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Commitbab7823

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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 parentb899ccb commitbab7823

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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
@@ -866,46 +866,83 @@ XLogRecPtr
866866
XLogFindNextRecord(XLogReaderState*state,XLogRecPtrRecPtr)
867867
{
868868
XLogReaderStatesaved_state=*state;
869-
XLogRecPtrtargetPagePtr;
870869
XLogRecPtrtmpRecPtr;
871-
inttargetRecOff;
872870
XLogRecPtrfound=InvalidXLogRecPtr;
873-
uint32pageHeaderSize;
874871
XLogPageHeaderheader;
875-
intreadLen;
876872
char*errormsg;
877873

878874
Assert(!XLogRecPtrIsInvalid(RecPtr));
879875

880-
targetRecOff=RecPtr %XLOG_BLCKSZ;
876+
/*
877+
* skip over potential continuation data, keeping in mind that it may span
878+
* multiple pages
879+
*/
880+
tmpRecPtr=RecPtr;
881+
while (true)
882+
{
883+
XLogRecPtrtargetPagePtr;
884+
inttargetRecOff;
885+
uint32pageHeaderSize;
886+
intreadLen;
881887

882-
/* scroll back to page boundary */
883-
targetPagePtr=RecPtr-targetRecOff;
888+
/*
889+
* Compute targetRecOff. It should typically be equal or greater than
890+
* short page-header since a valid record can't start anywhere before
891+
* that, except when caller has explicitly specified the offset that
892+
* falls somewhere there or when we are skipping multi-page
893+
* continuation record. It doesn't matter though because
894+
* ReadPageInternal() is prepared to handle that and will read at least
895+
* short page-header worth of data
896+
*/
897+
targetRecOff=tmpRecPtr %XLOG_BLCKSZ;
884898

885-
/* Read the page containing the record */
886-
readLen=ReadPageInternal(state,targetPagePtr,targetRecOff);
887-
if (readLen<0)
888-
gotoerr;
899+
/* scroll back to page boundary */
900+
targetPagePtr=tmpRecPtr-targetRecOff;
889901

890-
header= (XLogPageHeader)state->readBuf;
902+
/* Read the page containing the record */
903+
readLen=ReadPageInternal(state,targetPagePtr,targetRecOff);
904+
if (readLen<0)
905+
gotoerr;
891906

892-
pageHeaderSize=XLogPageHeaderSize(header);
907+
header=(XLogPageHeader)state->readBuf;
893908

894-
/* make sure we have enough data for the page header */
895-
readLen=ReadPageInternal(state,targetPagePtr,pageHeaderSize);
896-
if (readLen<0)
897-
gotoerr;
909+
pageHeaderSize=XLogPageHeaderSize(header);
898910

899-
/* skip over potential continuation data */
900-
if (header->xlp_info&XLP_FIRST_IS_CONTRECORD)
901-
{
902-
/* record headers are MAXALIGN'ed */
903-
tmpRecPtr=targetPagePtr+pageHeaderSize
904-
+MAXALIGN(header->xlp_rem_len);
905-
}
906-
else
907-
{
908-
tmpRecPtr=targetPagePtr+pageHeaderSize;
911+
/* make sure we have enough data for the page header */
912+
readLen=ReadPageInternal(state,targetPagePtr,pageHeaderSize);
913+
if (readLen<0)
914+
gotoerr;
915+
916+
/* skip over potential continuation data */
917+
if (header->xlp_info&XLP_FIRST_IS_CONTRECORD)
918+
{
919+
/*
920+
* If the length of the remaining continuation data is more than
921+
* what can fit in this page, the continuation record crosses over
922+
* this page. Read the next page and try again. xlp_rem_len in the
923+
* next page header will contain the remaining length of the
924+
* continuation data
925+
*
926+
* Note that record headers are MAXALIGN'ed
927+
*/
928+
if (MAXALIGN(header->xlp_rem_len)> (XLOG_BLCKSZ-pageHeaderSize))
929+
tmpRecPtr=targetPagePtr+XLOG_BLCKSZ;
930+
else
931+
{
932+
/*
933+
* The previous continuation record ends in this page. Set
934+
* tmpRecPtr to point to the first valid record
935+
*/
936+
tmpRecPtr=targetPagePtr+pageHeaderSize
937+
+MAXALIGN(header->xlp_rem_len);
938+
break;
939+
}
940+
}
941+
else
942+
{
943+
tmpRecPtr=targetPagePtr+pageHeaderSize;
944+
break;
945+
}
909946
}
910947

911948
/*

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