pg_rewind.c

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/*-------------------------------------------------------------------------
 *
 * pg_rewind.c
 *    Synchronizes a PostgreSQL data directory to a new timeline
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 *
 *-------------------------------------------------------------------------
 */
#include "postgres_fe.h"

#include <sys/stat.h>
#include <fcntl.h>
#include <time.h>
#include <unistd.h>

#include "access/timeline.h"
#include "access/xlog_internal.h"
#include "catalog/catversion.h"
#include "catalog/pg_control.h"
#include "common/controldata_utils.h"
#include "common/file_perm.h"
#include "common/restricted_token.h"
#include "common/string.h"
#include "fe_utils/recovery_gen.h"
#include "file_ops.h"
#include "filemap.h"
#include "getopt_long.h"
#include "pg_rewind.h"
#include "rewind_source.h"
#include "storage/bufpage.h"

static void usage(const char *progname);

static void perform_rewind(filemap_t *filemap, rewind_source *source,
                           XLogRecPtr chkptrec,
                           TimeLineID chkpttli,
                           XLogRecPtr chkptredo);

static void createBackupLabel(XLogRecPtr startpoint, TimeLineID starttli,
                              XLogRecPtr checkpointloc);

static void digestControlFile(ControlFileData *ControlFile,
                              const char *content, size_t size);
static void getRestoreCommand(const char *argv0);
static void sanityChecks(void);
static void findCommonAncestorTimeline(XLogRecPtr *recptr, int *tliIndex);
static void ensureCleanShutdown(const char *argv0);
static void disconnect_atexit(void);

static ControlFileData ControlFile_target;
static ControlFileData ControlFile_source;
static ControlFileData ControlFile_source_after;

const char *progname;
int         WalSegSz;

/* Configuration options */
char       *datadir_target = NULL;
char       *datadir_source = NULL;
char       *connstr_source = NULL;
char       *restore_command = NULL;

static bool debug = false;
bool        showprogress = false;
bool        dry_run = false;
bool        do_sync = true;
bool        restore_wal = false;

/* Target history */
TimeLineHistoryEntry *targetHistory;
int         targetNentries;

/* Progress counters */
uint64      fetch_size;
uint64      fetch_done;

static PGconn *conn;
static rewind_source *source;

static void
usage(const char *progname)
{
    printf(_("%s resynchronizes a PostgreSQL cluster with another copy of the cluster.\n\n"), progname);
    printf(_("Usage:\n  %s [OPTION]...\n\n"), progname);
    printf(_("Options:\n"));
    printf(_("  -c, --restore-target-wal       use restore_command in target configuration to\n"
             "                                 retrieve WAL files from archives\n"));
    printf(_("  -D, --target-pgdata=DIRECTORY  existing data directory to modify\n"));
    printf(_("      --source-pgdata=DIRECTORY  source data directory to synchronize with\n"));
    printf(_("      --source-server=CONNSTR    source server to synchronize with\n"));
    printf(_("  -n, --dry-run                  stop before modifying anything\n"));
    printf(_("  -N, --no-sync                  do not wait for changes to be written\n"
             "                                 safely to disk\n"));
    printf(_("  -P, --progress                 write progress messages\n"));
    printf(_("  -R, --write-recovery-conf      write configuration for replication\n"
             "                                 (requires --source-server)\n"));
    printf(_("      --debug                    write a lot of debug messages\n"));
    printf(_("      --no-ensure-shutdown       do not automatically fix unclean shutdown\n"));
    printf(_("  -V, --version                  output version information, then exit\n"));
    printf(_("  -?, --help                     show this help, then exit\n"));
    printf(_("\nReport bugs to <%s>.\n"), PACKAGE_BUGREPORT);
    printf(_("%s home page: <%s>\n"), PACKAGE_NAME, PACKAGE_URL);
}


