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Commit4d82750

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Remove remants of "snapshot too old"
Remove the 'whenTaken' and 'lsn' fields from SnapshotData. After theremoval of the "snapshot too old" feature, they were never set to anon-zero value.This largely reverts commit3e2f3c2, which added theOldestActiveSnapshot 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 andSnapshotAny. I kept a thin get_toast_snapshot() wrapper aroundSnapshotToast though, to check that you have a registered or activesnapshot. That's still a useful sanity check.Reviewed-by: Nathan Bossart, Andres Freund, Tom LaneDiscussion:https://www.postgresql.org/message-id/cd4b4f8c-e63a-41c0-95f6-6e6cd9b83f6d@iki.fi
1 parentf64ec81 commit4d82750

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10 files changed

+28
-114
lines changed

10 files changed

+28
-114
lines changed

‎contrib/amcheck/verify_heapam.c

Lines changed: 1 addition & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1767,7 +1767,6 @@ check_tuple_attribute(HeapCheckContext *ctx)
17671767
staticvoid
17681768
check_toasted_attribute(HeapCheckContext*ctx,ToastedAttribute*ta)
17691769
{
1770-
SnapshotDataSnapshotToast;
17711770
ScanKeyDatatoastkey;
17721771
SysScanDesctoastscan;
17731772
boolfound_toasttup;
@@ -1791,10 +1790,9 @@ check_toasted_attribute(HeapCheckContext *ctx, ToastedAttribute *ta)
17911790
* Check if any chunks for this toasted object exist in the toast table,
17921791
* accessible via the index.
17931792
*/
1794-
init_toast_snapshot(&SnapshotToast);
17951793
toastscan=systable_beginscan_ordered(ctx->toast_rel,
17961794
ctx->valid_toast_index,
1797-
&SnapshotToast,1,
1795+
get_toast_snapshot(),1,
17981796
&toastkey);
17991797
found_toasttup= false;
18001798
while ((toasttup=

‎src/backend/access/common/toast_internals.c

Lines changed: 18 additions & 33 deletions
Original file line numberDiff line numberDiff line change
@@ -393,7 +393,6 @@ toast_delete_datum(Relation rel, Datum value, bool is_speculative)
393393
HeapTupletoasttup;
394394
intnum_indexes;
395395
intvalidIndex;
396-
SnapshotDataSnapshotToast;
397396

398397
if (!VARATT_IS_EXTERNAL_ONDISK(attr))
399398
return;
@@ -425,9 +424,8 @@ toast_delete_datum(Relation rel, Datum value, bool is_speculative)
425424
* sequence or not, but since we've already locked the index we might as
426425
* well use systable_beginscan_ordered.)
427426
*/
428-
init_toast_snapshot(&SnapshotToast);
429427
toastscan=systable_beginscan_ordered(toastrel,toastidxs[validIndex],
430-
&SnapshotToast,1,&toastkey);
428+
get_toast_snapshot(),1,&toastkey);
431429
while ((toasttup=systable_getnext_ordered(toastscan,ForwardScanDirection))!=NULL)
432430
{
433431
/*
@@ -631,41 +629,28 @@ toast_close_indexes(Relation *toastidxs, int num_indexes, LOCKMODE lock)
631629
}
632630

633631
/* ----------
634-
*init_toast_snapshot
632+
*get_toast_snapshot
635633
*
636-
*Initialize an appropriate TOAST snapshot. We must use an MVCC snapshot
637-
*to initialize the TOAST snapshot; since we don't know which one to use,
638-
*just use the oldest one.
634+
*Return the TOAST snapshot. Detoasting *must* happen in the same
635+
*transaction that originally fetched the toast pointer.
639636
*/
640-
void
641-
init_toast_snapshot(Snapshottoast_snapshot)
637+
Snapshot
638+
get_toast_snapshot(void)
642639
{
643-
Snapshotsnapshot=GetOldestSnapshot();
644-
645640
/*
646-
* GetOldestSnapshot returns NULL if the session has no active snapshots.
647-
* We can get that if, for example, a procedure fetches a toasted value
648-
* into a local variable, commits, and then tries to detoast the value.
649-
* Such coding is unsafe, because once we commit there is nothing to
650-
* prevent the toast data from being deleted. Detoasting *must* happen in
651-
* the same transaction that originally fetched the toast pointer. Hence,
652-
* rather than trying to band-aid over the problem, throw an error. (This
653-
* is not very much protection, because in many scenarios the procedure
654-
* would have already created a new transaction snapshot, preventing us
655-
* from detecting the problem. But it's better than nothing, and for sure
656-
* we shouldn't expend code on masking the problem more.)
641+
* We cannot directly check that detoasting happens in the same
642+
* transaction that originally fetched the toast pointer, but at least
643+
* check that the session has some active snapshots. It might not if, for
644+
* example, a procedure fetches a toasted value into a local variable,
645+
* commits, and then tries to detoast the value. Such coding is unsafe,
646+
* because once we commit there is nothing to prevent the toast data from
647+
* being deleted. (This is not very much protection, because in many
648+
* scenarios the procedure would have already created a new transaction
649+
* snapshot, preventing us from detecting the problem. But it's better
650+
* than nothing.)
657651
*/
658-
if (snapshot==NULL)
652+
if (!HaveRegisteredOrActiveSnapshot())
659653
elog(ERROR,"cannot fetch toast data without an active snapshot");
660654

