basebackup_incremental.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * basebackup_incremental.c
 *    code for incremental backup support
 *
 * This code isn't actually in charge of taking an incremental backup;
 * the actual construction of the incremental backup happens in
 * basebackup.c. Here, we're concerned with providing the necessary
 * supports for that operation. In particular, we need to parse the
 * backup manifest supplied by the user taking the incremental backup
 * and extract the required information from it.
 *
 * Portions Copyright (c) 2010-2025, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/backup/basebackup_incremental.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/timeline.h"
#include "access/xlog.h"
#include "backup/basebackup_incremental.h"
#include "backup/walsummary.h"
#include "common/blkreftable.h"
#include "common/hashfn.h"
#include "common/int.h"
#include "common/parse_manifest.h"
#include "postmaster/walsummarizer.h"
 
#define BLOCKS_PER_READ         512
 
/*
 * We expect to find the last lines of the manifest, including the checksum,
 * in the last MIN_CHUNK bytes of the manifest. We trigger an incremental
 * parse step if we are about to overflow MAX_CHUNK bytes.
 */
#define MIN_CHUNK  1024
#define MAX_CHUNK (128 *  1024)
 
/*
 * Details extracted from the WAL ranges present in the supplied backup manifest.
 */
typedef struct
{
    TimeLineID  tli;
    XLogRecPtr  start_lsn;
    XLogRecPtr  end_lsn;
} backup_wal_range;
 
/*
 * Details extracted from the file list present in the supplied backup manifest.
 */
typedef struct
{
    uint32      status;
    const char *path;
    uint64      size;
} backup_file_entry;
 
static uint32 hash_string_pointer(const char *s);
#define SH_PREFIX               backup_file
#define SH_ELEMENT_TYPE         backup_file_entry
#define SH_KEY_TYPE             const char *
#define SH_KEY                  path
#define SH_HASH_KEY(tb, key)    hash_string_pointer(key)
#define SH_EQUAL(tb, a, b)      (strcmp(a, b) == 0)
#define SH_SCOPE                static inline
#define SH_DECLARE
#define SH_DEFINE
#include "lib/simplehash.h"
 
struct IncrementalBackupInfo
{
    /* Memory context for this object and its subsidiary objects. */
    MemoryContext mcxt;
 
    /* Temporary buffer for storing the manifest while parsing it. */
    StringInfoData buf;
 
    /* WAL ranges extracted from the backup manifest. */
    List       *manifest_wal_ranges;
 
    /*
     * Files extracted from the backup manifest.
     *
     * We don't really need this information, because we use WAL summaries to
     * figure out what's changed. It would be unsafe to just rely on the list
     * of files that existed before, because it's possible for a file to be
     * removed and a new one created with the same name and different
     * contents. In such cases, the whole file must still be sent. We can tell
     * from the WAL summaries whether that happened, but not from the file
     * list.
     *
     * Nonetheless, this data is useful for sanity checking. If a file that we
     * think we shouldn't need to send is not present in the manifest for the
     * prior backup, something has gone terribly wrong. We retain the file
     * names and sizes, but not the checksums or last modified times, for
     * which we have no use.
     *
     * One significant downside of storing this data is that it consumes
     * memory. If that turns out to be a problem, we might have to decide not
     * to retain this information, or to make it optional.
     */
    backup_file_hash *manifest_files;
 
    /*
     * Block-reference table for the incremental backup.
     *
     * It's possible that storing the entire block-reference table in memory
     * will be a problem for some users. The in-memory format that we're using
     * here is pretty efficient, converging to little more than 1 bit per
     * block for relation forks with large numbers of modified blocks. It's
     * possible, however, that if you try to perform an incremental backup of
     * a database with a sufficiently large number of relations on a
     * sufficiently small machine, you could run out of memory here. If that
     * turns out to be a problem in practice, we'll need to be more clever.
     */
    BlockRefTable *brtab;
 
    /*
     * State object for incremental JSON parsing
     */
    JsonManifestParseIncrementalState *inc_state;
};
 
static void manifest_process_version(JsonManifestParseContext *context,
                                     int manifest_version);
static void manifest_process_system_identifier(JsonManifestParseContext *context,
                                               uint64 manifest_system_identifier);
static void manifest_process_file(JsonManifestParseContext *context,
                                  const char *pathname,
                                  uint64 size,
                                  pg_checksum_type checksum_type,
                                  int checksum_length,
                                  uint8 *checksum_payload);
static void manifest_process_wal_range(JsonManifestParseContext *context,
                                       TimeLineID tli,
                                       XLogRecPtr start_lsn,
                                       XLogRecPtr end_lsn);
pg_noreturn static void manifest_report_error(JsonManifestParseContext *context,
                                              const char *fmt,...)
            pg_attribute_printf(2, 3);
static int  compare_block_numbers(const void *a, const void *b);
 