int
main(int argc, char **argv)
{
    static struct option long_options[] = {
        {"help", no_argument, NULL, '?'},
        {"target-pgdata", required_argument, NULL, 'D'},
        {"write-recovery-conf", no_argument, NULL, 'R'},
        {"source-pgdata", required_argument, NULL, 1},
        {"source-server", required_argument, NULL, 2},
        {"no-ensure-shutdown", no_argument, NULL, 4},
        {"version", no_argument, NULL, 'V'},
        {"restore-target-wal", no_argument, NULL, 'c'},
        {"dry-run", no_argument, NULL, 'n'},
        {"no-sync", no_argument, NULL, 'N'},
        {"progress", no_argument, NULL, 'P'},
        {"debug", no_argument, NULL, 3},
        {NULL, 0, NULL, 0}
    };
    int         option_index;
    int         c;
    XLogRecPtr  divergerec;
    int         lastcommontliIndex;
    XLogRecPtr  chkptrec;
    TimeLineID  chkpttli;
    XLogRecPtr  chkptredo;
    XLogRecPtr  target_wal_endrec;
    size_t      size;
    char       *buffer;
    bool        no_ensure_shutdown = false;
    bool        rewind_needed;
    bool        writerecoveryconf = false;
    filemap_t  *filemap;

    pg_logging_init(argv[0]);
    set_pglocale_pgservice(argv[0], PG_TEXTDOMAIN("pg_rewind"));
    progname = get_progname(argv[0]);

    /* Process command-line arguments */
    if (argc > 1)
    {
        if (strcmp(argv[1], "--help") == 0 || strcmp(argv[1], "-?") == 0)
        {
            usage(progname);
            exit(0);
        }
        if (strcmp(argv[1], "--version") == 0 || strcmp(argv[1], "-V") == 0)
        {
            puts("pg_rewind (PostgreSQL) " PG_VERSION);
            exit(0);
        }
    }

    while ((c = getopt_long(argc, argv, "cD:nNPR", long_options, &option_index)) != -1)
    {
        switch (c)
        {
            case '?':
                fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
                exit(1);

            case 'c':
                restore_wal = true;
                break;

            case 'P':
                showprogress = true;
                break;

            case 'n':
                dry_run = true;
                break;

            case 'N':
                do_sync = false;
                break;

            case 'R':
                writerecoveryconf = true;
                break;

            case 3:
                debug = true;
                pg_logging_increase_verbosity();
                break;

            case 'D':           /* -D or --target-pgdata */
                datadir_target = pg_strdup(optarg);
                break;

            case 1:             /* --source-pgdata */
                datadir_source = pg_strdup(optarg);
                break;

            case 2:             /* --source-server */
                connstr_source = pg_strdup(optarg);
                break;

            case 4:
                no_ensure_shutdown = true;
                break;
        }
    }

    if (datadir_source == NULL && connstr_source == NULL)
    {
        pg_log_error("no source specified (--source-pgdata or --source-server)");
        fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
        exit(1);
    }

    if (datadir_source != NULL && connstr_source != NULL)
    {
        pg_log_error("only one of --source-pgdata or --source-server can be specified");
        fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
        exit(1);
    }

    if (datadir_target == NULL)
    {
        pg_log_error("no target data directory specified (--target-pgdata)");
        fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
        exit(1);
    }

    if (writerecoveryconf && connstr_source == NULL)
    {
        pg_log_error("no source server information (--source-server) specified for --write-recovery-conf");
        fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
        exit(1);
    }

    if (optind < argc)
    {
        pg_log_error("too many command-line arguments (first is \"%s\")",
                     argv[optind]);
        fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
        exit(1);
    }

    /*
     * Don't allow pg_rewind to be run as root, to avoid overwriting the
     * ownership of files in the data directory. We need only check for root
     * -- any other user won't have sufficient permissions to modify files in
     * the data directory.
     */
#ifndef WIN32
    if (geteuid() == 0)
    {
        pg_log_error("cannot be executed by \"root\"");
        fprintf(stderr, _("You must run %s as the PostgreSQL superuser.\n"),
                progname);
        exit(1);
    }
#endif

    get_restricted_token();

    /* Set mask based on PGDATA permissions */
    if (!GetDataDirectoryCreatePerm(datadir_target))
    {
        pg_log_error("could not read permissions of directory \"%s\": %m",
                     datadir_target);
        exit(1);
    }

    umask(pg_mode_mask);

    getRestoreCommand(argv[0]);

    atexit(disconnect_atexit);

    /*
     * Ok, we have all the options and we're ready to start. First, connect to
     * remote server.
     */
    if (connstr_source)
    {
        conn = PQconnectdb(connstr_source);

        if (PQstatus(conn) == CONNECTION_BAD)
            pg_fatal("%s", PQerrorMessage(conn));

        if (showprogress)
            pg_log_info("connected to server");

        source = init_libpq_source(conn);
    }
    else
        source = init_local_source(datadir_source);

