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Commit7dfb9b4

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Fix pg_xlogdump so that it handles cross-page XLP_FIRST_IS_CONTRECORD record.
Previously pg_xlogdump failed to dump the contents of the WAL fileif the file starts with the continuation WAL record which spansmore than one pages. Since pg_xlogdump assumed that the continuationrecord always fits on a page, it could not find the valid WAL record tostart reading from in that case.This patch changes pg_xlogdump so that it can handle a continuationWAL record which crosses a page boundary and find the valid recordto start reading from.Back-patch to 9.3 where pg_xlogdump was introduced.Author: Pavan DeolaseeReviewed-By: Michael Paquier and Craig RingerDiscussion: CABOikdPsPByMiG6J01DKq6om2+BNkxHTPkOyqHM2a4oYwGKsqQ@mail.gmail.com
1 parentb09caec commit7dfb9b4

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+64
-27
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+64
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‎src/backend/access/transam/xlogreader.c

Lines changed: 64 additions & 27 deletions
Original file line numberDiff line numberDiff line change
@@ -844,46 +844,83 @@ XLogRecPtr
844844
XLogFindNextRecord(XLogReaderState*state,XLogRecPtrRecPtr)
845845
{
846846
XLogReaderStatesaved_state=*state;
847-
XLogRecPtrtargetPagePtr;
848847
XLogRecPtrtmpRecPtr;
849-
inttargetRecOff;
850848
XLogRecPtrfound=InvalidXLogRecPtr;
851-
uint32pageHeaderSize;
852849
XLogPageHeaderheader;
853-
intreadLen;
854850
char*errormsg;
855851

856852
Assert(!XLogRecPtrIsInvalid(RecPtr));
857853

858-
targetRecOff=RecPtr %XLOG_BLCKSZ;
854+
/*
855+
* skip over potential continuation data, keeping in mind that it may span
856+
* multiple pages
857+
*/
858+
tmpRecPtr=RecPtr;
859+
while (true)
860+
{
861+
XLogRecPtrtargetPagePtr;
862+
inttargetRecOff;
863+
uint32pageHeaderSize;
864+
intreadLen;
859865

860-
/* scroll back to page boundary */
861-
targetPagePtr=RecPtr-targetRecOff;
866+
/*
867+
* Compute targetRecOff. It should typically be equal or greater than
868+
* short page-header since a valid record can't start anywhere before
869+
* that, except when caller has explicitly specified the offset that
870+
* falls somewhere there or when we are skipping multi-page
871+
* continuation record. It doesn't matter though because
872+
* ReadPageInternal() is prepared to handle that and will read at least
873+
* short page-header worth of data
874+
*/
875+
targetRecOff=tmpRecPtr %XLOG_BLCKSZ;
862876

863-
/* Read the page containing the record */
864-
readLen=ReadPageInternal(state,targetPagePtr,targetRecOff);
865-
if (readLen<0)
866-
gotoerr;
877+
/* scroll back to page boundary */
878+
targetPagePtr=tmpRecPtr-targetRecOff;
867879

868-
header= (XLogPageHeader)state->readBuf;
880+
/* Read the page containing the record */
881+
readLen=ReadPageInternal(state,targetPagePtr,targetRecOff);
882+
if (readLen<0)
883+
gotoerr;
869884

870-
pageHeaderSize=XLogPageHeaderSize(header);
885+
header=(XLogPageHeader)state->readBuf;
871886

872-
/* make sure we have enough data for the page header */
873-
readLen=ReadPageInternal(state,targetPagePtr,pageHeaderSize);
874-
if (readLen<0)
875-
gotoerr;
887+
pageHeaderSize=XLogPageHeaderSize(header);
876888

877-
/* skip over potential continuation data */
878-
if (header->xlp_info&XLP_FIRST_IS_CONTRECORD)
879-
{
880-
/* record headers are MAXALIGN'ed */
881-
tmpRecPtr=targetPagePtr+pageHeaderSize
882-
+MAXALIGN(header->xlp_rem_len);
883-
}
884-
else
885-
{
886-
tmpRecPtr=targetPagePtr+pageHeaderSize;
889+
/* make sure we have enough data for the page header */
890+
readLen=ReadPageInternal(state,targetPagePtr,pageHeaderSize);
891+
if (readLen<0)
892+
gotoerr;
893+
894+
/* skip over potential continuation data */
895+
if (header->xlp_info&XLP_FIRST_IS_CONTRECORD)
896+
{
897+
/*
898+
* If the length of the remaining continuation data is more than
899+
* what can fit in this page, the continuation record crosses over
900+
* this page. Read the next page and try again. xlp_rem_len in the
901+
* next page header will contain the remaining length of the
902+
* continuation data
903+
*
904+
* Note that record headers are MAXALIGN'ed
905+
*/
906+
if (MAXALIGN(header->xlp_rem_len)> (XLOG_BLCKSZ-pageHeaderSize))
907+
tmpRecPtr=targetPagePtr+XLOG_BLCKSZ;
908+
else
909+
{
910+
/*
911+
* The previous continuation record ends in this page. Set
912+
* tmpRecPtr to point to the first valid record
913+
*/
914+
tmpRecPtr=targetPagePtr+pageHeaderSize
915+
+MAXALIGN(header->xlp_rem_len);
916+
break;
917+
}
918+
}
919+
else
920+
{
921+
tmpRecPtr=targetPagePtr+pageHeaderSize;
922+
break;
923+
}
887924
}
888925

889926
/*

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