/*
 * Create a new object for storing information extracted from the manifest
 * supplied when creating an incremental backup.
 */
IncrementalBackupInfo *
CreateIncrementalBackupInfo(MemoryContext mcxt)
{
    IncrementalBackupInfo *ib;
    MemoryContext oldcontext;
    JsonManifestParseContext *context;
 
    oldcontext = MemoryContextSwitchTo(mcxt);
 
    ib = palloc0(sizeof(IncrementalBackupInfo));
    ib->mcxt = mcxt;
    initStringInfo(&ib->buf);
 
    /*
     * It's hard to guess how many files a "typical" installation will have in
     * the data directory, but a fresh initdb creates almost 1000 files as of
     * this writing, so it seems to make sense for our estimate to
     * substantially higher.
     */
    ib->manifest_files = backup_file_create(mcxt, 10000, NULL);
 
    context = palloc0(sizeof(JsonManifestParseContext));
    /* Parse the manifest. */
    context->private_data = ib;
    context->version_cb = manifest_process_version;
    context->system_identifier_cb = manifest_process_system_identifier;
    context->per_file_cb = manifest_process_file;
    context->per_wal_range_cb = manifest_process_wal_range;
    context->error_cb = manifest_report_error;
 
    ib->inc_state = json_parse_manifest_incremental_init(context);
 
    MemoryContextSwitchTo(oldcontext);
 
    return ib;
}
 
/*
 * Before taking an incremental backup, the caller must supply the backup
 * manifest from a prior backup. Each chunk of manifest data received
 * from the client should be passed to this function.
 */
void
AppendIncrementalManifestData(IncrementalBackupInfo *ib, const char *data,
                              int len)
{
    MemoryContext oldcontext;
 
    /* Switch to our memory context. */
    oldcontext = MemoryContextSwitchTo(ib->mcxt);
 
    if (ib->buf.len > MIN_CHUNK && ib->buf.len + len > MAX_CHUNK)
    {
        /*
         * time for an incremental parse. We'll do all but the last MIN_CHUNK
         * so that we have enough left for the final piece.
         */
        json_parse_manifest_incremental_chunk(ib->inc_state, ib->buf.data,
                                              ib->buf.len - MIN_CHUNK, false);
        /* now remove what we just parsed  */
        memmove(ib->buf.data, ib->buf.data + (ib->buf.len - MIN_CHUNK),
                MIN_CHUNK + 1);
        ib->buf.len = MIN_CHUNK;
    }
 
    appendBinaryStringInfo(&ib->buf, data, len);
 
    /* Switch back to previous memory context. */
    MemoryContextSwitchTo(oldcontext);
}
 
/*
 * Finalize an IncrementalBackupInfo object after all manifest data has
 * been supplied via calls to AppendIncrementalManifestData.
 */
void
FinalizeIncrementalManifest(IncrementalBackupInfo *ib)
{
    MemoryContext oldcontext;
 
    /* Switch to our memory context. */
    oldcontext = MemoryContextSwitchTo(ib->mcxt);
 
    /* Parse the last chunk of the manifest */
    json_parse_manifest_incremental_chunk(ib->inc_state, ib->buf.data,
                                          ib->buf.len, true);
 
    /* Done with the buffer, so release memory. */
    pfree(ib->buf.data);
    ib->buf.data = NULL;
 
    /* Done with inc_state, so release that memory too */
    json_parse_manifest_incremental_shutdown(ib->inc_state);
 
    /* Switch back to previous memory context. */
    MemoryContextSwitchTo(oldcontext);
}
 
/*
 * Prepare to take an incremental backup.
 *
 * Before this function is called, AppendIncrementalManifestData and
 * FinalizeIncrementalManifest should have already been called to pass all
 * the manifest data to this object.
 *
 * This function performs sanity checks on the data extracted from the
 * manifest and figures out for which WAL ranges we need summaries, and
 * whether those summaries are available. Then, it reads and combines the
 * data from those summary files. It also updates the backup_state with the
 * reference TLI and LSN for the prior backup.
 */
void
PrepareForIncrementalBackup(IncrementalBackupInfo *ib,
                            BackupState *backup_state)
{
    MemoryContext oldcontext;
    List       *expectedTLEs;
    List       *all_wslist,
               *required_wslist = NIL;
    ListCell   *lc;
    TimeLineHistoryEntry **tlep;
    int         num_wal_ranges;
    int         i;
    bool        found_backup_start_tli = false;
    TimeLineID  earliest_wal_range_tli = 0;
    XLogRecPtr  earliest_wal_range_start_lsn = InvalidXLogRecPtr;
    TimeLineID  latest_wal_range_tli = 0;
 