    /*
     * Check the status of the target instance.
     *
     * If the target instance was not cleanly shut down, start and stop the
     * target cluster once in single-user mode to enforce recovery to finish,
     * ensuring that the cluster can be used by pg_rewind.  Note that if
     * no_ensure_shutdown is specified, pg_rewind ignores this step, and users
     * need to make sure by themselves that the target cluster is in a clean
     * state.
     */
    buffer = slurpFile(datadir_target, "global/pg_control", &size);
    digestControlFile(&ControlFile_target, buffer, size);
    pg_free(buffer);

    if (!no_ensure_shutdown &&
        ControlFile_target.state != DB_SHUTDOWNED &&
        ControlFile_target.state != DB_SHUTDOWNED_IN_RECOVERY)
    {
        ensureCleanShutdown(argv[0]);

        buffer = slurpFile(datadir_target, "global/pg_control", &size);
        digestControlFile(&ControlFile_target, buffer, size);
        pg_free(buffer);
    }

    buffer = source->fetch_file(source, "global/pg_control", &size);
    digestControlFile(&ControlFile_source, buffer, size);
    pg_free(buffer);

    sanityChecks();

    /*
     * Find the common ancestor timeline between the clusters.
     *
     * If both clusters are already on the same timeline, there's nothing to
     * do.
     */
    if (ControlFile_target.checkPointCopy.ThisTimeLineID ==
        ControlFile_source.checkPointCopy.ThisTimeLineID)
    {
        pg_log_info("source and target cluster are on the same timeline");
        rewind_needed = false;
        target_wal_endrec = 0;
    }
    else
    {
        XLogRecPtr  chkptendrec;

        findCommonAncestorTimeline(&divergerec, &lastcommontliIndex);
        pg_log_info("servers diverged at WAL location %X/%X on timeline %u",
                    (uint32) (divergerec >> 32), (uint32) divergerec,
                    targetHistory[lastcommontliIndex].tli);

        /*
         * Determine the end-of-WAL on the target.
         *
         * The WAL ends at the last shutdown checkpoint, or at
         * minRecoveryPoint if it was a standby. (If we supported rewinding a
         * server that was not shut down cleanly, we would need to replay
         * until we reach the first invalid record, like crash recovery does.)
         */

        /* read the checkpoint record on the target to see where it ends. */
        chkptendrec = readOneRecord(datadir_target,
                                    ControlFile_target.checkPoint,
                                    targetNentries - 1,
                                    restore_command);

        if (ControlFile_target.minRecoveryPoint > chkptendrec)
        {
            target_wal_endrec = ControlFile_target.minRecoveryPoint;
        }
        else
        {
            target_wal_endrec = chkptendrec;
        }

        /*
         * Check for the possibility that the target is in fact a direct
         * ancestor of the source. In that case, there is no divergent history
         * in the target that needs rewinding.
         */
        if (target_wal_endrec > divergerec)
        {
            rewind_needed = true;
        }
        else
        {
            /* the last common checkpoint record must be part of target WAL */
            Assert(target_wal_endrec == divergerec);

            rewind_needed = false;
        }
    }

    if (!rewind_needed)
    {
        pg_log_info("no rewind required");
        if (writerecoveryconf && !dry_run)
            WriteRecoveryConfig(conn, datadir_target,
                                GenerateRecoveryConfig(conn, NULL));
        exit(0);
    }

    findLastCheckpoint(datadir_target, divergerec, lastcommontliIndex,
                       &chkptrec, &chkpttli, &chkptredo, restore_command);
    pg_log_info("rewinding from last common checkpoint at %X/%X on timeline %u",
                (uint32) (chkptrec >> 32), (uint32) chkptrec,
                chkpttli);

    /* Initialize the hash table to track the status of each file */
    filehash_init();

    /*
     * Collect information about all files in the both data directories.
     */
    if (showprogress)
        pg_log_info("reading source file list");
    source->traverse_files(source, &process_source_file);

    if (showprogress)
        pg_log_info("reading target file list");
    traverse_datadir(datadir_target, &process_target_file);

    /*
     * Read the target WAL from last checkpoint before the point of fork, to
     * extract all the pages that were modified on the target cluster after
     * the fork.
     */
    if (showprogress)
        pg_log_info("reading WAL in target");
    extractPageMap(datadir_target, chkptrec, lastcommontliIndex,
                   target_wal_endrec, restore_command);

    /*
     * We have collected all information we need from both systems. Decide
     * what to do with each file.
     */
    filemap = decide_file_actions();
    if (showprogress)
        calculate_totals(filemap);

