parsexlog.c

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/*-------------------------------------------------------------------------
 *
 * parsexlog.c
 *    Functions for reading Write-Ahead-Log
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres_fe.h"
 
#include <unistd.h>
 
#include "access/rmgr.h"
#include "access/xact.h"
#include "access/xlog_internal.h"
#include "access/xlogreader.h"
#include "catalog/pg_control.h"
#include "catalog/storage_xlog.h"
#include "commands/dbcommands_xlog.h"
#include "fe_utils/archive.h"
#include "filemap.h"
#include "pg_rewind.h"
 
/*
 * RmgrNames is an array of the built-in resource manager names, to make error
 * messages a bit nicer.
 */
#define PG_RMGR(symname,name,redo,desc,identify,startup,cleanup,mask,decode) \
  name,
 
static const char *const RmgrNames[RM_MAX_ID + 1] = {
#include "access/rmgrlist.h"
};
 
#define RmgrName(rmid) (((rmid) <= RM_MAX_BUILTIN_ID) ? \
                        RmgrNames[rmid] : "custom")
 
static void extractPageInfo(XLogReaderState *record);
 
static int  xlogreadfd = -1;
static XLogSegNo xlogreadsegno = 0;
static char xlogfpath[MAXPGPATH];
 
typedef struct XLogPageReadPrivate
{
    const char *restoreCommand;
    int         tliIndex;
} XLogPageReadPrivate;
 
static int  SimpleXLogPageRead(XLogReaderState *xlogreader,
                               XLogRecPtr targetPagePtr,
                               int reqLen, XLogRecPtr targetRecPtr, char *readBuf);
 
/*
 * Read WAL from the datadir/pg_wal, starting from 'startpoint' on timeline
 * index 'tliIndex' in target timeline history, until 'endpoint'. Make note of
 * the data blocks touched by the WAL records, and return them in a page map.
 *
 * 'endpoint' is the end of the last record to read. The record starting at
 * 'endpoint' is the first one that is not read.
 */
void
extractPageMap(const char *datadir, XLogRecPtr startpoint, int tliIndex,
               XLogRecPtr endpoint, const char *restoreCommand)
{
    XLogRecord *record;
    XLogReaderState *xlogreader;
    char       *errormsg;
    XLogPageReadPrivate private;
 
    private.tliIndex = tliIndex;
    private.restoreCommand = restoreCommand;
    xlogreader = XLogReaderAllocate(WalSegSz, datadir,
                                    XL_ROUTINE(.page_read = &SimpleXLogPageRead),
                                    &private);
    if (xlogreader == NULL)
        pg_fatal("out of memory while allocating a WAL reading processor");
 
    XLogBeginRead(xlogreader, startpoint);
    do
    {
        record = XLogReadRecord(xlogreader, &errormsg);
 
        if (record == NULL)
        {
            XLogRecPtr  errptr = xlogreader->EndRecPtr;
 
            if (errormsg)
                pg_fatal("could not read WAL record at %X/%X: %s",
                         LSN_FORMAT_ARGS(errptr),
                         errormsg);
            else
                pg_fatal("could not read WAL record at %X/%X",
                         LSN_FORMAT_ARGS(errptr));
        }
 
        extractPageInfo(xlogreader);
    } while (xlogreader->EndRecPtr < endpoint);
 
    /*
     * If 'endpoint' didn't point exactly at a record boundary, the caller
     * messed up.
     */
    if (xlogreader->EndRecPtr != endpoint)
        pg_fatal("end pointer %X/%X is not a valid end point; expected %X/%X",
                 LSN_FORMAT_ARGS(endpoint), LSN_FORMAT_ARGS(xlogreader->EndRecPtr));
 
    XLogReaderFree(xlogreader);
    if (xlogreadfd != -1)
    {
        close(xlogreadfd);
        xlogreadfd = -1;
    }
}
 
