->xmin when
* the active snapshot stack is empty; this allows us not to need to track
* which active snapshot is oldest.
*
* Note: it's tempting to use GetOldestSnapshot() here so that we can include
* active snapshots in the calculation. However, that compares by LSN not
* xmin so it's not entirely clear that it's the same thing. Also, we'd be
* critically dependent on the assumption that the bottommost active snapshot
* stack entry has the oldest xmin. (Current uses of GetOldestSnapshot() are
* not actually critical, but this would be.)
*/
static void
SnapshotResetXmin(void)
@@ -980,8 +935,6 @@ AtSubAbort_Snapshot(int level)
pfree(ActiveSnapshot);
ActiveSnapshot = next;
if (ActiveSnapshot == NULL)
OldestActiveSnapshot = NULL;
}
SnapshotResetXmin();
@@ -1065,7 +1018,6 @@ AtEOXact_Snapshot(bool isCommit, bool resetXmin)
* it'll go away with TopTransactionContext.
*/
ActiveSnapshot = NULL;
OldestActiveSnapshot = NULL;
pairingheap_reset(&RegisteredSnapshots);
CurrentSnapshot = NULL;
@@ -1727,8 +1679,6 @@ SerializeSnapshot(Snapshot snapshot, char *start_address)
serialized_snapshot.suboverflowed = snapshot->suboverflowed;
serialized_snapshot.takenDuringRecovery = snapshot->takenDuringRecovery;
serialized_snapshot.curcid = snapshot->curcid;
serialized_snapshot.whenTaken = snapshot->whenTaken;
serialized_snapshot.lsn = snapshot->lsn;
/*
* Ignore the SubXID array if it has overflowed, unless the snapshot was
@@ -1801,8 +1751,6 @@ RestoreSnapshot(char *start_address)
snapshot->suboverflowed = serialized_snapshot.suboverflowed;
snapshot->takenDuringRecovery = serialized_snapshot.takenDuringRecovery;
snapshot->curcid = serialized_snapshot.curcid;
snapshot->whenTaken = serialized_snapshot.whenTaken;
snapshot->lsn = serialized_snapshot.lsn;
snapshot->snapXactCompletionCount = 0;
/* Copy XIDs, if present. */