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Refactor XLogReadRecord(), adding XLogBeginRead() function.

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The signature of XLogReadRecord() required the caller to pass the starting
WAL position as argument, or InvalidXLogRecPtr to continue reading at the
end of previous record. That's slightly awkward to the callers, as most
of them don't want to randomly jump around in the WAL stream, but start
reading at one position and then read everything from that point onwards.
Remove the 'RecPtr' argument and add a new function XLogBeginRead() to
specify the starting position instead. That's more convenient for the
callers. Also, xlogreader holds state that is reset when you change the
starting position, so having a separate function for doing that feels like
a more natural fit.

This changes XLogFindNextRecord() function so that it doesn't reset the
xlogreader's state to what it was before the call anymore. Instead, it
positions the xlogreader to the found record, like XLogBeginRead().

Reviewed-by: Kyotaro Horiguchi, Alvaro Herrera
Discussion: https://www.postgresql.org/message-id/5382a7a3-debe-be31-c860-cb810c08f366%40iki.fi
This commit is contained in:
Heikki Linnakangas
2020-01-26 11:39:00 +02:00
parent 1001368497
commit 38a957316d
10 changed files with 102 additions and 97 deletions
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19
src/include/access/xlogreader.h
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@ -13,7 +13,9 @@
* how to use the XLogReader infrastructure.
*
* The basic idea is to allocate an XLogReaderState via
* XLogReaderAllocate(), and call XLogReadRecord() until it returns NULL.
* XLogReaderAllocate(), position the reader to the first record with
* XLogBeginRead() or XLogFindNextRecord(), and call XLogReadRecord()
* until it returns NULL.
*
* After reading a record with XLogReadRecord(), it's decomposed into
* the per-block and main data parts, and the parts can be accessed
@ -126,7 +128,8 @@ struct XLogReaderState
/*
* Start and end point of last record read. EndRecPtr is also used as the
* position to read next, if XLogReadRecord receives an invalid recptr.
* position to read next. Calling XLogBeginRead() sets EndRecPtr to the
* starting position and ReadRecPtr to invalid.
*/
XLogRecPtr ReadRecPtr; /* start of last record read */
XLogRecPtr EndRecPtr; /* end+1 of last record read */
@ -239,18 +242,20 @@ typedef int (*WALSegmentOpen) (XLogSegNo nextSegNo, WALSegmentContext *segcxt,
extern void WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt,
int segsize, const char *waldir);
/* Position the XLogReader to given record */
extern void XLogBeginRead(XLogReaderState *state, XLogRecPtr RecPtr);
#ifdef FRONTEND
extern XLogRecPtr XLogFindNextRecord(XLogReaderState *state, XLogRecPtr RecPtr);
#endif /* FRONTEND */
/* Read the next XLog record. Returns NULL on end-of-WAL or failure */
extern struct XLogRecord *XLogReadRecord(XLogReaderState *state,
XLogRecPtr recptr, char **errormsg);
char **errormsg);
/* Validate a page */
extern bool XLogReaderValidatePageHeader(XLogReaderState *state,
XLogRecPtr recptr, char *phdr);
#ifdef FRONTEND
extern XLogRecPtr XLogFindNextRecord(XLogReaderState *state, XLogRecPtr RecPtr);
#endif /* FRONTEND */
/*
* Error information from WALRead that both backend and frontend caller can
* process. Currently only errors from pg_pread can be reported.