    Assert(ib->buf.data == NULL);
 
    /* Switch to our memory context. */
    oldcontext = MemoryContextSwitchTo(ib->mcxt);
 
    /*
     * A valid backup manifest must always contain at least one WAL range
     * (usually exactly one, unless the backup spanned a timeline switch).
     */
    num_wal_ranges = list_length(ib->manifest_wal_ranges);
    if (num_wal_ranges == 0)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("manifest contains no required WAL ranges")));
 
    /*
     * Match up the TLIs that appear in the WAL ranges of the backup manifest
     * with those that appear in this server's timeline history. We expect
     * every backup_wal_range to match to a TimeLineHistoryEntry; if it does
     * not, that's an error.
     *
     * This loop also decides which of the WAL ranges is the manifest is most
     * ancient and which one is the newest, according to the timeline history
     * of this server, and stores TLIs of those WAL ranges into
     * earliest_wal_range_tli and latest_wal_range_tli. It also updates
     * earliest_wal_range_start_lsn to the start LSN of the WAL range for
     * earliest_wal_range_tli.
     *
     * Note that the return value of readTimeLineHistory puts the latest
     * timeline at the beginning of the list, not the end. Hence, the earliest
     * TLI is the one that occurs nearest the end of the list returned by
     * readTimeLineHistory, and the latest TLI is the one that occurs closest
     * to the beginning.
     */
    expectedTLEs = readTimeLineHistory(backup_state->starttli);
    tlep = palloc0(num_wal_ranges * sizeof(TimeLineHistoryEntry *));
    for (i = 0; i < num_wal_ranges; ++i)
    {
        backup_wal_range *range = list_nth(ib->manifest_wal_ranges, i);
        bool        saw_earliest_wal_range_tli = false;
        bool        saw_latest_wal_range_tli = false;
 
        /* Search this server's history for this WAL range's TLI. */
        foreach(lc, expectedTLEs)
        {
            TimeLineHistoryEntry *tle = lfirst(lc);
 
            if (tle->tli == range->tli)
            {
                tlep[i] = tle;
                break;
            }
 
            if (tle->tli == earliest_wal_range_tli)
                saw_earliest_wal_range_tli = true;
            if (tle->tli == latest_wal_range_tli)
                saw_latest_wal_range_tli = true;
        }
 
        /*
         * An incremental backup can only be taken relative to a backup that
         * represents a previous state of this server. If the backup requires
         * WAL from a timeline that's not in our history, that definitely
         * isn't the case.
         */
        if (tlep[i] == NULL)
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("timeline %u found in manifest, but not in this server's history",
                            range->tli)));
 
        /*
         * If we found this TLI in the server's history before encountering
         * the latest TLI seen so far in the server's history, then this TLI
         * is the latest one seen so far.
         *
         * If on the other hand we saw the earliest TLI seen so far before
         * finding this TLI, this TLI is earlier than the earliest one seen so
         * far. And if this is the first TLI for which we've searched, it's
         * also the earliest one seen so far.
         *
         * On the first loop iteration, both things should necessarily be
         * true.
         */
        if (!saw_latest_wal_range_tli)
            latest_wal_range_tli = range->tli;
        if (earliest_wal_range_tli == 0 || saw_earliest_wal_range_tli)
        {
            earliest_wal_range_tli = range->tli;
            earliest_wal_range_start_lsn = range->start_lsn;
        }
    }
 
    /*
     * Propagate information about the prior backup into the backup_label that
     * will be generated for this backup.
     */
    backup_state->istartpoint = earliest_wal_range_start_lsn;
    backup_state->istarttli = earliest_wal_range_tli;
 
