xlogreader.h

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * xlogreader.h
 *      Definitions for the generic XLog reading facility
 *
 * Portions Copyright (c) 2013-2021, 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(), position the reader to the first record with
 *      XLogBeginRead() or XLogFindNextRecord(), and call XLogReadRecord()
 *      until it returns NULL.
 *
 *      Callers supply a page_read callback if they want to call
 *      XLogReadRecord or XLogFindNextRecord; it can be passed in as NULL
 *      otherwise.  The WALRead function can be used as a helper to write
 *      page_read callbacks, but it is not mandatory; callers that use it,
 *      must supply segment_open callbacks.  The segment_close callback
 *      must always be supplied.
 *
 *      After reading a record with XLogReadRecord(), it's decomposed into
 *      the per-block and main data parts, and the parts can be accessed
 *      with the XLogRec* macros and functions. You can also decode a
 *      record that's already constructed in memory, without reading from
 *      disk, by calling the DecodeXLogRecord() function.
 *-------------------------------------------------------------------------
 */
#ifndef XLOGREADER_H
#define XLOGREADER_H

#ifndef FRONTEND
#include "access/transam.h"
#endif

#include "access/xlogrecord.h"

/* WALOpenSegment represents a WAL segment being read. */
typedef struct WALOpenSegment
{
    int         ws_file;        /* segment file descriptor */
    XLogSegNo   ws_segno;       /* segment number */
    TimeLineID  ws_tli;         /* timeline ID of the currently open file */
} WALOpenSegment;

/* WALSegmentContext carries context information about WAL segments to read */
typedef struct WALSegmentContext
{
    char        ws_dir[MAXPGPATH];
    int         ws_segsize;
} WALSegmentContext;

typedef struct XLogReaderState XLogReaderState;

/* Function type definitions for various xlogreader interactions */
typedef int (*XLogPageReadCB) (XLogReaderState *xlogreader,
                               XLogRecPtr targetPagePtr,
                               int reqLen,
                               XLogRecPtr targetRecPtr,
                               char *readBuf);
typedef void (*WALSegmentOpenCB) (XLogReaderState *xlogreader,
                                  XLogSegNo nextSegNo,
                                  TimeLineID *tli_p);
typedef void (*WALSegmentCloseCB) (XLogReaderState *xlogreader);

typedef struct XLogReaderRoutine
{
    /*
     * Data input callback
     *
     * 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 ->seg.ws_tli to the TLI of the file the page was
     * read from.
     */
    XLogPageReadCB page_read;

    /*
     * Callback to open the specified WAL segment for reading.  ->seg.ws_file
     * shall be set to the file descriptor of the opened segment.  In case of
     * failure, an error shall be raised by the callback and it shall not
     * return.
     *
     * "nextSegNo" is the number of the segment to be opened.
     *
     * "tli_p" is an input/output argument. WALRead() 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.
     */
    WALSegmentOpenCB segment_open;

    /*
     * WAL segment close callback.  ->seg.ws_file shall be set to a negative
     * number.
     */
    WALSegmentCloseCB segment_close;
} XLogReaderRoutine;

#define XL_ROUTINE(...) &(XLogReaderRoutine){__VA_ARGS__}

typedef struct
{
    /* Is this block ref in use? */
    bool        in_use;

    /* Identify the block this refers to */
    RelFileNode rnode;
    ForkNumber  forknum;
    BlockNumber blkno;

    /* copy of the fork_flags field from the XLogRecordBlockHeader */
    uint8       flags;

    /* Information on full-page image, if any */
    bool        has_image;      /* has image, even for consistency checking */
    bool        apply_image;    /* has image that should be restored */
    char       *bkp_image;
    uint16      hole_offset;
    uint16      hole_length;
    uint16      bimg_len;
    uint8       bimg_info;

    /* Buffer holding the rmgr-specific data associated with this block */
    bool        has_data;
    char       *data;
    uint16      data_len;
    uint16      data_bufsz;
} DecodedBkpBlock;

struct XLogReaderState
{
    /*
     * Operational callbacks
     */
    XLogReaderRoutine routine;

    /* ----------------------------------------
     * Public parameters
     * ----------------------------------------
     */

    /*
     * 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.  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 */