    /* this is too verbose even for verbose mode */
    if (debug)
        print_filemap(filemap);

    /*
     * Ok, we're ready to start copying things over.
     */
    if (showprogress)
    {
        pg_log_info("need to copy %lu MB (total source directory size is %lu MB)",
                    (unsigned long) (filemap->fetch_size / (1024 * 1024)),
                    (unsigned long) (filemap->total_size / (1024 * 1024)));

        fetch_size = filemap->fetch_size;
        fetch_done = 0;
    }

    /*
     * We have now collected all the information we need from both systems,
     * and we are ready to start modifying the target directory.
     *
     * This is the point of no return. Once we start copying things, there is
     * no turning back!
     */
    perform_rewind(filemap, source, chkptrec, chkpttli, chkptredo);

    if (showprogress)
        pg_log_info("syncing target data directory");
    sync_target_dir();

    /* Also update the standby configuration, if requested. */
    if (writerecoveryconf && !dry_run)
        WriteRecoveryConfig(conn, datadir_target,
                            GenerateRecoveryConfig(conn, NULL));

    /* don't need the source connection anymore */
    source->destroy(source);
    if (conn)
    {
        PQfinish(conn);
        conn = NULL;
    }

    pg_log_info("Done!");

    return 0;
}

/*
 * Perform the rewind.
 *
 * We have already collected all the information we need from the
 * target and the source.
 */
static void
perform_rewind(filemap_t *filemap, rewind_source *source,
               XLogRecPtr chkptrec,
               TimeLineID chkpttli,
               XLogRecPtr chkptredo)
{
    XLogRecPtr  endrec;
    TimeLineID  endtli;
    ControlFileData ControlFile_new;
    size_t      size;
    char       *buffer;

    /*
     * Execute the actions in the file map, fetching data from the source
     * system as needed.
     */
    for (int i = 0; i < filemap->nentries; i++)
    {
        file_entry_t *entry = filemap->entries[i];

        /*
         * If this is a relation file, copy the modified blocks.
         *
         * This is in addition to any other changes.
         */
        if (entry->target_pages_to_overwrite.bitmapsize > 0)
        {
            datapagemap_iterator_t *iter;
            BlockNumber blkno;
            off_t       offset;

            iter = datapagemap_iterate(&entry->target_pages_to_overwrite);
            while (datapagemap_next(iter, &blkno))
            {
                offset = blkno * BLCKSZ;
                source->queue_fetch_range(source, entry->path, offset, BLCKSZ);
            }
            pg_free(iter);
        }

        switch (entry->action)
        {
            case FILE_ACTION_NONE:
                /* nothing else to do */
                break;

            case FILE_ACTION_COPY:
                /* Truncate the old file out of the way, if any */
                open_target_file(entry->path, true);
                source->queue_fetch_range(source, entry->path,
                                          0, entry->source_size);
                break;

            case FILE_ACTION_TRUNCATE:
                truncate_target_file(entry->path, entry->source_size);
                break;

            case FILE_ACTION_COPY_TAIL:
                source->queue_fetch_range(source, entry->path,
                                          entry->target_size,
                                          entry->source_size - entry->target_size);
                break;

            case FILE_ACTION_REMOVE:
                remove_target(entry);
                break;

            case FILE_ACTION_CREATE:
                create_target(entry);
                break;

            case FILE_ACTION_UNDECIDED:
                pg_fatal("no action decided for \"%s\"", entry->path);
                break;
        }
    }

    /* Complete any remaining range-fetches that we queued up above. */
    source->finish_fetch(source);

    close_target_file();

    progress_report(true);

    /*
     * Fetch the control file from the source last. This ensures that the
     * minRecoveryPoint is up-to-date.
     */
    buffer = source->fetch_file(source, "global/pg_control", &size);
    digestControlFile(&ControlFile_source_after, buffer, size);
    pg_free(buffer);

    /*
     * Sanity check: If the source is a local system, the control file should
     * not have changed since we started.
     *
     * XXX: We assume it hasn't been modified, but actually, what could go
     * wrong? The logic handles a libpq source that's modified concurrently,
     * why not a local datadir?
     */
    if (datadir_source &&
        memcmp(&ControlFile_source, &ControlFile_source_after,
               sizeof(ControlFileData)) != 0)
    {
        pg_fatal("source system was modified while pg_rewind was running");
    }

    if (showprogress)
        pg_log_info("creating backup label and updating control file");