    /*
     * Sanity check start and end LSNs for the WAL ranges in the manifest.
     *
     * Commonly, there won't be any timeline switches during the prior backup
     * at all, but if there are, they should happen at the same LSNs that this
     * server switched timelines.
     *
     * Whether there are any timeline switches during the prior backup or not,
     * the prior backup shouldn't require any WAL from a timeline prior to the
     * start of that timeline. It also shouldn't require any WAL from later
     * than the start of this backup.
     *
     * If any of these sanity checks fail, one possible explanation is that
     * the user has generated WAL on the same timeline with the same LSNs more
     * than once. For instance, if two standbys running on timeline 1 were
     * both promoted and (due to a broken archiving setup) both selected new
     * timeline ID 2, then it's possible that one of these checks might trip.
     *
     * Note that there are lots of ways for the user to do something very bad
     * without tripping any of these checks, and they are not intended to be
     * comprehensive. It's pretty hard to see how we could be certain of
     * anything here. However, if there's a problem staring us right in the
     * face, it's best to report it, so we do.
     */
    for (i = 0; i < num_wal_ranges; ++i)
    {
        backup_wal_range *range = list_nth(ib->manifest_wal_ranges, i);
 
        if (range->tli == earliest_wal_range_tli)
        {
            if (range->start_lsn < tlep[i]->begin)
                ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("manifest requires WAL from initial timeline %u starting at %X/%X, but that timeline begins at %X/%X",
                                range->tli,
                                LSN_FORMAT_ARGS(range->start_lsn),
                                LSN_FORMAT_ARGS(tlep[i]->begin))));
        }
        else
        {
            if (range->start_lsn != tlep[i]->begin)
                ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("manifest requires WAL from continuation timeline %u starting at %X/%X, but that timeline begins at %X/%X",
                                range->tli,
                                LSN_FORMAT_ARGS(range->start_lsn),
                                LSN_FORMAT_ARGS(tlep[i]->begin))));
        }
 
        if (range->tli == latest_wal_range_tli)
        {
            if (range->end_lsn > backup_state->startpoint)
                ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("manifest requires WAL from final timeline %u ending at %X/%X, but this backup starts at %X/%X",
                                range->tli,
                                LSN_FORMAT_ARGS(range->end_lsn),
                                LSN_FORMAT_ARGS(backup_state->startpoint)),
                         errhint("This can happen for incremental backups on a standby if there was little activity since the previous backup.")));
        }
        else
        {
            if (range->end_lsn != tlep[i]->end)
                ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("manifest requires WAL from non-final timeline %u ending at %X/%X, but this server switched timelines at %X/%X",
                                range->tli,
                                LSN_FORMAT_ARGS(range->end_lsn),
                                LSN_FORMAT_ARGS(tlep[i]->end))));
        }
 
    }
 
    /*
     * Wait for WAL summarization to catch up to the backup start LSN. This
     * will throw an error if the WAL summarizer appears to be stuck. If WAL
     * summarization gets disabled while we're waiting, this will return
     * immediately, and we'll error out further down if the WAL summaries are
     * incomplete.
     */
    WaitForWalSummarization(backup_state->startpoint);
 
    /*
     * Retrieve a list of all WAL summaries on any timeline that overlap with
     * the LSN range of interest. We could instead call GetWalSummaries() once
     * per timeline in the loop that follows, but that would involve reading
     * the directory multiple times. It should be mildly faster - and perhaps
     * a bit safer - to do it just once.
     */
    all_wslist = GetWalSummaries(0, earliest_wal_range_start_lsn,
                                 backup_state->startpoint);
 
    /*
     * We need WAL summaries for everything that happened during the prior
     * backup and everything that happened afterward up until the point where
     * the current backup started.
     */
    foreach(lc, expectedTLEs)
    {
        TimeLineHistoryEntry *tle = lfirst(lc);
        XLogRecPtr  tli_start_lsn = tle->begin;
        XLogRecPtr  tli_end_lsn = tle->end;
        XLogRecPtr  tli_missing_lsn = InvalidXLogRecPtr;
        List       *tli_wslist;
 
        /*
         * Working through the history of this server from the current
         * timeline backwards, we skip everything until we find the timeline
         * where this backup started. Most of the time, this means we won't
         * skip anything at all, as it's unlikely that the timeline has
         * changed since the beginning of the backup moments ago.
         */
        if (tle->tli == backup_state->starttli)
        {
            found_backup_start_tli = true;
            tli_end_lsn = backup_state->startpoint;
        }
        else if (!found_backup_start_tli)
            continue;
 
        /*
         * Find the summaries that overlap the LSN range of interest for this
         * timeline. If this is the earliest timeline involved, the range of
         * interest begins with the start LSN of the prior backup; otherwise,
         * it begins at the LSN at which this timeline came into existence. If
         * this is the latest TLI involved, the range of interest ends at the
         * start LSN of the current backup; otherwise, it ends at the point
         * where we switched from this timeline to the next one.
         */
        if (tle->tli == earliest_wal_range_tli)
            tli_start_lsn = earliest_wal_range_start_lsn;
        tli_wslist = FilterWalSummaries(all_wslist, tle->tli,
                                        tli_start_lsn, tli_end_lsn);
 