661-
/*
662-
* Catalog snapshots can be returned by GetOldestSnapshot() even if not
663-
* registered or active. That easily hides bugs around not having a
664-
* snapshot set up - most of the time there is a valid catalog snapshot.
665-
* So additionally insist that the current snapshot is registered or
666-
* active.
667-
*/
668-
Assert(HaveRegisteredOrActiveSnapshot());
669-
670-
InitToastSnapshot(*toast_snapshot,snapshot->lsn,snapshot->whenTaken);
655+
return&SnapshotToastData;
671656
}

‎src/backend/access/heap/heaptoast.c

Lines changed: 1 addition & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -639,7 +639,6 @@ heap_fetch_toast_slice(Relation toastrel, Oid valueid, int32 attrsize,
639639
intendchunk;
640640
intnum_indexes;
641641
intvalidIndex;
642-
SnapshotDataSnapshotToast;
643642

644643
/* Look for the valid index of toast relation */
645644
validIndex=toast_open_indexes(toastrel,
@@ -685,9 +684,8 @@ heap_fetch_toast_slice(Relation toastrel, Oid valueid, int32 attrsize,
685684
}
686685

687686
/* Prepare for scan */
688-
init_toast_snapshot(&SnapshotToast);
689687
toastscan=systable_beginscan_ordered(toastrel,toastidxs[validIndex],
690-
&SnapshotToast,nscankeys,toastkey);
688+
get_toast_snapshot(),nscankeys,toastkey);
691689

692690
/*
693691
* Read the chunks by index

‎src/backend/storage/ipc/procarray.c

Lines changed: 0 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -2135,8 +2135,6 @@ GetSnapshotDataReuse(Snapshot snapshot)
21352135
snapshot->active_count=0;
21362136
snapshot->regd_count=0;
21372137
snapshot->copied= false;
2138-
snapshot->lsn=InvalidXLogRecPtr;
2139-
snapshot->whenTaken=0;
21402138

21412139
return true;
21422140
}
@@ -2516,8 +2514,6 @@ GetSnapshotData(Snapshot snapshot)
25162514
snapshot->active_count=0;
25172515
snapshot->regd_count=0;
25182516
snapshot->copied= false;
2519-
snapshot->lsn=InvalidXLogRecPtr;
2520-
snapshot->whenTaken=0;
25212517

25222518
returnsnapshot;
25232519
}

‎src/backend/utils/time/snapmgr.c

Lines changed: 1 addition & 53 deletions
Original file line numberDiff line numberDiff line change
@@ -83,6 +83,7 @@ static SnapshotData SecondarySnapshotData = {SNAPSHOT_MVCC};
8383
SnapshotDataCatalogSnapshotData= {SNAPSHOT_MVCC};
8484
SnapshotDataSnapshotSelfData= {SNAPSHOT_SELF};
8585
SnapshotDataSnapshotAnyData= {SNAPSHOT_ANY};
86+
SnapshotDataSnapshotToastData= {SNAPSHOT_TOAST};
8687

8788
/* Pointers to valid snapshots */
8889
staticSnapshotCurrentSnapshot=NULL;
@@ -119,9 +120,6 @@ typedef struct ActiveSnapshotElt
119120
/* Top of the stack of active snapshots */
120121
staticActiveSnapshotElt*ActiveSnapshot=NULL;
121122

122-
/* Bottom of the stack of active snapshots */
123-
staticActiveSnapshotElt*OldestActiveSnapshot=NULL;
124-
125123
/*
126124
* Currently registered Snapshots. Ordered in a heap by xmin, so that we can
127125
* quickly find the one with lowest xmin, to advance our MyProc->xmin.
@@ -199,8 +197,6 @@ typedef struct SerializedSnapshotData
199197
boolsuboverflowed;
200198
booltakenDuringRecovery;
201199
CommandIdcurcid;
202-
TimestampTzwhenTaken;
203-
XLogRecPtrlsn;
204200
}SerializedSnapshotData;
205201

206202
/*
@@ -313,36 +309,6 @@ GetLatestSnapshot(void)
313309
returnSecondarySnapshot;
314310
}
315311