    /*
     * Create a backup label file, to tell the target where to begin the WAL
     * replay. Normally, from the last common checkpoint between the source
     * and the target. But if the source is a standby server, it's possible
     * that the last common checkpoint is *after* the standby's restartpoint.
     * That implies that the source server has applied the checkpoint record,
     * but hasn't perfomed a corresponding restartpoint yet. Make sure we
     * start at the restartpoint's redo point in that case.
     *
     * Use the old version of the source's control file for this. The server
     * might have finished the restartpoint after we started copying files,
     * but we must begin from the redo point at the time that started copying.
     */
    if (ControlFile_source.checkPointCopy.redo < chkptredo)
    {
        chkptredo = ControlFile_source.checkPointCopy.redo;
        chkpttli = ControlFile_source.checkPointCopy.ThisTimeLineID;
        chkptrec = ControlFile_source.checkPoint;
    }
    createBackupLabel(chkptredo, chkpttli, chkptrec);

    /*
     * Update control file of target, to tell the target how far it must
     * replay the WAL (minRecoveryPoint).
     */
    if (connstr_source)
    {
        /*
         * The source is a live server. Like in an online backup, it's
         * important that we recover all the WAL that was generated while we
         * were copying files.
         */
        if (ControlFile_source_after.state == DB_IN_ARCHIVE_RECOVERY)
        {
            /*
             * Source is a standby server. We must replay to its
             * minRecoveryPoint.
             */
            endrec = ControlFile_source_after.minRecoveryPoint;
            endtli = ControlFile_source_after.minRecoveryPointTLI;
        }
        else
        {
            /*
             * Source is a production, non-standby, server. We must replay to
             * the last WAL insert location.
             */
            if (ControlFile_source_after.state != DB_IN_PRODUCTION)
                pg_fatal("source system was in unexpected state at end of rewind");

            endrec = source->get_current_wal_insert_lsn(source);
            endtli = ControlFile_source_after.checkPointCopy.ThisTimeLineID;
        }
    }
    else
    {
        /*
         * Source is a local data directory. It should've shut down cleanly,
         * and we must replay to the latest shutdown checkpoint.
         */
        endrec = ControlFile_source_after.checkPoint;
        endtli = ControlFile_source_after.checkPointCopy.ThisTimeLineID;
    }

    memcpy(&ControlFile_new, &ControlFile_source_after, sizeof(ControlFileData));
    ControlFile_new.minRecoveryPoint = endrec;
    ControlFile_new.minRecoveryPointTLI = endtli;
    ControlFile_new.state = DB_IN_ARCHIVE_RECOVERY;
    if (!dry_run)
        update_controlfile(datadir_target, &ControlFile_new, do_sync);
}

static void
sanityChecks(void)
{
    /* TODO Check that there's no backup_label in either cluster */

    /* Check system_identifier match */
    if (ControlFile_target.system_identifier != ControlFile_source.system_identifier)
        pg_fatal("source and target clusters are from different systems");

    /* check version */
    if (ControlFile_target.pg_control_version != PG_CONTROL_VERSION ||
        ControlFile_source.pg_control_version != PG_CONTROL_VERSION ||
        ControlFile_target.catalog_version_no != CATALOG_VERSION_NO ||
        ControlFile_source.catalog_version_no != CATALOG_VERSION_NO)
    {
        pg_fatal("clusters are not compatible with this version of pg_rewind");
    }

    /*
     * Target cluster need to use checksums or hint bit wal-logging, this to
     * prevent from data corruption that could occur because of hint bits.
     */
    if (ControlFile_target.data_checksum_version != PG_DATA_CHECKSUM_VERSION &&
        !ControlFile_target.wal_log_hints)
    {
        pg_fatal("target server needs to use either data checksums or \"wal_log_hints = on\"");
    }

    /*
     * Target cluster better not be running. This doesn't guard against
     * someone starting the cluster concurrently. Also, this is probably more
     * strict than necessary; it's OK if the target node was not shut down
     * cleanly, as long as it isn't running at the moment.
     */
    if (ControlFile_target.state != DB_SHUTDOWNED &&
        ControlFile_target.state != DB_SHUTDOWNED_IN_RECOVERY)
        pg_fatal("target server must be shut down cleanly");