        /*
         * There is no guarantee that the WAL summaries we found cover the
         * entire range of LSNs for which summaries are required, or indeed
         * that we found any WAL summaries at all. Check whether we have a
         * problem of that sort.
         */
        if (!WalSummariesAreComplete(tli_wslist, tli_start_lsn, tli_end_lsn,
                                     &tli_missing_lsn))
        {
            if (XLogRecPtrIsInvalid(tli_missing_lsn))
                ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("WAL summaries are required on timeline %u from %X/%X to %X/%X, but no summaries for that timeline and LSN range exist",
                                tle->tli,
                                LSN_FORMAT_ARGS(tli_start_lsn),
                                LSN_FORMAT_ARGS(tli_end_lsn))));
            else
                ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("WAL summaries are required on timeline %u from %X/%X to %X/%X, but the summaries for that timeline and LSN range are incomplete",
                                tle->tli,
                                LSN_FORMAT_ARGS(tli_start_lsn),
                                LSN_FORMAT_ARGS(tli_end_lsn)),
                         errdetail("The first unsummarized LSN in this range is %X/%X.",
                                   LSN_FORMAT_ARGS(tli_missing_lsn))));
        }
 
        /*
         * Remember that we need to read these summaries.
         *
         * Technically, it's possible that this could read more files than
         * required, since tli_wslist in theory could contain redundant
         * summaries. For instance, if we have a summary from 0/10000000 to
         * 0/20000000 and also one from 0/00000000 to 0/30000000, then the
         * latter subsumes the former and the former could be ignored.
         *
         * We ignore this possibility because the WAL summarizer only tries to
         * generate summaries that do not overlap. If somehow they exist,
         * we'll do a bit of extra work but the results should still be
         * correct.
         */
        required_wslist = list_concat(required_wslist, tli_wslist);
 
        /*
         * Timelines earlier than the one in which the prior backup began are
         * not relevant.
         */
        if (tle->tli == earliest_wal_range_tli)
            break;
    }
 
    /*
     * Read all of the required block reference table files and merge all of
     * the data into a single in-memory block reference table.
     *
     * See the comments for struct IncrementalBackupInfo for some thoughts on
     * memory usage.
     */
    ib->brtab = CreateEmptyBlockRefTable();
    foreach(lc, required_wslist)
    {
        WalSummaryFile *ws = lfirst(lc);
        WalSummaryIO wsio;
        BlockRefTableReader *reader;
        RelFileLocator rlocator;
        ForkNumber  forknum;
        BlockNumber limit_block;
        BlockNumber blocks[BLOCKS_PER_READ];
 
        wsio.file = OpenWalSummaryFile(ws, false);
        wsio.filepos = 0;
        ereport(DEBUG1,
                (errmsg_internal("reading WAL summary file \"%s\"",
                                 FilePathName(wsio.file))));
        reader = CreateBlockRefTableReader(ReadWalSummary, &wsio,
                                           FilePathName(wsio.file),
                                           ReportWalSummaryError, NULL);
        while (BlockRefTableReaderNextRelation(reader, &rlocator, &forknum,
                                               &limit_block))
        {
            BlockRefTableSetLimitBlock(ib->brtab, &rlocator,
                                       forknum, limit_block);
 
            while (1)
            {
                unsigned    nblocks;
                unsigned    i;
 
                nblocks = BlockRefTableReaderGetBlocks(reader, blocks,
                                                       BLOCKS_PER_READ);
                if (nblocks == 0)
                    break;
 
                for (i = 0; i < nblocks; ++i)
                    BlockRefTableMarkBlockModified(ib->brtab, &rlocator,
                                                   forknum, blocks[i]);
            }
        }
        DestroyBlockRefTableReader(reader);
        FileClose(wsio.file);
    }
 
    /* Switch back to previous memory context. */
    MemoryContextSwitchTo(oldcontext);
}
 
