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Commit 0dc8ead

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alvherrealvherre
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Refactor WAL file-reading code into WALRead()
XLogReader, walsender and pg_waldump all had their own routines to read data from WAL files to memory, with slightly different approaches according to the particular conditions of each environment. There's a lot of commonality, so we can refactor that into a single routine WALRead in XLogReader, and move the differences to a separate (simpler) callback that just opens the next WAL-segment. This results in a clearer (ahem) code flow. The error reporting needs are covered by filling in a new error-info struct, WALReadError, and it's the caller's responsibility to act on it. The backend has WALReadRaiseError() to do so. We no longer ever need to seek in this interface; switch to using pg_pread(). Author: Antonin Houska, with contributions from Álvaro Herrera Reviewed-by: Michaël Paquier, Kyotaro Horiguchi Discussion: https://postgr.es/m/14984.1554998742@spoje.net
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‎src/backend/access/transam/xlogreader.c

Copy file name to clipboardExpand all lines: src/backend/access/transam/xlogreader.c
+100-6Lines changed: 100 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -17,6 +17,8 @@
1717
*/
1818
#include "postgres.h"
1919

20+
#include <unistd.h>
21+
2022
#include "access/transam.h"
2123
#include "access/xlog_internal.h"
2224
#include "access/xlogreader.h"
@@ -27,6 +29,7 @@
2729

2830
#ifndef FRONTEND
2931
#include "miscadmin.h"
32+
#include "pgstat.h"
3033
#include "utils/memutils.h"
3134
#endif
3235

@@ -208,7 +211,6 @@ WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt,
208211
{
209212
seg->ws_file = -1;
210213
seg->ws_segno = 0;
211-
seg->ws_off = 0;
212214
seg->ws_tli = 0;
213215

214216
segcxt->ws_segsize = segsize;
@@ -295,8 +297,7 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
295297
* byte to cover the whole record header, or at least the part of it that
296298
* fits on the same page.
297299
*/
298-
readOff = ReadPageInternal(state,
299-
targetPagePtr,
300+
readOff = ReadPageInternal(state, targetPagePtr,
300301
Min(targetRecOff + SizeOfXLogRecord, XLOG_BLCKSZ));
301302
if (readOff < 0)
302303
goto err;
@@ -556,7 +557,7 @@ ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen)
556557

557558
/* check whether we have all the requested data already */
558559
if (targetSegNo == state->seg.ws_segno &&
559-
targetPageOff == state->seg.ws_off && reqLen <= state->readLen)
560+
targetPageOff == state->segoff && reqLen <= state->readLen)
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return state->readLen;
561562

562563
/*
@@ -627,7 +628,7 @@ ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen)
627628

628629
/* update read state information */
629630
state->seg.ws_segno = targetSegNo;
630-
state->seg.ws_off = targetPageOff;
631+
state->segoff = targetPageOff;
631632
state->readLen = readLen;
632633

633634
return readLen;
@@ -644,7 +645,7 @@ static void
644645
XLogReaderInvalReadState(XLogReaderState *state)
645646
{
646647
state->seg.ws_segno = 0;
647-
state->seg.ws_off = 0;
648+
state->segoff = 0;
648649
state->readLen = 0;
649650
}
650651

@@ -1015,6 +1016,99 @@ XLogFindNextRecord(XLogReaderState *state, XLogRecPtr RecPtr)
10151016

10161017
#endif /* FRONTEND */
10171018

1019+
/*
1020+
* Read 'count' bytes into 'buf', starting at location 'startptr', from WAL
1021+
* fetched from timeline 'tli'.
1022+
*
1023+
* 'seg/segcxt' identify the last segment used. 'openSegment' is a callback
1024+
* to open the next segment, if necessary.
1025+
*
1026+
* Returns true if succeeded, false if an error occurs, in which case
1027+
* 'errinfo' receives error details.
1028+
*
1029+
* XXX probably this should be improved to suck data directly from the
1030+
* WAL buffers when possible.
1031+
*/
1032+
bool
1033+
WALRead(char *buf, XLogRecPtr startptr, Size count, TimeLineID tli,
1034+
WALOpenSegment *seg, WALSegmentContext *segcxt,
1035+
WALSegmentOpen openSegment, WALReadError *errinfo)
1036+
{
1037+
char *p;
1038+
XLogRecPtr recptr;
1039+
Size nbytes;
1040+
1041+
p = buf;
1042+
recptr = startptr;
1043+
nbytes = count;
1044+
1045+
while (nbytes > 0)
1046+
{
1047+
uint32 startoff;
1048+
int segbytes;
1049+
int readbytes;
1050+
1051+
startoff = XLogSegmentOffset(recptr, segcxt->ws_segsize);
1052+
1053+
/*
1054+
* If the data we want is not in a segment we have open, close what we
1055+
* have (if anything) and open the next one, using the caller's
1056+
* provided openSegment callback.
1057+
*/
1058+
if (seg->ws_file < 0 ||
1059+
!XLByteInSeg(recptr, seg->ws_segno, segcxt->ws_segsize) ||
1060+
tli != seg->ws_tli)
1061+
{
1062+
XLogSegNo nextSegNo;
1063+
1064+
if (seg->ws_file >= 0)
1065+
close(seg->ws_file);
1066+
1067+
XLByteToSeg(recptr, nextSegNo, segcxt->ws_segsize);
1068+
seg->ws_file = openSegment(nextSegNo, segcxt, &tli);
1069+
1070+
/* Update the current segment info. */
1071+
seg->ws_tli = tli;
1072+
seg->ws_segno = nextSegNo;
1073+
}
1074+
1075+
/* How many bytes are within this segment? */
1076+
if (nbytes > (segcxt->ws_segsize - startoff))
1077+
segbytes = segcxt->ws_segsize - startoff;
1078+
else
1079+
segbytes = nbytes;
1080+
1081+
#ifndef FRONTEND
1082+
pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
1083+
#endif
1084+
1085+
/* Reset errno first; eases reporting non-errno-affecting errors */
1086+
errno = 0;
1087+
readbytes = pg_pread(seg->ws_file, p, segbytes, (off_t) startoff);
1088+
1089+
#ifndef FRONTEND
1090+
pgstat_report_wait_end();
1091+
#endif
1092+
1093+
if (readbytes <= 0)
1094+
{
1095+
errinfo->wre_errno = errno;
1096+
errinfo->wre_req = segbytes;
1097+
errinfo->wre_read = readbytes;
1098+
errinfo->wre_off = startoff;
1099+
errinfo->wre_seg = *seg;
1100+
return false;
1101+
}
1102+
1103+
/* Update state for read */
1104+
recptr += readbytes;
1105+
nbytes -= readbytes;
1106+
p += readbytes;
1107+
}
1108+
1109+
return true;
1110+
}
1111+
10181112
/* ----------------------------------------
10191113
* Functions for decoding the data and block references in a record.
10201114
* ----------------------------------------

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