    /*
     * When the source is a data directory, also require that the source
     * server is shut down. There isn't any very strong reason for this
     * limitation, but better safe than sorry.
     */
    if (datadir_source &&
        ControlFile_source.state != DB_SHUTDOWNED &&
        ControlFile_source.state != DB_SHUTDOWNED_IN_RECOVERY)
        pg_fatal("source data directory must be shut down cleanly");
}

/*
 * Print a progress report based on the fetch_size and fetch_done variables.
 *
 * Progress report is written at maximum once per second, except that the
 * last progress report is always printed.
 *
 * If finished is set to true, this is the last progress report. The cursor
 * is moved to the next line.
 */
void
progress_report(bool finished)
{
    static pg_time_t last_progress_report = 0;
    int         percent;
    char        fetch_done_str[32];
    char        fetch_size_str[32];
    pg_time_t   now;

    if (!showprogress)
        return;

    now = time(NULL);
    if (now == last_progress_report && !finished)
        return;                 /* Max once per second */

    last_progress_report = now;
    percent = fetch_size ? (int) ((fetch_done) * 100 / fetch_size) : 0;

    /*
     * Avoid overflowing past 100% or the full size. This may make the total
     * size number change as we approach the end of the backup (the estimate
     * will always be wrong if WAL is included), but that's better than having
     * the done column be bigger than the total.
     */
    if (percent > 100)
        percent = 100;
    if (fetch_done > fetch_size)
        fetch_size = fetch_done;

    /*
     * Separate step to keep platform-dependent format code out of
     * translatable strings.  And we only test for INT64_FORMAT availability
     * in snprintf, not fprintf.
     */
    snprintf(fetch_done_str, sizeof(fetch_done_str), INT64_FORMAT,
             fetch_done / 1024);
    snprintf(fetch_size_str, sizeof(fetch_size_str), INT64_FORMAT,
             fetch_size / 1024);

    fprintf(stderr, _("%*s/%s kB (%d%%) copied"),
            (int) strlen(fetch_size_str), fetch_done_str, fetch_size_str,
            percent);

    /*
     * Stay on the same line if reporting to a terminal and we're not done
     * yet.
     */
    fputc((!finished && isatty(fileno(stderr))) ? '\r' : '\n', stderr);
}

/*
 * Find minimum from two WAL locations assuming InvalidXLogRecPtr means
 * infinity as src/include/access/timeline.h states. This routine should
 * be used only when comparing WAL locations related to history files.
 */
static XLogRecPtr
MinXLogRecPtr(XLogRecPtr a, XLogRecPtr b)
{
    if (XLogRecPtrIsInvalid(a))
        return b;
    else if (XLogRecPtrIsInvalid(b))
        return a;
    else
        return Min(a, b);
}

/*
 * Retrieve timeline history for given control file which should behold
 * either source or target.
 */
static TimeLineHistoryEntry *
getTimelineHistory(ControlFileData *controlFile, int *nentries)
{
    TimeLineHistoryEntry *history;
    TimeLineID  tli;

    tli = controlFile->checkPointCopy.ThisTimeLineID;

    /*
     * Timeline 1 does not have a history file, so there is no need to check
     * and fake an entry with infinite start and end positions.
     */
    if (tli == 1)
    {
        history = (TimeLineHistoryEntry *) pg_malloc(sizeof(TimeLineHistoryEntry));
        history->tli = tli;
        history->begin = history->end = InvalidXLogRecPtr;
        *nentries = 1;
    }
    else
    {
        char        path[MAXPGPATH];
        char       *histfile;

        TLHistoryFilePath(path, tli);

        /* Get history file from appropriate source */
        if (controlFile == &ControlFile_source)
            histfile = source->fetch_file(source, path, NULL);
        else if (controlFile == &ControlFile_target)
            histfile = slurpFile(datadir_target, path, NULL);
        else
            pg_fatal("invalid control file");

        history = rewind_parseTimeLineHistory(histfile, tli, nentries);
        pg_free(histfile);
    }

    if (debug)
    {
        int         i;

        if (controlFile == &ControlFile_source)
            pg_log_debug("Source timeline history:");
        else if (controlFile == &ControlFile_target)
            pg_log_debug("Target timeline history:");
        else
            Assert(false);

