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-rw-r--r--src/backend/access/common/toast_internals.c51
-rw-r--r--src/backend/access/heap/heaptoast.c4
-rw-r--r--src/backend/storage/ipc/procarray.c4
-rw-r--r--src/backend/utils/time/snapmgr.c54
4 files changed, 20 insertions, 93 deletions
diff --git a/src/backend/access/common/toast_internals.c b/src/backend/access/common/toast_internals.c
index 90d0654e629..1939cfb4d2a 100644
--- a/src/backend/access/common/toast_internals.c
+++ b/src/backend/access/common/toast_internals.c
@@ -393,7 +393,6 @@ toast_delete_datum(Relation rel, Datum value, bool is_speculative)
HeapTuple toasttup;
int num_indexes;
int validIndex;
- SnapshotData SnapshotToast;
if (!VARATT_IS_EXTERNAL_ONDISK(attr))
return;
@@ -425,9 +424,8 @@ toast_delete_datum(Relation rel, Datum value, bool is_speculative)
* sequence or not, but since we've already locked the index we might as
* well use systable_beginscan_ordered.)
*/
- init_toast_snapshot(&SnapshotToast);
toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
- &SnapshotToast, 1, &toastkey);
+ get_toast_snapshot(), 1, &toastkey);
while ((toasttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
{
/*
@@ -631,41 +629,28 @@ toast_close_indexes(Relation *toastidxs, int num_indexes, LOCKMODE lock)
}
/* ----------
- * init_toast_snapshot
+ * get_toast_snapshot
*
- * Initialize an appropriate TOAST snapshot. We must use an MVCC snapshot
- * to initialize the TOAST snapshot; since we don't know which one to use,
- * just use the oldest one.
+ * Return the TOAST snapshot. Detoasting *must* happen in the same
+ * transaction that originally fetched the toast pointer.
*/
-void
-init_toast_snapshot(Snapshot toast_snapshot)
+Snapshot
+get_toast_snapshot(void)
{
- Snapshot snapshot = GetOldestSnapshot();
-
/*
- * GetOldestSnapshot returns NULL if the session has no active snapshots.
- * We can get that if, for example, a procedure fetches a toasted value
- * into a local variable, commits, and then tries to detoast the value.
- * Such coding is unsafe, because once we commit there is nothing to
- * prevent the toast data from being deleted. Detoasting *must* happen in
- * the same transaction that originally fetched the toast pointer. Hence,
- * rather than trying to band-aid over the problem, throw an error. (This
- * is not very much protection, because in many scenarios the procedure
- * would have already created a new transaction snapshot, preventing us
- * from detecting the problem. But it's better than nothing, and for sure
- * we shouldn't expend code on masking the problem more.)
+ * We cannot directly check that detoasting happens in the same
+ * transaction that originally fetched the toast pointer, but at least
+ * check that the session has some active snapshots. It might not if, for
+ * example, a procedure fetches a toasted value into a local variable,
+ * commits, and then tries to detoast the value. Such coding is unsafe,
+ * because once we commit there is nothing to prevent the toast data from
+ * being deleted. (This is not very much protection, because in many
+ * scenarios the procedure would have already created a new transaction
+ * snapshot, preventing us from detecting the problem. But it's better
+ * than nothing.)
*/
- if (snapshot == NULL)
+ if (!HaveRegisteredOrActiveSnapshot())
elog(ERROR, "cannot fetch toast data without an active snapshot");
- /*
- * Catalog snapshots can be returned by GetOldestSnapshot() even if not
- * registered or active. That easily hides bugs around not having a
- * snapshot set up - most of the time there is a valid catalog snapshot.
- * So additionally insist that the current snapshot is registered or
- * active.
- */
- Assert(HaveRegisteredOrActiveSnapshot());
-
- InitToastSnapshot(*toast_snapshot, snapshot->lsn, snapshot->whenTaken);
+ return &SnapshotToastData;
}
diff --git a/src/backend/access/heap/heaptoast.c b/src/backend/access/heap/heaptoast.c
index a420e165304..aae72bc2abf 100644
--- a/src/backend/access/heap/heaptoast.c
+++ b/src/backend/access/heap/heaptoast.c
@@ -639,7 +639,6 @@ heap_fetch_toast_slice(Relation toastrel, Oid valueid, int32 attrsize,
int endchunk;
int num_indexes;
int validIndex;
- SnapshotData SnapshotToast;
/* Look for the valid index of toast relation */
validIndex = toast_open_indexes(toastrel,
@@ -685,9 +684,8 @@ heap_fetch_toast_slice(Relation toastrel, Oid valueid, int32 attrsize,
}
/* Prepare for scan */
- init_toast_snapshot(&SnapshotToast);
toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
- &SnapshotToast, nscankeys, toastkey);
+ get_toast_snapshot(), nscankeys, toastkey);
/*
* Read the chunks by index
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 36610a1c7e7..c769b1aa3ef 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -2135,8 +2135,6 @@ GetSnapshotDataReuse(Snapshot snapshot)
snapshot->active_count = 0;
snapshot->regd_count = 0;
snapshot->copied = false;
- snapshot->lsn = InvalidXLogRecPtr;
- snapshot->whenTaken = 0;
return true;
}
@@ -2516,8 +2514,6 @@ GetSnapshotData(Snapshot snapshot)
snapshot->active_count = 0;
snapshot->regd_count = 0;
snapshot->copied = false;
- snapshot->lsn = InvalidXLogRecPtr;
- snapshot->whenTaken = 0;
return snapshot;
}
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 7d2b34d4f20..a1a0c2adeb6 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -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;
/*
@@ -314,36 +310,6 @@ GetLatestSnapshot(void)
}
/*
- * 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
* system catalog with the specified OID.
@@ -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. */
generated by cgit v1.2.3 (git 2.25.1) at 2025-08-03 09:26:21 +0000