/*
 * Get the pathname that should be used when a file is sent incrementally.
 *
 * The result is a palloc'd string.
 */
char *
GetIncrementalFilePath(Oid dboid, Oid spcoid, RelFileNumber relfilenumber,
                       ForkNumber forknum, unsigned segno)
{
    RelPathStr  path;
    char       *lastslash;
    char       *ipath;
 
    path = GetRelationPath(dboid, spcoid, relfilenumber, INVALID_PROC_NUMBER,
                           forknum);
 
    lastslash = strrchr(path.str, '/');
    Assert(lastslash != NULL);
    *lastslash = '\0';
 
    if (segno > 0)
        ipath = psprintf("%s/INCREMENTAL.%s.%u", path.str, lastslash + 1, segno);
    else
        ipath = psprintf("%s/INCREMENTAL.%s", path.str, lastslash + 1);
 
    return ipath;
}
 
/*
 * How should we back up a particular file as part of an incremental backup?
 *
 * If the return value is BACK_UP_FILE_FULLY, caller should back up the whole
 * file just as if this were not an incremental backup.  The contents of the
 * relative_block_numbers array are unspecified in this case.
 *
 * If the return value is BACK_UP_FILE_INCREMENTALLY, caller should include
 * an incremental file in the backup instead of the entire file. On return,
 * *num_blocks_required will be set to the number of blocks that need to be
 * sent, and the actual block numbers will have been stored in
 * relative_block_numbers, which should be an array of at least RELSEG_SIZE.
 * In addition, *truncation_block_length will be set to the value that should
 * be included in the incremental file.
 */
FileBackupMethod
GetFileBackupMethod(IncrementalBackupInfo *ib, const char *path,
                    Oid dboid, Oid spcoid,
                    RelFileNumber relfilenumber, ForkNumber forknum,
                    unsigned segno, size_t size,
                    unsigned *num_blocks_required,
                    BlockNumber *relative_block_numbers,
                    unsigned *truncation_block_length)
{
    BlockNumber limit_block;
    BlockNumber start_blkno;
    BlockNumber stop_blkno;
    RelFileLocator rlocator;
    BlockRefTableEntry *brtentry;
    unsigned    i;
    unsigned    nblocks;
 
    /* Should only be called after PrepareForIncrementalBackup. */
    Assert(ib->buf.data == NULL);
 
    /*
     * dboid could be InvalidOid if shared rel, but spcoid and relfilenumber
     * should have legal values.
     */
    Assert(OidIsValid(spcoid));
    Assert(RelFileNumberIsValid(relfilenumber));
 
    /*
     * If the file size is too large or not a multiple of BLCKSZ, then
     * something weird is happening, so give up and send the whole file.
     */
    if ((size % BLCKSZ) != 0 || size / BLCKSZ > RELSEG_SIZE)
        return BACK_UP_FILE_FULLY;
 
    /*
     * The free-space map fork is not properly WAL-logged, so we need to
     * backup the entire file every time.
     */
    if (forknum == FSM_FORKNUM)
        return BACK_UP_FILE_FULLY;
 
    /*
     * If this file was not part of the prior backup, back it up fully.
     *
     * If this file was created after the prior backup and before the start of
     * the current backup, then the WAL summary information will tell us to
     * back up the whole file. However, if this file was created after the
     * start of the current backup, then the WAL summary won't know anything
     * about it. Without this logic, we would erroneously conclude that it was
     * OK to send it incrementally.
     *
     * Note that the file could have existed at the time of the prior backup,
     * gotten deleted, and then a new file with the same name could have been
     * created.  In that case, this logic won't prevent the file from being
     * backed up incrementally. But, if the deletion happened before the start
     * of the current backup, the limit block will be 0, inducing a full
     * backup. If the deletion happened after the start of the current backup,
     * reconstruction will erroneously combine blocks from the current
     * lifespan of the file with blocks from the previous lifespan -- but in
     * this type of case, WAL replay to reach backup consistency should remove
     * and recreate the file anyway, so the initial bogus contents should not
     * matter.
     */
    if (backup_file_lookup(ib->manifest_files, path) == NULL)
    {
        char       *ipath;
 
        ipath = GetIncrementalFilePath(dboid, spcoid, relfilenumber,
                                       forknum, segno);
        if (backup_file_lookup(ib->manifest_files, ipath) == NULL)
            return BACK_UP_FILE_FULLY;
    }
 
    /*
     * Look up the special block reference table entry for the database as a
     * whole.
     */
    rlocator.spcOid = spcoid;
    rlocator.dbOid = dboid;
    rlocator.relNumber = 0;
    if (BlockRefTableGetEntry(ib->brtab, &rlocator, MAIN_FORKNUM,
                              &limit_block) != NULL)
    {
        /*
         * According to the WAL summary, this database OID/tablespace OID
         * pairing has been created since the previous backup. So, everything
         * in it must be backed up fully.
         */
        return BACK_UP_FILE_FULLY;
    }
 
    /* Look up the block reference table entry for this relfilenode. */
    rlocator.relNumber = relfilenumber;
    brtentry = BlockRefTableGetEntry(ib->brtab, &rlocator, forknum,
                                     &limit_block);
 