        /*
         * Print the target timeline history.
         */
        for (i = 0; i < targetNentries; i++)
        {
            TimeLineHistoryEntry *entry;

            entry = &history[i];
            pg_log_debug("%d: %X/%X - %X/%X", entry->tli,
                         (uint32) (entry->begin >> 32), (uint32) (entry->begin),
                         (uint32) (entry->end >> 32), (uint32) (entry->end));
        }
    }

    return history;
}

/*
 * Determine the TLI of the last common timeline in the timeline history of the
 * two clusters. targetHistory is filled with target timeline history and
 * targetNentries is number of items in targetHistory. *tliIndex is set to the
 * index of last common timeline in targetHistory array, and *recptr is set to
 * the position where the timeline history diverged (ie. the first WAL record
 * that's not the same in both clusters).
 *
 * Control files of both clusters must be read into ControlFile_target/source
 * before calling this routine.
 */
static void
findCommonAncestorTimeline(XLogRecPtr *recptr, int *tliIndex)
{
    TimeLineHistoryEntry *sourceHistory;
    int         sourceNentries;
    int         i,
                n;

    /* Retrieve timelines for both source and target */
    sourceHistory = getTimelineHistory(&ControlFile_source, &sourceNentries);
    targetHistory = getTimelineHistory(&ControlFile_target, &targetNentries);

    /*
     * Trace the history forward, until we hit the timeline diverge. It may
     * still be possible that the source and target nodes used the same
     * timeline number in their history but with different start position
     * depending on the history files that each node has fetched in previous
     * recovery processes. Hence check the start position of the new timeline
     * as well and move down by one extra timeline entry if they do not match.
     */
    n = Min(sourceNentries, targetNentries);
    for (i = 0; i < n; i++)
    {
        if (sourceHistory[i].tli != targetHistory[i].tli ||
            sourceHistory[i].begin != targetHistory[i].begin)
            break;
    }

    if (i > 0)
    {
        i--;
        *recptr = MinXLogRecPtr(sourceHistory[i].end, targetHistory[i].end);
        *tliIndex = i;

        pg_free(sourceHistory);
        return;
    }
    else
    {
        pg_fatal("could not find common ancestor of the source and target cluster's timelines");
    }
}


/*
 * Create a backup_label file that forces recovery to begin at the last common
 * checkpoint.
 */
static void
createBackupLabel(XLogRecPtr startpoint, TimeLineID starttli, XLogRecPtr checkpointloc)
{
    XLogSegNo   startsegno;
    time_t      stamp_time;
    char        strfbuf[128];
    char        xlogfilename[MAXFNAMELEN];
    struct tm  *tmp;
    char        buf[1000];
    int         len;

    XLByteToSeg(startpoint, startsegno, WalSegSz);
    XLogFileName(xlogfilename, starttli, startsegno, WalSegSz);

    /*
     * Construct backup label file
     */
    stamp_time = time(NULL);
    tmp = localtime(&stamp_time);
    strftime(strfbuf, sizeof(strfbuf), "%Y-%m-%d %H:%M:%S %Z", tmp);

    len = snprintf(buf, sizeof(buf),
                   "START WAL LOCATION: %X/%X (file %s)\n"
                   "CHECKPOINT LOCATION: %X/%X\n"
                   "BACKUP METHOD: pg_rewind\n"
                   "BACKUP FROM: standby\n"
                   "START TIME: %s\n",
    /* omit LABEL: line */
                   (uint32) (startpoint >> 32), (uint32) startpoint, xlogfilename,
                   (uint32) (checkpointloc >> 32), (uint32) checkpointloc,
                   strfbuf);
    if (len >= sizeof(buf))
        pg_fatal("backup label buffer too small");  /* shouldn't happen */

    /* TODO: move old file out of the way, if any. */
    open_target_file("backup_label", true); /* BACKUP_LABEL_FILE */
    write_target_range(buf, 0, len);
    close_target_file();
}

/*
 * Check CRC of control file
 */
static void
checkControlFile(ControlFileData *ControlFile)
{
    pg_crc32c   crc;

    /* Calculate CRC */
    INIT_CRC32C(crc);
    COMP_CRC32C(crc, (char *) ControlFile, offsetof(ControlFileData, crc));
    FIN_CRC32C(crc);

    /* And simply compare it */
    if (!EQ_CRC32C(crc, ControlFile->crc))
        pg_fatal("unexpected control file CRC");
}

/*
 * Verify control file contents in the buffer 'content', and copy it to
 * *ControlFile.
 */
static void
digestControlFile(ControlFileData *ControlFile, const char *content,
                  size_t size)
{
    if (size != PG_CONTROL_FILE_SIZE)
        pg_fatal("unexpected control file size %d, expected %d",
                 (int) size, PG_CONTROL_FILE_SIZE);

    memcpy(ControlFile, content, sizeof(ControlFileData));

