xlogreader.h

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/*-------------------------------------------------------------------------
 *
 * 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"
#include "storage/buf.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;
typedef struct XLogFindNextRecordState XLogFindNextRecordState;

/* Function type definition for the segment cleanup callback */
typedef void (*WALSegmentCleanupCB) (XLogReaderState *xlogreader);

/* Function type definition for the open/close callbacks for WALRead() */
typedef void (*WALSegmentOpenCB) (XLogReaderState *xlogreader,
                                  XLogSegNo nextSegNo,
                                  TimeLineID *tli_p);
typedef void (*WALSegmentCloseCB) (XLogReaderState *xlogreader);

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

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

    /* Workspace for remembering last known buffer holding this block. */
    Buffer      recent_buffer;

    /* 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;

/* Return code from XLogReadRecord */
typedef enum XLogReadRecordResult
{
    XLREAD_SUCCESS,             /* record is successfully read */
    XLREAD_NEED_DATA,           /* need more data. see XLogReadRecord. */
    XLREAD_FULL,        /* cannot hold more data while reading ahead */
    XLREAD_FAIL                 /* failed during reading a record */
}           XLogReadRecordResult;

/*
 * internal state of XLogReadRecord
 *
 * XLogReadState runs a state machine while reading a record. Theses states
 * are not seen outside the function. Each state may repeat several times
 * exiting requesting caller for new data. See the comment of XLogReadRecrod
 * for details.
 */
typedef enum XLogReadRecordState
{
    XLREAD_NEXT_RECORD,
    XLREAD_TOT_LEN,
    XLREAD_FIRST_FRAGMENT,
    XLREAD_CONTINUATION
}           XLogReadRecordState;

/*
 * The decoded contents of a record.  This occupies a contiguous region of
 * memory, with main_data and blocks[n].data pointing to memory after the
 * members declared here.
 */
typedef struct DecodedXLogRecord
{
    /* Private member used for resource management. */
    size_t      size;           /* total size of decoded record */
    bool        oversized;      /* outside the regular decode buffer? */
    struct DecodedXLogRecord *next; /* decoded record queue  link */

    /* Public members. */
    XLogRecPtr  lsn;            /* location */
    XLogRecPtr  next_lsn;       /* location of next record */
    XLogRecord  header;         /* header */
    RepOriginId record_origin;
    TransactionId toplevel_xid; /* XID of top-level transaction */
    char       *main_data;      /* record's main data portion */
    uint32      main_data_len;  /* main data portion's length */
    int         max_block_id;   /* highest block_id in use (-1 if none) */
    DecodedBkpBlock blocks[FLEXIBLE_ARRAY_MEMBER];
} DecodedXLogRecord;

struct XLogReaderState
{
    /*
     * Operational callbacks
     */
    WALSegmentCleanupCB cleanup_cb;

    /* ----------------------------------------
     * 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;

    /*
     * 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.
     *
     * Start and end point of last record returned by XLogReadRecord().  These
     * are also available as record->lsn and record->next_lsn.
     */
    XLogRecPtr  ReadRecPtr;     /* start of last record read or being read */
    XLogRecPtr  EndRecPtr;      /* end+1 of last record read */

    /* ----------------------------------------
     * Communication with page reader
     * readBuf is XLOG_BLCKSZ bytes, valid up to at least reqLen bytes.
     * ----------------------------------------
     */
    /* variables the clients of xlogreader can examine */
    XLogRecPtr  readPagePtr;    /* page pointer to read */
    int32       reqLen;         /* bytes requested to the caller */
    char       *readBuf;        /* buffer to store data */
    bool        page_verified;  /* is the page header on the buffer verified? */
    bool        record_verified;/* is the current record header verified? */

    /* variables set by the client of xlogreader */
    int32       readLen;        /* actual bytes copied into readBuf by client,
                                 * which should be >= reqLen.  Client should
                                 * use XLogReaderSetInputData() to set. */

    /* ----------------------------------------
     * Decoded representation of current record
     *
     * Use XLogRecGet* functions to investigate the record; these fields
     * should not be accessed directly.
     * ----------------------------------------
     * Start and end point of the last record read and decoded by
     * XLogReadRecordInternal().  NextRecPtr is also used as the position to
     * decode next.  Calling XLogBeginRead() sets NextRecPtr and EndRecPtr to
     * the requested starting position.
     */
    XLogRecPtr  DecodeRecPtr;   /* start of last record decoded */
    XLogRecPtr  NextRecPtr;     /* end+1 of last record decoded */
    XLogRecPtr  PrevRecPtr;     /* start of previous record decoded */

    /* Last record returned by XLogReadRecord(). */
    DecodedXLogRecord *record;

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

    /*
     * Buffer for decoded records.  This is a circular buffer, though
     * individual records can't be split in the middle, so some space is often
     * wasted at the end.  Oversized records that don't fit in this space are
     * allocated separately.
     */
    char       *decode_buffer;
    size_t      decode_buffer_size;
    bool        free_decode_buffer;     /* need to free? */
    char       *decode_buffer_head;     /* write head */
    char       *decode_buffer_tail;     /* read head */

