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Commited57cac

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pg_walinspect: fix case where flush LSN is in the middle of a record.
Instability in the test for pg_walinspect revealed thatpg_get_wal_records_info_till_end_of_wal(x) would try to decode all therecords with a start LSN earlier than the flush LSN, even though thatmight include a partial record at the end of the range. In that case,read_local_xlog_page_no_wait() would return NULL when it tried to readpast the flush LSN, which would be interpreted as an error by thecaller. That caused a test failure only on a BF animal that had beenrestarted recently, but could be expected to happen in the wild quiteeasily depending on the alignment of various parameters.Fix by using private data in read_local_xlog_page_no_wait() to signalend-of-wal to the caller, so that it can be properly distinguishedfrom a real error.Discussion:https://postgr.es/m/Ymd/e5eeZMNAkrXo%40paquier.xyzDiscussion:https://postgr.es/m/111657.1650910309@sss.pgh.pa.usAuthors: Thomas Munro, Bharath Rupireddy.
1 parentccd10a9 commited57cac

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3 files changed

+50
-26
lines changed

3 files changed

+50
-26
lines changed

‎contrib/pg_walinspect/pg_walinspect.c

Lines changed: 33 additions & 26 deletions
Original file line numberDiff line numberDiff line change
@@ -89,6 +89,7 @@ static XLogReaderState *
8989
InitXLogReaderState(XLogRecPtrlsn,XLogRecPtr*first_record)
9090
{
9191
XLogReaderState*xlogreader;
92+
ReadLocalXLogPageNoWaitPrivate*private_data;
9293

9394
/*
9495
* Reading WAL below the first page of the first segments isn't allowed.
@@ -100,11 +101,14 @@ InitXLogReaderState(XLogRecPtr lsn, XLogRecPtr *first_record)
100101
(errmsg("could not read WAL at LSN %X/%X",
101102
LSN_FORMAT_ARGS(lsn))));
102103

104+
private_data= (ReadLocalXLogPageNoWaitPrivate*)
105+
palloc0(sizeof(ReadLocalXLogPageNoWaitPrivate));
106+
103107
xlogreader=XLogReaderAllocate(wal_segment_size,NULL,
104108
XL_ROUTINE(.page_read=&read_local_xlog_page_no_wait,
105109
.segment_open=&wal_segment_open,
106110
.segment_close=&wal_segment_close),
107-
NULL);
111+
private_data);
108112

109113
if (xlogreader==NULL)
110114
ereport(ERROR,
@@ -132,7 +136,8 @@ InitXLogReaderState(XLogRecPtr lsn, XLogRecPtr *first_record)
132136
*
133137
* We guard against ordinary errors trying to read WAL that hasn't been
134138
* written yet by limiting end_lsn to the flushed WAL, but that can also
135-
* encounter errors if the flush pointer falls in the middle of a record.
139+
* encounter errors if the flush pointer falls in the middle of a record. In
140+
* that case we'll return NULL.
136141
*/
137142
staticXLogRecord*
138143
ReadNextXLogRecord(XLogReaderState*xlogreader,XLogRecPtrfirst_record)
@@ -144,6 +149,15 @@ ReadNextXLogRecord(XLogReaderState *xlogreader, XLogRecPtr first_record)
144149

145150
if (record==NULL)
146151
{
152+
ReadLocalXLogPageNoWaitPrivate*private_data;
153+
154+
/* return NULL, if end of WAL is reached */
155+
private_data= (ReadLocalXLogPageNoWaitPrivate*)
156+
xlogreader->private_data;
157+
158+
if (private_data->end_of_wal)
159+
returnNULL;
160+
147161
if (errormsg)
148162
ereport(ERROR,
149163
(errcode_for_file_access(),
@@ -246,14 +260,19 @@ pg_get_wal_record_info(PG_FUNCTION_ARGS)
246260

247261
xlogreader=InitXLogReaderState(lsn,&first_record);
248262

249-
(void)ReadNextXLogRecord(xlogreader,first_record);
263+
if (!ReadNextXLogRecord(xlogreader,first_record))
264+
ereport(ERROR,
265+
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
266+
errmsg("could not read WAL at %X/%X",
267+
LSN_FORMAT_ARGS(first_record))));
250268