316-
/*
317-
* GetOldestSnapshot
318-
*
319-
*Get the transaction's oldest known snapshot, as judged by the LSN.
320-
*Will return NULL if there are no active or registered snapshots.
321-
*/
322-
Snapshot
323-
GetOldestSnapshot(void)
324-
{
325-
SnapshotOldestRegisteredSnapshot=NULL;
326-
XLogRecPtrRegisteredLSN=InvalidXLogRecPtr;
327-
328-
if (!pairingheap_is_empty(&RegisteredSnapshots))
329-
{
330-
OldestRegisteredSnapshot=pairingheap_container(SnapshotData,ph_node,
331-
pairingheap_first(&RegisteredSnapshots));
332-
RegisteredLSN=OldestRegisteredSnapshot->lsn;
333-
}
334-
335-
if (OldestActiveSnapshot!=NULL)
336-
{
337-
XLogRecPtrActiveLSN=OldestActiveSnapshot->as_snap->lsn;
338-
339-
if (XLogRecPtrIsInvalid(RegisteredLSN)||RegisteredLSN>ActiveLSN)
340-
returnOldestActiveSnapshot->as_snap;
341-
}
342-
343-
returnOldestRegisteredSnapshot;
344-
}
345-
346312
/*
347313
* GetCatalogSnapshot
348314
*Get a snapshot that is sufficiently up-to-date for scan of the
@@ -684,8 +650,6 @@ PushActiveSnapshotWithLevel(Snapshot snapshot, int snap_level)
684650
newactive->as_snap->active_count++;
685651

686652
ActiveSnapshot=newactive;
687-
if (OldestActiveSnapshot==NULL)
688-
OldestActiveSnapshot=ActiveSnapshot;
689653
}
690654

691655
/*
@@ -756,8 +720,6 @@ PopActiveSnapshot(void)
756720

757721
pfree(ActiveSnapshot);
758722
ActiveSnapshot=newstack;
759-
if (ActiveSnapshot==NULL)
760-
OldestActiveSnapshot=NULL;
761723

762724
SnapshotResetXmin();
763725
}
@@ -902,13 +864,6 @@ xmin_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg)
902864
* dropped. For efficiency, we only consider recomputing PGPROC->xmin when
903865
* the active snapshot stack is empty; this allows us not to need to track
904866
* which active snapshot is oldest.
905-
*
906-
* Note: it's tempting to use GetOldestSnapshot() here so that we can include
907-
* active snapshots in the calculation. However, that compares by LSN not
908-
* xmin so it's not entirely clear that it's the same thing. Also, we'd be
909-
* critically dependent on the assumption that the bottommost active snapshot
910-
* stack entry has the oldest xmin. (Current uses of GetOldestSnapshot() are
911-
* not actually critical, but this would be.)
912867
*/
913868
staticvoid
914869
SnapshotResetXmin(void)
@@ -980,8 +935,6 @@ AtSubAbort_Snapshot(int level)
980935
pfree(ActiveSnapshot);
981936

982937
ActiveSnapshot=next;
983-
if (ActiveSnapshot==NULL)
984-
OldestActiveSnapshot=NULL;
985938
}
986939

987940
SnapshotResetXmin();
@@ -1065,7 +1018,6 @@ AtEOXact_Snapshot(bool isCommit, bool resetXmin)
10651018
* it'll go away with TopTransactionContext.
10661019
*/
10671020
ActiveSnapshot=NULL;
1068-
OldestActiveSnapshot=NULL;
10691021
pairingheap_reset(&RegisteredSnapshots);
10701022

10711023
CurrentSnapshot=NULL;
@@ -1727,8 +1679,6 @@ SerializeSnapshot(Snapshot snapshot, char *start_address)
17271679
serialized_snapshot.suboverflowed=snapshot->suboverflowed;
17281680
serialized_snapshot.takenDuringRecovery=snapshot->takenDuringRecovery;
17291681
serialized_snapshot.curcid=snapshot->curcid;
1730-
serialized_snapshot.whenTaken=snapshot->whenTaken;
1731-
serialized_snapshot.lsn=snapshot->lsn;
17321682

17331683
/*
17341684
* Ignore the SubXID array if it has overflowed, unless the snapshot was
@@ -1801,8 +1751,6 @@ RestoreSnapshot(char *start_address)
18011751
snapshot->suboverflowed=serialized_snapshot.suboverflowed;
18021752
snapshot->takenDuringRecovery=serialized_snapshot.takenDuringRecovery;
18031753
snapshot->curcid=serialized_snapshot.curcid;
1804-
snapshot->whenTaken=serialized_snapshot.whenTaken;
1805-
snapshot->lsn=serialized_snapshot.lsn;
18061754
snapshot->snapXactCompletionCount=0;
18071755

