xlogreader.h

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/*-------------------------------------------------------------------------
 *
 * xlogreader.h
 *      Definitions for the generic XLog reading facility
 *
 * Portions Copyright (c) 2013, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *      src/include/access/xlogreader.h
 *
 * NOTES
 *      See the definition of the XLogReaderState struct for instructions on
 *      how to use the XLogReader infrastructure.
 *
 *      The basic idea is to allocate an XLogReaderState via
 *      XLogReaderAllocate(), and call XLogReadRecord() until it returns NULL.
 *-------------------------------------------------------------------------
 */
#ifndef XLOGREADER_H
#define XLOGREADER_H

#include "access/xlog_internal.h"

typedef struct XLogReaderState XLogReaderState;

/* Function type definition for the read_page callback */
typedef int (*XLogPageReadCB) (XLogReaderState *xlogreader,
                                           XLogRecPtr targetPagePtr,
                                           int reqLen,
                                           XLogRecPtr targetRecPtr,
                                           char *readBuf,
                                           TimeLineID *pageTLI);

struct XLogReaderState
{
    /* ----------------------------------------
     * Public parameters
     * ----------------------------------------
     */

    /*
     * Data input callback (mandatory).
     *
     * This callback shall read at least reqLen valid bytes of the xlog page
     * starting at targetPagePtr, and store them in readBuf.  The callback
     * shall return the number of bytes read (never more than XLOG_BLCKSZ), or
     * -1 on failure.  The callback shall sleep, if necessary, to wait for the
     * requested bytes to become available.  The callback will not be invoked
     * again for the same page unless more than the returned number of bytes
     * are needed.
     *
     * targetRecPtr is the position of the WAL record we're reading.  Usually
     * it is equal to targetPagePtr + reqLen, but sometimes xlogreader needs
     * to read and verify the page or segment header, before it reads the
     * actual WAL record it's interested in.  In that case, targetRecPtr can
     * be used to determine which timeline to read the page from.
     *
     * The callback shall set *pageTLI to the TLI of the file the page was
     * read from.  It is currently used only for error reporting purposes, to
     * reconstruct the name of the WAL file where an error occurred.
     */
    XLogPageReadCB read_page;

    /*
     * System identifier of the xlog files we're about to read.  Set to zero
     * (the default value) if unknown or unimportant.
     */
    uint64      system_identifier;

    /*
     * Opaque data for callbacks to use.  Not used by XLogReader.
     */
    void       *private_data;

    /*
     * Start and end point of last record read.  EndRecPtr is also used as the
     * position to read next, if XLogReadRecord receives an invalid recptr.
     */
    XLogRecPtr  ReadRecPtr;     /* start of last record read */
    XLogRecPtr  EndRecPtr;      /* end+1 of last record read */

    /* ----------------------------------------
     * private/internal state
     * ----------------------------------------
     */

    /* Buffer for currently read page (XLOG_BLCKSZ bytes) */
    char       *readBuf;

    /* last read segment, segment offset, read length, TLI */
    XLogSegNo   readSegNo;
    uint32      readOff;
    uint32      readLen;
    TimeLineID  readPageTLI;

    /* beginning of last page read, and its TLI  */
    XLogRecPtr  latestPagePtr;
    TimeLineID  latestPageTLI;

    /* beginning of the WAL record being read. */
    XLogRecPtr  currRecPtr;

    /* Buffer for current ReadRecord result (expandable) */
    char       *readRecordBuf;
    uint32      readRecordBufSize;

    /* Buffer to hold error message */
    char       *errormsg_buf;
};

/* Get a new XLogReader */
extern XLogReaderState *XLogReaderAllocate(XLogPageReadCB pagereadfunc,
                   void *private_data);

/* Free an XLogReader */
extern void XLogReaderFree(XLogReaderState *state);

/* Read the next XLog record. Returns NULL on end-of-WAL or failure */
extern struct XLogRecord *XLogReadRecord(XLogReaderState *state,
               XLogRecPtr recptr, char **errormsg);

#ifdef FRONTEND
extern XLogRecPtr XLogFindNextRecord(XLogReaderState *state, XLogRecPtr RecPtr);
#endif   /* FRONTEND */

#endif   /* XLOGREADER_H */