    /* set and validate WalSegSz */
    WalSegSz = ControlFile->xlog_seg_size;

    if (!IsValidWalSegSize(WalSegSz))
        pg_fatal(ngettext("WAL segment size must be a power of two between 1 MB and 1 GB, but the control file specifies %d byte",
                          "WAL segment size must be a power of two between 1 MB and 1 GB, but the control file specifies %d bytes",
                          WalSegSz),
                 WalSegSz);

    /* Additional checks on control file */
    checkControlFile(ControlFile);
}

/*
 * Get value of GUC parameter restore_command from the target cluster.
 *
 * This uses a logic based on "postgres -C" to get the value from the
 * cluster.
 */
static void
getRestoreCommand(const char *argv0)
{
    int         rc;
    char        postgres_exec_path[MAXPGPATH],
                postgres_cmd[MAXPGPATH],
                cmd_output[MAXPGPATH];

    if (!restore_wal)
        return;

    /* find postgres executable */
    rc = find_other_exec(argv0, "postgres",
                         PG_BACKEND_VERSIONSTR,
                         postgres_exec_path);

    if (rc < 0)
    {
        char        full_path[MAXPGPATH];

        if (find_my_exec(argv0, full_path) < 0)
            strlcpy(full_path, progname, sizeof(full_path));

        if (rc == -1)
            pg_log_error("The program \"%s\" is needed by %s but was not found in the\n"
                         "same directory as \"%s\".\n"
                         "Check your installation.",
                         "postgres", progname, full_path);
        else
            pg_log_error("The program \"%s\" was found by \"%s\"\n"
                         "but was not the same version as %s.\n"
                         "Check your installation.",
                         "postgres", full_path, progname);
        exit(1);
    }

    /*
     * Build a command able to retrieve the value of GUC parameter
     * restore_command, if set.
     */
    snprintf(postgres_cmd, sizeof(postgres_cmd),
             "\"%s\" -D \"%s\" -C restore_command",
             postgres_exec_path, datadir_target);

    if (!pipe_read_line(postgres_cmd, cmd_output, sizeof(cmd_output)))
        exit(1);

    (void) pg_strip_crlf(cmd_output);

    if (strcmp(cmd_output, "") == 0)
        pg_fatal("restore_command is not set in the target cluster");

    restore_command = pg_strdup(cmd_output);

    pg_log_debug("using for rewind restore_command = \'%s\'",
                 restore_command);
}


/*
 * Ensure clean shutdown of target instance by launching single-user mode
 * postgres to do crash recovery.
 */
static void
ensureCleanShutdown(const char *argv0)
{
    int         ret;
#define MAXCMDLEN (2 * MAXPGPATH)
    char        exec_path[MAXPGPATH];
    char        cmd[MAXCMDLEN];

    /* locate postgres binary */
    if ((ret = find_other_exec(argv0, "postgres",
                               PG_BACKEND_VERSIONSTR,
                               exec_path)) < 0)
    {
        char        full_path[MAXPGPATH];

        if (find_my_exec(argv0, full_path) < 0)
            strlcpy(full_path, progname, sizeof(full_path));

        if (ret == -1)
            pg_fatal("The program \"%s\" is needed by %s but was not found in the\n"
                     "same directory as \"%s\".\n"
                     "Check your installation.",
                     "postgres", progname, full_path);
        else
            pg_fatal("The program \"%s\" was found by \"%s\"\n"
                     "but was not the same version as %s.\n"
                     "Check your installation.",
                     "postgres", full_path, progname);
    }

    pg_log_info("executing \"%s\" for target server to complete crash recovery",
                exec_path);

    /*
     * Skip processing if requested, but only after ensuring presence of
     * postgres.
     */
    if (dry_run)
        return;

    /*
     * Finally run postgres in single-user mode.  There is no need to use
     * fsync here.  This makes the recovery faster, and the target data folder
     * is synced at the end anyway.
     */
    snprintf(cmd, MAXCMDLEN, "\"%s\" --single -F -D \"%s\" template1 < \"%s\"",
             exec_path, datadir_target, DEVNULL);

    if (system(cmd) != 0)
    {
        pg_log_error("postgres single-user mode in target cluster failed");
        pg_fatal("Command was: %s", cmd);
    }
}

static void
disconnect_atexit(void)
{
    if (conn != NULL)
        PQfinish(conn);
}