18081756
/* Copy XIDs, if present. */

‎src/include/access/genam.h

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -14,9 +14,11 @@
1414
#ifndefGENAM_H
1515
#defineGENAM_H
1616

17+
#include"access/htup.h"
1718
#include"access/sdir.h"
1819
#include"access/skey.h"
1920
#include"nodes/tidbitmap.h"
21+
#include"storage/buf.h"
2022
#include"storage/lockdefs.h"
2123
#include"utils/relcache.h"
2224
#include"utils/snapshot.h"

‎src/include/access/toast_internals.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -58,6 +58,6 @@ extern inttoast_open_indexes(Relation toastrel,
5858
int*num_indexes);
5959
externvoidtoast_close_indexes(Relation*toastidxs,intnum_indexes,
6060
LOCKMODElock);
61-
externvoidinit_toast_snapshot(Snapshottoast_snapshot);
61+
externSnapshotget_toast_snapshot(void);
6262

6363
#endif/* TOAST_INTERNALS_H */

‎src/include/storage/predicate.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -14,6 +14,7 @@
1414
#ifndefPREDICATE_H
1515
#definePREDICATE_H
1616

17+
#include"storage/itemptr.h"
1718
#include"storage/lock.h"
1819
#include"utils/relcache.h"
1920
#include"utils/snapshot.h"

‎src/include/utils/snapmgr.h

Lines changed: 3 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -27,11 +27,14 @@ extern PGDLLIMPORT TransactionId RecentXmin;
2727
/* Variables representing various special snapshot semantics */
2828
externPGDLLIMPORTSnapshotDataSnapshotSelfData;
2929
externPGDLLIMPORTSnapshotDataSnapshotAnyData;
30+
externPGDLLIMPORTSnapshotDataSnapshotToastData;
3031
externPGDLLIMPORTSnapshotDataCatalogSnapshotData;
3132

3233
#defineSnapshotSelf(&SnapshotSelfData)
3334
#defineSnapshotAny(&SnapshotAnyData)
3435

36+
/* Use get_toast_snapshot() for the TOAST snapshot */
37+
3538
/*
3639
* We don't provide a static SnapshotDirty variable because it would be
3740
* non-reentrant. Instead, users of that snapshot type should declare a
@@ -49,15 +52,6 @@ extern PGDLLIMPORT SnapshotData CatalogSnapshotData;
4952
((snapshotdata).snapshot_type = SNAPSHOT_NON_VACUUMABLE, \
5053
(snapshotdata).vistest = (vistestp))
5154

52-
/*
53-
* Similarly, some initialization is required for SnapshotToast. We need
54-
* to set lsn and whenTaken correctly to support snapshot_too_old.
55-
*/
56-
#defineInitToastSnapshot(snapshotdata,l,w) \
57-
((snapshotdata).snapshot_type = SNAPSHOT_TOAST, \
58-
(snapshotdata).lsn = (l),\
59-
(snapshotdata).whenTaken = (w))
60-
6155
/* This macro encodes the knowledge of which snapshots are MVCC-safe */
6256
#defineIsMVCCSnapshot(snapshot) \
6357
((snapshot)->snapshot_type == SNAPSHOT_MVCC || \
@@ -66,7 +60,6 @@ extern PGDLLIMPORT SnapshotData CatalogSnapshotData;
6660
externSnapshotGetTransactionSnapshot(void);
6761
externSnapshotGetLatestSnapshot(void);
6862
externvoidSnapshotSetCommandId(CommandIdcurcid);
69-
externSnapshotGetOldestSnapshot(void);
7063

7164
externSnapshotGetCatalogSnapshot(Oidrelid);
7265
externSnapshotGetNonHistoricCatalogSnapshot(Oidrelid);

‎src/include/utils/snapshot.h

Lines changed: 0 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -13,11 +13,7 @@
1313
#ifndefSNAPSHOT_H
1414
#defineSNAPSHOT_H
1515

16-
#include"access/htup.h"
17-
#include"access/xlogdefs.h"
18-
#include"datatype/timestamp.h"
1916
#include"lib/pairingheap.h"
20-
#include"storage/buf.h"
2117

2218

2319
/*
@@ -205,9 +201,6 @@ typedef struct SnapshotData
205201
uint32regd_count;/* refcount on RegisteredSnapshots */
206202
pairingheap_nodeph_node;/* link in the RegisteredSnapshots heap */
207203

208-
TimestampTzwhenTaken;/* timestamp when snapshot was taken */
209-
XLogRecPtrlsn;/* position in the WAL stream when taken */
210-
211204
/*
212205
* The transaction completion count at the time GetSnapshotData() built
213206
* this snapshot. Allows to avoid re-computing static snapshots when no

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