/*
 * Reads one WAL record. Returns the end position of the record, without
 * doing anything with the record itself.
 */
XLogRecPtr
readOneRecord(const char *datadir, XLogRecPtr ptr, int tliIndex,
              const char *restoreCommand)
{
    XLogRecord *record;
    XLogReaderState *xlogreader;
    char       *errormsg;
    XLogPageReadPrivate private;
    XLogRecPtr  endptr;
 
    private.tliIndex = tliIndex;
    private.restoreCommand = restoreCommand;
    xlogreader = XLogReaderAllocate(WalSegSz, datadir,
                                    XL_ROUTINE(.page_read = &SimpleXLogPageRead),
                                    &private);
    if (xlogreader == NULL)
        pg_fatal("out of memory while allocating a WAL reading processor");
 
    XLogBeginRead(xlogreader, ptr);
    record = XLogReadRecord(xlogreader, &errormsg);
    if (record == NULL)
    {
        if (errormsg)
            pg_fatal("could not read WAL record at %X/%X: %s",
                     LSN_FORMAT_ARGS(ptr), errormsg);
        else
            pg_fatal("could not read WAL record at %X/%X",
                     LSN_FORMAT_ARGS(ptr));
    }
    endptr = xlogreader->EndRecPtr;
 
    XLogReaderFree(xlogreader);
    if (xlogreadfd != -1)
    {
        close(xlogreadfd);
        xlogreadfd = -1;
    }
 
    return endptr;
}
 
/*
 * Find the previous checkpoint preceding given WAL location.
 */
void
findLastCheckpoint(const char *datadir, XLogRecPtr forkptr, int tliIndex,
                   XLogRecPtr *lastchkptrec, TimeLineID *lastchkpttli,
                   XLogRecPtr *lastchkptredo, const char *restoreCommand)
{
    /* Walk backwards, starting from the given record */
    XLogRecord *record;
    XLogRecPtr  searchptr;
    XLogReaderState *xlogreader;
    char       *errormsg;
    XLogPageReadPrivate private;
    XLogSegNo   current_segno = 0;
    TimeLineID  current_tli = 0;
 
    /*
     * The given fork pointer points to the end of the last common record,
     * which is not necessarily the beginning of the next record, if the
     * previous record happens to end at a page boundary. Skip over the page
     * header in that case to find the next record.
     */
    if (forkptr % XLOG_BLCKSZ == 0)
    {
        if (XLogSegmentOffset(forkptr, WalSegSz) == 0)
            forkptr += SizeOfXLogLongPHD;
        else
            forkptr += SizeOfXLogShortPHD;
    }
 
    private.tliIndex = tliIndex;
    private.restoreCommand = restoreCommand;
    xlogreader = XLogReaderAllocate(WalSegSz, datadir,
                                    XL_ROUTINE(.page_read = &SimpleXLogPageRead),
                                    &private);
    if (xlogreader == NULL)
        pg_fatal("out of memory while allocating a WAL reading processor");
 
    searchptr = forkptr;
    for (;;)
    {
        uint8       info;
 
        XLogBeginRead(xlogreader, searchptr);
        record = XLogReadRecord(xlogreader, &errormsg);
 
        if (record == NULL)
        {
            if (errormsg)
                pg_fatal("could not find previous WAL record at %X/%X: %s",
                         LSN_FORMAT_ARGS(searchptr),
                         errormsg);
            else
                pg_fatal("could not find previous WAL record at %X/%X",
                         LSN_FORMAT_ARGS(searchptr));
        }
 
        /* Detect if a new WAL file has been opened */
        if (xlogreader->seg.ws_tli != current_tli ||
            xlogreader->seg.ws_segno != current_segno)
        {
            char        xlogfname[MAXFNAMELEN];
 
            snprintf(xlogfname, MAXFNAMELEN, XLOGDIR "/");
 
            /* update current values */
            current_tli = xlogreader->seg.ws_tli;
            current_segno = xlogreader->seg.ws_segno;
 
            XLogFileName(xlogfname + sizeof(XLOGDIR),
                         current_tli, current_segno, WalSegSz);
 
            /* Track this filename as one to not remove */
            keepwal_add_entry(xlogfname);
        }
 
