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hlinnakahlinnaka
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Handle duplicate XIDs in txid_snapshot.
The proc array can contain duplicate XIDs, when a transaction is just being prepared for two-phase commit. To cope, remove any duplicates in txid_current_snapshot(). Also ignore duplicates in the input functions, so that if e.g. you have an old pg_dump file that already contains duplicates, it will be accepted. Report and fix by Jan Wieck. Backpatch to all supported versions.
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3 files changed

+54
-17
lines changed

3 files changed

+54
-17
lines changed

‎src/backend/utils/adt/txid.c

Lines changed: 47 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -130,16 +130,34 @@ cmp_txid(const void *aa, const void *bb)
130130
}
131131

132132
/*
133-
* sort a snapshot's txids, so we can use bsearch() later.
133+
* Sort a snapshot's txids, so we can use bsearch() later. Also remove
134+
* any duplicates.
134135
*
135136
* For consistency of on-disk representation, we always sort even if bsearch
136137
* will not be used.
137138
*/
138139
static void
139140
sort_snapshot(TxidSnapshot *snap)
140141
{
142+
txid last = 0;
143+
int nxip, idx1, idx2;
144+
141145
if (snap->nxip > 1)
146+
{
142147
qsort(snap->xip, snap->nxip, sizeof(txid), cmp_txid);
148+
149+
/* remove duplicates */
150+
nxip = snap->nxip;
151+
idx1 = idx2 = 0;
152+
while (idx1 < nxip)
153+
{
154+
if (snap->xip[idx1] != last)
155+
last = snap->xip[idx2++] = snap->xip[idx1];
156+
else
157+
snap->nxip--;
158+
idx1++;
159+
}
160+
}
143161
}
144162

145163
/*
@@ -294,10 +312,12 @@ parse_snapshot(const char *str)
294312
str = endp;
295313

296314
/* require the input to be in order */
297-
if (val < xmin || val >= xmax || val <= last_val)
315+
if (val < xmin || val >= xmax || val < last_val)
298316
goto bad_format;
299317

300-
buf_add_txid(buf, val);
318+
/* skip duplicates */
319+
if (val != last_val)
320+
buf_add_txid(buf, val);
301321
last_val = val;
302322

303323
if (*str == ',')
@@ -360,8 +380,7 @@ txid_current_snapshot(PG_FUNCTION_ARGS)
360380
{
361381
TxidSnapshot *snap;
362382
uint32 nxip,
363-
i,
364-
size;
383+
i;
365384
TxidEpoch state;
366385
Snapshot cur;
367386

@@ -373,9 +392,7 @@ txid_current_snapshot(PG_FUNCTION_ARGS)
373392

374393
/* allocate */
375394
nxip = cur->xcnt;
376-
size = TXID_SNAPSHOT_SIZE(nxip);
377-
snap = palloc(size);
378-
SET_VARSIZE(snap, size);
395+
snap = palloc(TXID_SNAPSHOT_SIZE(nxip));
379396

380397
/* fill */
381398
snap->xmin = convert_xid(cur->xmin, &state);
@@ -384,9 +401,18 @@ txid_current_snapshot(PG_FUNCTION_ARGS)
384401
for (i = 0; i < nxip; i++)
385402
snap->xip[i] = convert_xid(cur->xip[i], &state);
386403

387-
/* we want them guaranteed to be in ascending order */
404+
/*
405+
* We want them guaranteed to be in ascending order. This also removes
406+
* any duplicate xids. Normally, an XID can only be assigned to one
407+
* backend, but when preparing a transaction for two-phase commit, there
408+
* is a transient state when both the original backend and the dummy
409+
* PGPROC entry reserved for the prepared transaction hold the same XID.
410+
*/
388411
sort_snapshot(snap);
389412

413+
/* set size after sorting, because it may have removed duplicate xips */
414+
SET_VARSIZE(snap, TXID_SNAPSHOT_SIZE(snap->nxip));
415+
390416
PG_RETURN_POINTER(snap);
391417
}
392418

@@ -464,18 +490,27 @@ txid_snapshot_recv(PG_FUNCTION_ARGS)
464490
snap = palloc(TXID_SNAPSHOT_SIZE(nxip));
465491
snap->xmin = xmin;
466492
snap->xmax = xmax;
467-
snap->nxip = nxip;
468-
SET_VARSIZE(snap, TXID_SNAPSHOT_SIZE(nxip));
469493

470494
for (i = 0; i < nxip; i++)
471495
{
472496
txid cur = pq_getmsgint64(buf);
473497

474-
if (cur <= last || cur < xmin || cur >= xmax)
498+
if (cur < last || cur < xmin || cur >= xmax)
475499
goto bad_format;
500+
501+
/* skip duplicate xips */
502+
if (cur == last)
503+
{
504+
i--;
505+
nxip--;
506+
continue;
507+
}
508+
476509
snap->xip[i] = cur;
477510
last = cur;
478511
}
512+
snap->nxip = nxip;
513+
SET_VARSIZE(snap, TXID_SNAPSHOT_SIZE(nxip));
479514
PG_RETURN_POINTER(snap);
480515

481516
bad_format:

‎src/test/regress/expected/txid.out

Lines changed: 6 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -12,6 +12,12 @@ select '12:18:14,16'::txid_snapshot;
1212
12:18:14,16
1313
(1 row)
1414

15+
select '12:16:14,14'::txid_snapshot;
16+
txid_snapshot
17+
---------------
18+
12:16:14
19+
(1 row)
20+
1521
-- errors
1622
select '31:12:'::txid_snapshot;
1723
ERROR: invalid input for txid_snapshot: "31:12:"
@@ -29,10 +35,6 @@ select '12:16:14,13'::txid_snapshot;
2935
ERROR: invalid input for txid_snapshot: "12:16:14,13"
3036
LINE 1: select '12:16:14,13'::txid_snapshot;
3137
^
32-
select '12:16:14,14'::txid_snapshot;
33-
ERROR: invalid input for txid_snapshot: "12:16:14,14"
34-
LINE 1: select '12:16:14,14'::txid_snapshot;
35-
^
3638
create temp table snapshot_test (
3739
nr integer,
3840
snap txid_snapshot

‎src/test/regress/sql/txid.sql

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -3,13 +3,13 @@
33
-- i/o
44
select '12:13:'::txid_snapshot;
55
select '12:18:14,16'::txid_snapshot;
6+
select '12:16:14,14'::txid_snapshot;
67

78
-- errors
89
select '31:12:'::txid_snapshot;
910
select '0:1:'::txid_snapshot;
1011
select '12:13:0'::txid_snapshot;
1112
select '12:16:14,13'::txid_snapshot;
12-
select '12:16:14,14'::txid_snapshot;
1313

1414
create temp table snapshot_test (
1515
nr integer,

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