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Commiteb5e8ba

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Fix sequence creation to set the t_xmin of a sequence's tuple to
FrozenTransactionId, not the XID of the creating transaction. Withoutthis it's possible for a reference to a long-gone CLOG record to occur,per Christian Meunier's bug report of 10-Jan-02. Worse, the sequencetuple would become invisible to SELECTs after 2 billion transactions.Since the fix is applied during sequence creation it does not helpexisting databases, unless you drop and recreate every sequence.However, we intend to force initdb for 7.2RC1 anyway, to fix a pg_procerror, so I see no need to do more for this problem.
1 parent051a4f2 commiteb5e8ba

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1 file changed

+45
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‎src/backend/commands/sequence.c‎

Lines changed: 45 additions & 9 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $Header: /cvsroot/pgsql/src/backend/commands/sequence.c,v 1.67 2001/11/05 17:46:24 momjian Exp $
11+
* $Header: /cvsroot/pgsql/src/backend/commands/sequence.c,v 1.68 2002/01/11 18:16:04 tgl Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -43,7 +43,7 @@
4343
#defineSEQ_MINVALUE(-SEQ_MAXVALUE)
4444

4545
/*
46-
* We don't want to log each fetchingvaluesfromsequences,
46+
* We don't want to log each fetchingof a valuefroma sequence,
4747
* so we pre-log a few fetches in advance. In the event of
4848
* crash we can lose as much as we pre-logged.
4949
*/
@@ -199,24 +199,59 @@ DefineSequence(CreateSeqStmt *seq)
199199
/* hack: ensure heap_insert will insert on the just-created page */
200200
rel->rd_targblock=0;
201201

202-
/* Now-form & insert sequence tuple */
202+
/* Now form & insert sequence tuple */
203203
tuple=heap_formtuple(tupDesc,value,null);
204204
heap_insert(rel,tuple);
205205

206+
Assert(ItemPointerGetOffsetNumber(&(tuple->t_self))==FirstOffsetNumber);
207+
206208
/*
207-
* After crash REDO of heap_insert above would re-init page and our
208-
* magic number would be lost. We have to log sequence creation. This
209-
* means two log records instead of one -:(
209+
* Two special hacks here:
210+
*
211+
* 1. Since VACUUM does not process sequences, we have to force the tuple
212+
* to have xmin = FrozenTransactionId now. Otherwise it would become
213+
* invisible to SELECTs after 2G transactions. It is okay to do this
214+
* because if the current transaction aborts, no other xact will ever
215+
* examine the sequence tuple anyway.
216+
*
217+
* 2. Even though heap_insert emitted a WAL log record, we have to emit
218+
* an XLOG_SEQ_LOG record too, since (a) the heap_insert record will
219+
* not have the right xmin, and (b) REDO of the heap_insert record
220+
* would re-init page and sequence magic number would be lost. This
221+
* means two log records instead of one :-(
210222
*/
211223
LockBuffer(buf,BUFFER_LOCK_EXCLUSIVE);
212224
START_CRIT_SECTION();
225+
226+
{
227+
/*
228+
* Note that the "tuple" structure is still just a local tuple record
229+
* created by heap_formtuple; its t_data pointer doesn't point at the
230+
* disk buffer. To scribble on the disk buffer we need to fetch the
231+
* item pointer. But do the same to the local tuple, since that will
232+
* be the source for the WAL log record, below.
233+
*/
234+
ItemIditemId;
235+
Itemitem;
236+
237+
itemId=PageGetItemId((Page)page,FirstOffsetNumber);
238+
item=PageGetItem((Page)page,itemId);
239+
240+
((HeapTupleHeader)item)->t_xmin=FrozenTransactionId;
241+
((HeapTupleHeader)item)->t_infomask |=HEAP_XMIN_COMMITTED;
242+
243+
tuple->t_data->t_xmin=FrozenTransactionId;
244+
tuple->t_data->t_infomask |=HEAP_XMIN_COMMITTED;
245+
}
246+
213247
{
214248
xl_seq_recxlrec;
215249
XLogRecPtrrecptr;
216250
XLogRecDatardata[2];
217251
Form_pg_sequencenewseq= (Form_pg_sequence)GETSTRUCT(tuple);
218252

219253
/* We do not log first nextval call, so "advance" sequence here */
254+
/* Note we are scribbling on local tuple, not the disk buffer */
220255
newseq->is_called= true;
221256
newseq->log_cnt=0;
222257

@@ -239,8 +274,8 @@ DefineSequence(CreateSeqStmt *seq)
239274
END_CRIT_SECTION();
240275

241276
LockBuffer(buf,BUFFER_LOCK_UNLOCK);
242-
ReleaseBuffer(buf);
243-
heap_close(rel,AccessExclusiveLock);
277+
WriteBuffer(buf);
278+
heap_close(rel,NoLock);
244279
}
245280

246281

@@ -628,7 +663,6 @@ read_info(char *caller, SeqTable elm, Buffer *buf)
628663
elm->increment=seq->increment_by;
629664

630665
returnseq;
631-
632666
}
633667

634668

@@ -858,6 +892,8 @@ seq_redo(XLogRecPtr lsn, XLogRecord *record)
858892

859893
page= (Page)BufferGetPage(buffer);
860894

895+
/* Always reinit the page and reinstall the magic number */
896+
/* See comments in DefineSequence */
861897
PageInit((Page)page,BufferGetPageSize(buffer),sizeof(sequence_magic));
862898
sm= (sequence_magic*)PageGetSpecialPointer(page);
863899
sm->magic=SEQ_MAGIC;

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