        /*
         * Check if it is a checkpoint record. This checkpoint record needs to
         * be the latest checkpoint before WAL forked and not the checkpoint
         * where the primary has been stopped to be rewound.
         */
        info = XLogRecGetInfo(xlogreader) & ~XLR_INFO_MASK;
        if (searchptr < forkptr &&
            XLogRecGetRmid(xlogreader) == RM_XLOG_ID &&
            (info == XLOG_CHECKPOINT_SHUTDOWN ||
             info == XLOG_CHECKPOINT_ONLINE))
        {
            CheckPoint  checkPoint;
 
            memcpy(&checkPoint, XLogRecGetData(xlogreader), sizeof(CheckPoint));
            *lastchkptrec = searchptr;
            *lastchkpttli = checkPoint.ThisTimeLineID;
            *lastchkptredo = checkPoint.redo;
            break;
        }
 
        /* Walk backwards to previous record. */
        searchptr = record->xl_prev;
    }
 
    XLogReaderFree(xlogreader);
    if (xlogreadfd != -1)
    {
        close(xlogreadfd);
        xlogreadfd = -1;
    }
}
 
/* XLogReader callback function, to read a WAL page */
static int
SimpleXLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr,
                   int reqLen, XLogRecPtr targetRecPtr, char *readBuf)
{
    XLogPageReadPrivate *private = (XLogPageReadPrivate *) xlogreader->private_data;
    uint32      targetPageOff;
    XLogRecPtr  targetSegEnd;
    XLogSegNo   targetSegNo;
    int         r;
 
    XLByteToSeg(targetPagePtr, targetSegNo, WalSegSz);
    XLogSegNoOffsetToRecPtr(targetSegNo + 1, 0, WalSegSz, targetSegEnd);
    targetPageOff = XLogSegmentOffset(targetPagePtr, WalSegSz);
 
    /*
     * See if we need to switch to a new segment because the requested record
     * is not in the currently open one.
     */
    if (xlogreadfd >= 0 &&
        !XLByteInSeg(targetPagePtr, xlogreadsegno, WalSegSz))
    {
        close(xlogreadfd);
        xlogreadfd = -1;
    }
 
    XLByteToSeg(targetPagePtr, xlogreadsegno, WalSegSz);
 
    if (xlogreadfd < 0)
    {
        char        xlogfname[MAXFNAMELEN];
 
        /*
         * Since incomplete segments are copied into next timelines, switch to
         * the timeline holding the required segment. Assuming this scan can
         * be done both forward and backward, consider also switching timeline
         * accordingly.
         */
        while (private->tliIndex < targetNentries - 1 &&
               targetHistory[private->tliIndex].end < targetSegEnd)
            private->tliIndex++;
        while (private->tliIndex > 0 &&
               targetHistory[private->tliIndex].begin >= targetSegEnd)
            private->tliIndex--;
 
        XLogFileName(xlogfname, targetHistory[private->tliIndex].tli,
                     xlogreadsegno, WalSegSz);
 
        snprintf(xlogfpath, MAXPGPATH, "%s/" XLOGDIR "/%s",
                 xlogreader->segcxt.ws_dir, xlogfname);
 
        xlogreadfd = open(xlogfpath, O_RDONLY | PG_BINARY, 0);
 
        if (xlogreadfd < 0)
        {
            /*
             * If we have no restore_command to execute, then exit.
             */
            if (private->restoreCommand == NULL)
            {
                pg_log_error("could not open file \"%s\": %m", xlogfpath);
                return -1;
            }
 
            /*
             * Since we have restore_command, then try to retrieve missing WAL
             * file from the archive.
             */
            xlogreadfd = RestoreArchivedFile(xlogreader->segcxt.ws_dir,
                                             xlogfname,
                                             WalSegSz,
                                             private->restoreCommand);
 
            if (xlogreadfd < 0)
                return -1;
            else
                pg_log_debug("using file \"%s\" restored from archive",
                             xlogfpath);
        }
    }
 
    /*
     * At this point, we have the right segment open.
     */
    Assert(xlogreadfd != -1);
 