    /*
     * If there is no entry, then there have been no WAL-logged changes to the
     * relation since the predecessor backup was taken, so we can back it up
     * incrementally and need not include any modified blocks.
     *
     * However, if the file is zero-length, we should do a full backup,
     * because an incremental file is always more than zero length, and it's
     * silly to take an incremental backup when a full backup would be
     * smaller.
     */
    if (brtentry == NULL)
    {
        if (size == 0)
            return BACK_UP_FILE_FULLY;
        *num_blocks_required = 0;
        *truncation_block_length = size / BLCKSZ;
        return BACK_UP_FILE_INCREMENTALLY;
    }
 
    /*
     * If the limit_block is less than or equal to the point where this
     * segment starts, send the whole file.
     */
    if (limit_block <= segno * RELSEG_SIZE)
        return BACK_UP_FILE_FULLY;
 
    /*
     * Get relevant entries from the block reference table entry.
     *
     * We shouldn't overflow computing the start or stop block numbers, but if
     * it manages to happen somehow, detect it and throw an error.
     */
    start_blkno = segno * RELSEG_SIZE;
    stop_blkno = start_blkno + (size / BLCKSZ);
    if (start_blkno / RELSEG_SIZE != segno || stop_blkno < start_blkno)
        ereport(ERROR,
                errcode(ERRCODE_INTERNAL_ERROR),
                errmsg_internal("overflow computing block number bounds for segment %u with size %zu",
                                segno, size));
 
    /*
     * This will write *absolute* block numbers into the output array, but
     * we'll transpose them below.
     */
    nblocks = BlockRefTableEntryGetBlocks(brtentry, start_blkno, stop_blkno,
                                          relative_block_numbers, RELSEG_SIZE);
    Assert(nblocks <= RELSEG_SIZE);
 
    /*
     * If we're going to have to send nearly all of the blocks, then just send
     * the whole file, because that won't require much extra storage or
     * transfer and will speed up and simplify backup restoration. It's not
     * clear what threshold is most appropriate here and perhaps it ought to
     * be configurable, but for now we're just going to say that if we'd need
     * to send 90% of the blocks anyway, give up and send the whole file.
     *
     * NB: If you change the threshold here, at least make sure to back up the
     * file fully when every single block must be sent, because there's
     * nothing good about sending an incremental file in that case.
     */
    if (nblocks * BLCKSZ > size * 0.9)
        return BACK_UP_FILE_FULLY;
 
    /*
     * Looks like we can send an incremental file, so sort the block numbers
     * and then transpose them from absolute block numbers to relative block
     * numbers if necessary.
     *
     * NB: If the block reference table was using the bitmap representation
     * for a given chunk, the block numbers in that chunk will already be
     * sorted, but when the array-of-offsets representation is used, we can
     * receive block numbers here out of order.
     */
    qsort(relative_block_numbers, nblocks, sizeof(BlockNumber),
          compare_block_numbers);
    if (start_blkno != 0)
    {
        for (i = 0; i < nblocks; ++i)
            relative_block_numbers[i] -= start_blkno;
    }
    *num_blocks_required = nblocks;
 
    /*
     * The truncation block length is the minimum length of the reconstructed
     * file. Any block numbers below this threshold that are not present in
     * the backup need to be fetched from the prior backup. At or above this
     * threshold, blocks should only be included in the result if they are
     * present in the backup. (This may require inserting zero blocks if the
     * blocks included in the backup are non-consecutive.)
     */
    *truncation_block_length = size / BLCKSZ;
    if (BlockNumberIsValid(limit_block))
    {
        unsigned    relative_limit = limit_block - segno * RELSEG_SIZE;
 
        if (*truncation_block_length < relative_limit)
            *truncation_block_length = relative_limit;
    }
 
    /* Send it incrementally. */
    return BACK_UP_FILE_INCREMENTALLY;
}
 
/*
 * Compute the size for a header of an incremental file containing a given
 * number of blocks. The header is rounded to a multiple of BLCKSZ, but
 * only if the file will store some block data.
 */
size_t
GetIncrementalHeaderSize(unsigned num_blocks_required)
{
    size_t      result;
 
    /* Make sure we're not going to overflow. */
    Assert(num_blocks_required <= RELSEG_SIZE);
 
    /*
     * Three four byte quantities (magic number, truncation block length,
     * block count) followed by block numbers.
     */
    result = 3 * sizeof(uint32) + (sizeof(BlockNumber) * num_blocks_required);
 