    /* ----------------------------------------
     * Decoded representation of current record
     *
     * Use XLogRecGet* functions to investigate the record; these fields
     * should not be accessed directly.
     * ----------------------------------------
     */
    XLogRecord *decoded_record; /* currently decoded record */

    char       *main_data;      /* record's main data portion */
    uint32      main_data_len;  /* main data portion's length */
    uint32      main_data_bufsz;    /* allocated size of the buffer */

    RepOriginId record_origin;

    TransactionId toplevel_xid; /* XID of top-level transaction */

    /* information about blocks referenced by the record. */
    DecodedBkpBlock blocks[XLR_MAX_BLOCK_ID + 1];

    int         max_block_id;   /* highest block_id in use (-1 if none) */

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

    /*
     * Buffer for currently read page (XLOG_BLCKSZ bytes, valid up to at least
     * readLen bytes)
     */
    char       *readBuf;
    uint32      readLen;

    /* 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
     * correspond to what's in readBuf; used for timeline sanity checks.
     */
    XLogRecPtr  latestPagePtr;
    TimeLineID  latestPageTLI;

    /* beginning of the WAL record being read. */
    XLogRecPtr  currRecPtr;
    /* timeline to read it from, 0 if a lookup is required */
    TimeLineID  currTLI;

    /*
     * Safe point to read to in currTLI if current TLI is historical
     * (tliSwitchPoint) or InvalidXLogRecPtr if on current timeline.
     *
     * Actually set to the start of the segment containing the timeline switch
     * that ends currTLI's validity, not the LSN of the switch its self, since
     * we can't assume the old segment will be present.
     */
    XLogRecPtr  currTLIValidUntil;

    /*
     * If currTLI is not the most recent known timeline, the next timeline to
     * read from when currTLIValidUntil is reached.
     */
    TimeLineID  nextTLI;

    /*
     * Buffer for current ReadRecord result (expandable), used when a record
     * crosses a page boundary.
     */
    char       *readRecordBuf;
    uint32      readRecordBufSize;

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

/* Get a new XLogReader */
extern XLogReaderState *XLogReaderAllocate(int wal_segment_size,
                                           const char *waldir,
                                           XLogReaderRoutine *routine,
                                           void *private_data);
extern XLogReaderRoutine *LocalXLogReaderRoutine(void);

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

/* 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,
                                         char **errormsg);

/* Validate a page */
extern bool XLogReaderValidatePageHeader(XLogReaderState *state,
                                         XLogRecPtr recptr, char *phdr);

/*
 * 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(XLogReaderState *state,
                    char *buf, XLogRecPtr startptr, Size count,
                    TimeLineID tli, WALReadError *errinfo);

/* Functions for decoding an XLogRecord */

extern bool DecodeXLogRecord(XLogReaderState *state, XLogRecord *record,
                             char **errmsg);

#define XLogRecGetTotalLen(decoder) ((decoder)->decoded_record->xl_tot_len)
#define XLogRecGetPrev(decoder) ((decoder)->decoded_record->xl_prev)
#define XLogRecGetInfo(decoder) ((decoder)->decoded_record->xl_info)
#define XLogRecGetRmid(decoder) ((decoder)->decoded_record->xl_rmid)
#define XLogRecGetXid(decoder) ((decoder)->decoded_record->xl_xid)
#define XLogRecGetOrigin(decoder) ((decoder)->record_origin)
#define XLogRecGetTopXid(decoder) ((decoder)->toplevel_xid)
#define XLogRecGetData(decoder) ((decoder)->main_data)
#define XLogRecGetDataLen(decoder) ((decoder)->main_data_len)
#define XLogRecHasAnyBlockRefs(decoder) ((decoder)->max_block_id >= 0)
#define XLogRecHasBlockRef(decoder, block_id) \
    ((decoder)->blocks[block_id].in_use)
#define XLogRecHasBlockImage(decoder, block_id) \
    ((decoder)->blocks[block_id].has_image)
#define XLogRecBlockImageApply(decoder, block_id) \
    ((decoder)->blocks[block_id].apply_image)

#ifndef FRONTEND
extern FullTransactionId XLogRecGetFullXid(XLogReaderState *record);
#endif

extern bool RestoreBlockImage(XLogReaderState *record, uint8 block_id, char *page);
extern char *XLogRecGetBlockData(XLogReaderState *record, uint8 block_id, Size *len);
extern bool XLogRecGetBlockTag(XLogReaderState *record, uint8 block_id,
                               RelFileNode *rnode, ForkNumber *forknum,
                               BlockNumber *blknum);

#endif                          /* XLOGREADER_H */