    /* Read the requested page */
    if (lseek(xlogreadfd, (off_t) targetPageOff, SEEK_SET) < 0)
    {
        pg_log_error("could not seek in file \"%s\": %m", xlogfpath);
        return -1;
    }
 
 
    r = read(xlogreadfd, readBuf, XLOG_BLCKSZ);
    if (r != XLOG_BLCKSZ)
    {
        if (r < 0)
            pg_log_error("could not read file \"%s\": %m", xlogfpath);
        else
            pg_log_error("could not read file \"%s\": read %d of %zu",
                         xlogfpath, r, (Size) XLOG_BLCKSZ);
 
        return -1;
    }
 
    Assert(targetSegNo == xlogreadsegno);
 
    xlogreader->seg.ws_tli = targetHistory[private->tliIndex].tli;
    return XLOG_BLCKSZ;
}
 
/*
 * Extract information on which blocks the current record modifies.
 */
static void
extractPageInfo(XLogReaderState *record)
{
    int         block_id;
    RmgrId      rmid = XLogRecGetRmid(record);
    uint8       info = XLogRecGetInfo(record);
    uint8       rminfo = info & ~XLR_INFO_MASK;
 
    /* Is this a special record type that I recognize? */
 
    if (rmid == RM_DBASE_ID && rminfo == XLOG_DBASE_CREATE_FILE_COPY)
    {
        /*
         * New databases can be safely ignored. It won't be present in the
         * source system, so it will be deleted. There's one corner-case,
         * though: if a new, different, database is also created in the source
         * system, we'll see that the files already exist and not copy them.
         * That's OK, though; WAL replay of creating the new database, from
         * the source systems's WAL, will re-copy the new database,
         * overwriting the database created in the target system.
         */
    }
    else if (rmid == RM_DBASE_ID && rminfo == XLOG_DBASE_CREATE_WAL_LOG)
    {
        /*
         * New databases can be safely ignored. It won't be present in the
         * source system, so it will be deleted.
         */
    }
    else if (rmid == RM_DBASE_ID && rminfo == XLOG_DBASE_DROP)
    {
        /*
         * An existing database was dropped. We'll see that the files don't
         * exist in the target data dir, and copy them in toto from the source
         * system. No need to do anything special here.
         */
    }
    else if (rmid == RM_SMGR_ID && rminfo == XLOG_SMGR_CREATE)
    {
        /*
         * We can safely ignore these. The file will be removed from the
         * target, if it doesn't exist in source system. If a file with same
         * name is created in source system, too, there will be WAL records
         * for all the blocks in it.
         */
    }
    else if (rmid == RM_SMGR_ID && rminfo == XLOG_SMGR_TRUNCATE)
    {
        /*
         * We can safely ignore these. When we compare the sizes later on,
         * we'll notice that they differ, and copy the missing tail from
         * source system.
         */
    }
    else if (rmid == RM_XACT_ID &&
             ((rminfo & XLOG_XACT_OPMASK) == XLOG_XACT_COMMIT ||
              (rminfo & XLOG_XACT_OPMASK) == XLOG_XACT_COMMIT_PREPARED ||
              (rminfo & XLOG_XACT_OPMASK) == XLOG_XACT_ABORT ||
              (rminfo & XLOG_XACT_OPMASK) == XLOG_XACT_ABORT_PREPARED))
    {
        /*
         * These records can include "dropped rels". We can safely ignore
         * them, we will see that they are missing and copy them from the
         * source.
         */
    }
    else if (info & XLR_SPECIAL_REL_UPDATE)
    {
        /*
         * This record type modifies a relation file in some special way, but
         * we don't recognize the type. That's bad - we don't know how to
         * track that change.
         */
        pg_fatal("WAL record modifies a relation, but record type is not recognized: "
                 "lsn: %X/%X, rmid: %d, rmgr: %s, info: %02X",
                 LSN_FORMAT_ARGS(record->ReadRecPtr),
                 rmid, RmgrName(rmid), info);
    }
 
    for (block_id = 0; block_id <= XLogRecMaxBlockId(record); block_id++)
    {
        RelFileLocator rlocator;
        ForkNumber  forknum;
        BlockNumber blkno;
 
        if (!XLogRecGetBlockTagExtended(record, block_id,
                                        &rlocator, &forknum, &blkno, NULL))
            continue;
 
        /* We only care about the main fork; others are copied in toto */
        if (forknum != MAIN_FORKNUM)
            continue;
 
        process_target_wal_block_change(forknum, rlocator, blkno);
    }
}