    /*
     * Round the header size to a multiple of BLCKSZ - when not a multiple of
     * BLCKSZ, add the missing fraction of a block. But do this only if the
     * file will store data for some blocks, otherwise keep it small.
     */
    if ((num_blocks_required > 0) && (result % BLCKSZ != 0))
        result += BLCKSZ - (result % BLCKSZ);
 
    return result;
}
 
/*
 * Compute the size for an incremental file containing a given number of blocks.
 */
size_t
GetIncrementalFileSize(unsigned num_blocks_required)
{
    size_t      result;
 
    /* Make sure we're not going to overflow. */
    Assert(num_blocks_required <= RELSEG_SIZE);
 
    /*
     * Header with three four byte quantities (magic number, truncation block
     * length, block count) followed by block numbers, rounded to a multiple
     * of BLCKSZ (for files with block data), followed by block contents.
     */
    result = GetIncrementalHeaderSize(num_blocks_required);
    result += BLCKSZ * num_blocks_required;
 
    return result;
}
 
/*
 * Helper function for filemap hash table.
 */
static uint32
hash_string_pointer(const char *s)
{
    unsigned char *ss = (unsigned char *) s;
 
    return hash_bytes(ss, strlen(s));
}
 
/*
 * This callback to validate the manifest version for incremental backup.
 */
static void
manifest_process_version(JsonManifestParseContext *context,
                         int manifest_version)
{
    /* Incremental backups don't work with manifest version 1 */
    if (manifest_version == 1)
        context->error_cb(context,
                          "backup manifest version 1 does not support incremental backup");
}
 
/*
 * This callback to validate the manifest system identifier against the current
 * database server.
 */
static void
manifest_process_system_identifier(JsonManifestParseContext *context,
                                   uint64 manifest_system_identifier)
{
    uint64      system_identifier;
 
    /* Get system identifier of current system */
    system_identifier = GetSystemIdentifier();
 
    if (manifest_system_identifier != system_identifier)
        context->error_cb(context,
                          "system identifier in backup manifest is %llu, but database system identifier is %llu",
                          (unsigned long long) manifest_system_identifier,
                          (unsigned long long) system_identifier);
}
 
/*
 * This callback is invoked for each file mentioned in the backup manifest.
 *
 * We store the path to each file and the size of each file for sanity-checking
 * purposes. For further details, see comments for IncrementalBackupInfo.
 */
static void
manifest_process_file(JsonManifestParseContext *context,
                      const char *pathname, uint64 size,
                      pg_checksum_type checksum_type,
                      int checksum_length,
                      uint8 *checksum_payload)
{
    IncrementalBackupInfo *ib = context->private_data;
    backup_file_entry *entry;
    bool        found;
 
    entry = backup_file_insert(ib->manifest_files, pathname, &found);
    if (!found)
    {
        entry->path = MemoryContextStrdup(ib->manifest_files->ctx,
                                          pathname);
        entry->size = size;
    }
}
 
/*
 * This callback is invoked for each WAL range mentioned in the backup
 * manifest.
 *
 * We're just interested in learning the oldest LSN and the corresponding TLI
 * that appear in any WAL range.
 */
static void
manifest_process_wal_range(JsonManifestParseContext *context,
                           TimeLineID tli, XLogRecPtr start_lsn,
                           XLogRecPtr end_lsn)
{
    IncrementalBackupInfo *ib = context->private_data;
    backup_wal_range *range = palloc(sizeof(backup_wal_range));
 
    range->tli = tli;
    range->start_lsn = start_lsn;
    range->end_lsn = end_lsn;
    ib->manifest_wal_ranges = lappend(ib->manifest_wal_ranges, range);
}
 
/*
 * This callback is invoked if an error occurs while parsing the backup
 * manifest.
 */
static void
manifest_report_error(JsonManifestParseContext *context, const char *fmt,...)
{
    StringInfoData errbuf;
 
    initStringInfo(&errbuf);
 
    for (;;)
    {
        va_list     ap;
        int         needed;
 
        va_start(ap, fmt);
        needed = appendStringInfoVA(&errbuf, fmt, ap);
        va_end(ap);
        if (needed == 0)
            break;
        enlargeStringInfo(&errbuf, needed);
    }
 
    ereport(ERROR,
            errmsg_internal("%s", errbuf.data));
}
 
/*
 * Quicksort comparator for block numbers.
 */
static int
compare_block_numbers(const void *a, const void *b)
{
    BlockNumber aa = *(BlockNumber *) a;
    BlockNumber bb = *(BlockNumber *) b;
 
    return pg_cmp_u32(aa, bb);
}