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Refactor WAL file-reading code into WALRead()

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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
This commit is contained in:
Alvaro Herrera
2019-11-25 15:04:54 -03:00
parent 5883f5fe27
commit 0dc8ead463
6 changed files with 387 additions and 433 deletions
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39
src/include/access/xlogreader.h
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@ -36,7 +36,6 @@ typedef struct WALOpenSegment
{
int ws_file; /* segment file descriptor */
XLogSegNo ws_segno; /* segment number */
uint32 ws_off; /* offset in the segment */
TimeLineID ws_tli; /* timeline ID of the currently open file */
} WALOpenSegment;
@ -168,6 +167,7 @@ struct XLogReaderState
/* last read XLOG position for data currently in readBuf */
WALSegmentContext segcxt;
WALOpenSegment seg;
uint32 segoff;
/*
* beginning of prior page read, and its TLI. Doesn't necessarily
@ -217,6 +217,24 @@ extern XLogReaderState *XLogReaderAllocate(int wal_segment_size,
/* Free an XLogReader */
extern void XLogReaderFree(XLogReaderState *state);
/*
* Callback to open the specified WAL segment for reading. Returns a valid
* file descriptor when the file was opened successfully.
*
* "nextSegNo" is the number of the segment to be opened.
*
* "segcxt" is additional information about the segment.
*
* "tli_p" is an input/output argument. XLogRead() uses it to pass the
* timeline in which the new segment should be found, but the callback can use
* it to return the TLI that it actually opened.
*
* BasicOpenFile() is the preferred way to open the segment file in backend
* code, whereas open(2) should be used in frontend.
*/
typedef int (*WALSegmentOpen) (XLogSegNo nextSegNo, WALSegmentContext *segcxt,
TimeLineID *tli_p);
/* Initialize supporting structures */
extern void WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt,
int segsize, const char *waldir);
@ -232,6 +250,25 @@ extern bool XLogReaderValidatePageHeader(XLogReaderState *state,
#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.
*/
typedef struct WALReadError
{
int wre_errno; /* errno set by the last pg_pread() */
int wre_off; /* Offset we tried to read from. */
int wre_req; /* Bytes requested to be read. */
int wre_read; /* Bytes read by the last read(). */
WALOpenSegment wre_seg; /* Segment we tried to read from. */
} WALReadError;
extern bool WALRead(char *buf, XLogRecPtr startptr, Size count,
TimeLineID tli, WALOpenSegment *seg,
WALSegmentContext *segcxt, WALSegmentOpen openSegment,
WALReadError *errinfo);
/* Functions for decoding an XLogRecord */
extern bool DecodeXLogRecord(XLogReaderState *state, XLogRecord *record,