    /*
     * Queue of records that have been decoded.  This is a linked list that
     * usually consists of consecutive records in decode_buffer, but may also
     * contain oversized records allocated with palloc().
     */
    DecodedXLogRecord *decode_queue_head;   /* newest decoded record */
    DecodedXLogRecord *decode_queue_tail;   /* oldest decoded record */

    /* 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;

    /* 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;

    /*
     * XLogReadRecordInternal() state
     */
    XLogReadRecordState readRecordState;    /* state machine state */
    int         recordGotLen;   /* amount of current record that has already
                                 * been read */
    int         recordRemainLen;    /* length of current record that remains */
    XLogRecPtr  recordContRecPtr;   /* where the current record continues */

    DecodedXLogRecord *decoding;    /* record currently being decoded */

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

struct XLogFindNextRecordState
{
    XLogReaderState *reader_state;
    XLogRecPtr      targetRecPtr;
    XLogRecPtr      currRecPtr;
};

/* Report that data is available for decoding. */
static inline void
XLogReaderSetInputData(XLogReaderState *state, int32 len)
{
    state->readLen = len;
}

/* Get a new XLogReader */
extern XLogReaderState *XLogReaderAllocate(int wal_segment_size,
                                           const char *waldir,
                                           WALSegmentCleanupCB cleanup_cb);

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

/* Optionally provide a circular decoding buffer to allow readahead. */
extern void XLogReaderSetDecodeBuffer(XLogReaderState *state,
                                      void *buffer,
                                      size_t size);

/* Position the XLogReader to given record */
extern void XLogBeginRead(XLogReaderState *state, XLogRecPtr RecPtr);
#ifdef FRONTEND
extern XLogFindNextRecordState *InitXLogFindNextRecord(XLogReaderState *reader_state, XLogRecPtr start_ptr);
extern bool XLogFindNextRecord(XLogFindNextRecordState *state);
#endif                          /* FRONTEND */

/* Read the next record's header.  Returns NULL on end-of-WAL or failure. */
extern XLogReadRecordResult XLogReadRecord(XLogReaderState *state,
                                           XLogRecord **record,
                                           char **errormsg);

/* Read the next decoded record.  Returns NULL on end-of-WAL or failure. */
extern XLogReadRecordResult XLogNextRecord(XLogReaderState *state,
                                           DecodedXLogRecord **record,
                                           char **errormsg);

/* Try to read ahead, if there is space in the decoding buffer. */
extern XLogReadRecordResult XLogReadAhead(XLogReaderState *state,
                                          DecodedXLogRecord **record,
                                          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,
                    WALSegmentOpenCB segopenfn, WALSegmentCloseCB sgclosefn,
                    char *buf, XLogRecPtr startptr, Size count,
                    TimeLineID tli, WALReadError *errinfo);

/* Functions for decoding an XLogRecord */

extern size_t DecodeXLogRecordRequiredSpace(size_t xl_tot_len);
extern bool DecodeXLogRecord(XLogReaderState *state,
                             DecodedXLogRecord *decoded,
                             XLogRecord *record,
                             XLogRecPtr lsn,
                             char **errmsg);

#define XLogRecGetTotalLen(decoder) ((decoder)->record->header.xl_tot_len)
#define XLogRecGetPrev(decoder) ((decoder)->record->header.xl_prev)
#define XLogRecGetInfo(decoder) ((decoder)->record->header.xl_info)
#define XLogRecGetRmid(decoder) ((decoder)->record->header.xl_rmid)
#define XLogRecGetXid(decoder) ((decoder)->record->header.xl_xid)
#define XLogRecGetOrigin(decoder) ((decoder)->record->record_origin)
#define XLogRecGetTopXid(decoder) ((decoder)->record->toplevel_xid)
#define XLogRecGetData(decoder) ((decoder)->record->main_data)
#define XLogRecGetDataLen(decoder) ((decoder)->record->main_data_len)
#define XLogRecHasAnyBlockRefs(decoder) ((decoder)->record->max_block_id >= 0)
#define XLogRecMaxBlockId(decoder) ((decoder)->record->max_block_id)
#define XLogRecGetBlock(decoder, i) (&(decoder)->record->blocks[(i)])
#define XLogRecHasBlockRef(decoder, block_id) \
    ((decoder)->record->max_block_id >= (block_id) && \
     (decoder)->record->blocks[block_id].in_use)
#define XLogRecHasBlockImage(decoder, block_id) \
    ((decoder)->record->blocks[block_id].has_image)
#define XLogRecBlockImageApply(decoder, block_id) \
    ((decoder)->record->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);
extern bool XLogRecGetRecentBuffer(XLogReaderState *record, uint8 block_id,
                                   RelFileNode *rnode, ForkNumber *forknum,
                                   BlockNumber *blknum, Buffer *recent_buffer);

#endif                          /* XLOGREADER_H */