251269
MemSet(values,0,sizeof(values));
252270
MemSet(nulls,0,sizeof(nulls));
253271

254272
GetWALRecordInfo(xlogreader,first_record,values,nulls,
255273
PG_GET_WAL_RECORD_INFO_COLS);
256274

275+
pfree(xlogreader->private_data);
257276
XLogReaderFree(xlogreader);
258277

259278
tuple=heap_form_tuple(tupdesc,values,nulls);
@@ -327,26 +346,19 @@ GetWALRecordsInfo(FunctionCallInfo fcinfo, XLogRecPtr start_lsn,
327346
MemSet(values,0,sizeof(values));
328347
MemSet(nulls,0,sizeof(nulls));
329348

330-
for (;;)
349+
while (ReadNextXLogRecord(xlogreader,first_record)&&
350+
xlogreader->EndRecPtr <=end_lsn)
331351
{
332-
(void)ReadNextXLogRecord(xlogreader,first_record);
333-
334-
if (xlogreader->EndRecPtr <=end_lsn)
335-
{
336-
GetWALRecordInfo(xlogreader,xlogreader->currRecPtr,values,nulls,
337-
PG_GET_WAL_RECORDS_INFO_COLS);
352+
GetWALRecordInfo(xlogreader,xlogreader->currRecPtr,values,nulls,
353+
PG_GET_WAL_RECORDS_INFO_COLS);
338354

339-
tuplestore_putvalues(rsinfo->setResult,rsinfo->setDesc,
340-
values,nulls);
341-
}
342-
343-
/* if we read up to end_lsn, we're done */
344-
if (xlogreader->EndRecPtr >=end_lsn)
345-
break;
355+
tuplestore_putvalues(rsinfo->setResult,rsinfo->setDesc,
356+
values,nulls);
346357

347358
CHECK_FOR_INTERRUPTS();
348359
}
349360

361+
pfree(xlogreader->private_data);
350362
XLogReaderFree(xlogreader);
351363

352364
#undef PG_GET_WAL_RECORDS_INFO_COLS
@@ -555,20 +567,15 @@ GetWalStats(FunctionCallInfo fcinfo, XLogRecPtr start_lsn,
555567

556568
MemSet(&stats,0,sizeof(stats));
557569

558-
for (;;)
570+
while (ReadNextXLogRecord(xlogreader,first_record)&&
571+
xlogreader->EndRecPtr <=end_lsn)
559572
{
560-
(void)ReadNextXLogRecord(xlogreader,first_record);
561-
562-
if (xlogreader->EndRecPtr <=end_lsn)
563-
XLogRecStoreStats(&stats,xlogreader);
564-
565-
/* if we read up to end_lsn, we're done */
566-
if (xlogreader->EndRecPtr >=end_lsn)
567-
break;
573+
XLogRecStoreStats(&stats,xlogreader);
568574

569575
CHECK_FOR_INTERRUPTS();
570576
}
571577

578+
pfree(xlogreader->private_data);
572579
XLogReaderFree(xlogreader);
573580

574581
MemSet(values,0,sizeof(values));

‎src/backend/access/transam/xlogutils.c

Lines changed: 11 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -957,7 +957,18 @@ read_local_xlog_page_guts(XLogReaderState *state, XLogRecPtr targetPagePtr,
957957

958958
/* If asked, let's not wait for future WAL. */
959959
if (!wait_for_wal)
960+
{
961+
ReadLocalXLogPageNoWaitPrivate*private_data;
962+
963+
/*
964+
* Inform the caller of read_local_xlog_page_no_wait that the
965+
* end of WAL has been reached.
966+
*/
967+
private_data= (ReadLocalXLogPageNoWaitPrivate*)
968+
state->private_data;
969+
private_data->end_of_wal= true;
960970
break;
971+
}
961972

962973
CHECK_FOR_INTERRUPTS();
963974
pg_usleep(1000L);

‎src/include/access/xlogutils.h

Lines changed: 6 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -75,6 +75,12 @@ typedef enum
7575
* need to be replayed) */
7676
}XLogRedoAction;
7777

78+
/* Private data of the read_local_xlog_page_no_wait callback. */
79+
typedefstructReadLocalXLogPageNoWaitPrivate
80+
{
81+
boolend_of_wal;/* true, when end of WAL is reached */
82+
}ReadLocalXLogPageNoWaitPrivate;
83+
7884
externXLogRedoActionXLogReadBufferForRedo(XLogReaderState*record,
7985
uint8buffer_id,Buffer*buf);
8086
externBufferXLogInitBufferForRedo(XLogReaderState*record,uint8block_id);

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