walmethods.c

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/*-------------------------------------------------------------------------
 *
 * walmethods.c - implementations of different ways to write received wal
 *
 * NOTE! The caller must ensure that only one method is instantiated in
 *       any given program, and that it's only instantiated once!
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *        src/bin/pg_basebackup/walmethods.c
 *-------------------------------------------------------------------------
 */

#include "postgres_fe.h"

#include <sys/stat.h>
#include <time.h>
#include <unistd.h>
#ifdef HAVE_LIBZ
#include <zlib.h>
#endif

#include "common/file_perm.h"
#include "common/file_utils.h"
#include "pgtar.h"
#include "receivelog.h"
#include "streamutil.h"

/* Size of zlib buffer for .tar.gz */
#define ZLIB_OUT_SIZE 4096

/*-------------------------------------------------------------------------
 * WalDirectoryMethod - write wal to a directory looking like pg_wal
 *-------------------------------------------------------------------------
 */

/*
 * Global static data for this method
 */
typedef struct DirectoryMethodData
{
    char       *basedir;
    int         compression;
    bool        sync;
} DirectoryMethodData;
static DirectoryMethodData *dir_data = NULL;

/*
 * Local file handle
 */
typedef struct DirectoryMethodFile
{
    int         fd;
    off_t       currpos;
    char       *pathname;
    char       *fullpath;
    char       *temp_suffix;
#ifdef HAVE_LIBZ
    gzFile      gzfp;
#endif
} DirectoryMethodFile;

static const char *
dir_getlasterror(void)
{
    /* Directory method always sets errno, so just use strerror */
    return strerror(errno);
}

static char *
dir_get_file_name(const char *pathname, const char *temp_suffix)
{
    char       *filename = pg_malloc0(MAXPGPATH * sizeof(char));

    snprintf(filename, MAXPGPATH, "%s%s%s",
             pathname, dir_data->compression > 0 ? ".gz" : "",
             temp_suffix ? temp_suffix : "");

    return filename;
}

static Walfile
dir_open_for_write(const char *pathname, const char *temp_suffix, size_t pad_to_size)
{
    static char tmppath[MAXPGPATH];
    char       *filename;
    int         fd;
    DirectoryMethodFile *f;
#ifdef HAVE_LIBZ
    gzFile      gzfp = NULL;
#endif

    filename = dir_get_file_name(pathname, temp_suffix);
    snprintf(tmppath, sizeof(tmppath), "%s/%s",
             dir_data->basedir, filename);
    pg_free(filename);

    /*
     * Open a file for non-compressed as well as compressed files. Tracking
     * the file descriptor is important for dir_sync() method as gzflush()
     * does not do any system calls to fsync() to make changes permanent on
     * disk.
     */
    fd = open(tmppath, O_WRONLY | O_CREAT | PG_BINARY, pg_file_create_mode);
    if (fd < 0)
        return NULL;

#ifdef HAVE_LIBZ
    if (dir_data->compression > 0)
    {
        gzfp = gzdopen(fd, "wb");
        if (gzfp == NULL)
        {
            close(fd);
            return NULL;
        }

        if (gzsetparams(gzfp, dir_data->compression,
                        Z_DEFAULT_STRATEGY) != Z_OK)
        {
            gzclose(gzfp);
            return NULL;
        }
    }
#endif

    /* Do pre-padding on non-compressed files */
    if (pad_to_size && dir_data->compression == 0)
    {
        PGAlignedXLogBlock zerobuf;
        int         bytes;

        memset(zerobuf.data, 0, XLOG_BLCKSZ);
        for (bytes = 0; bytes < pad_to_size; bytes += XLOG_BLCKSZ)
        {
            errno = 0;
            if (write(fd, zerobuf.data, XLOG_BLCKSZ) != XLOG_BLCKSZ)
            {
                int         save_errno = errno;

                close(fd);

                /*
                 * If write didn't set errno, assume problem is no disk space.
                 */
                errno = save_errno ? save_errno : ENOSPC;
                return NULL;
            }
        }

        if (lseek(fd, 0, SEEK_SET) != 0)
        {
            int         save_errno = errno;

            close(fd);
            errno = save_errno;
            return NULL;
        }
    }

    /*
     * fsync WAL file and containing directory, to ensure the file is
     * persistently created and zeroed (if padded). That's particularly
     * important when using synchronous mode, where the file is modified and
     * fsynced in-place, without a directory fsync.
     */
    if (dir_data->sync)
    {
        if (fsync_fname(tmppath, false) != 0 ||
            fsync_parent_path(tmppath) != 0)
        {
#ifdef HAVE_LIBZ
            if (dir_data->compression > 0)
                gzclose(gzfp);
            else
#endif
                close(fd);
            return NULL;
        }
    }

    f = pg_malloc0(sizeof(DirectoryMethodFile));
#ifdef HAVE_LIBZ
    if (dir_data->compression > 0)
        f->gzfp = gzfp;
#endif
    f->fd = fd;
    f->currpos = 0;
    f->pathname = pg_strdup(pathname);
    f->fullpath = pg_strdup(tmppath);
    if (temp_suffix)
        f->temp_suffix = pg_strdup(temp_suffix);

    return f;
}

static ssize_t
dir_write(Walfile f, const void *buf, size_t count)
{
    ssize_t     r;
    DirectoryMethodFile *df = (DirectoryMethodFile *) f;

    Assert(f != NULL);

#ifdef HAVE_LIBZ
    if (dir_data->compression > 0)
        r = (ssize_t) gzwrite(df->gzfp, buf, count);
    else
#endif
        r = write(df->fd, buf, count);
    if (r > 0)
        df->currpos += r;
    return r;
}

static off_t
dir_get_current_pos(Walfile f)
{
    Assert(f != NULL);

    /* Use a cached value to prevent lots of reseeks */
    return ((DirectoryMethodFile *) f)->currpos;
}

static int
dir_close(Walfile f, WalCloseMethod method)
{
    int         r;
    DirectoryMethodFile *df = (DirectoryMethodFile *) f;
    static char tmppath[MAXPGPATH];
    static char tmppath2[MAXPGPATH];

    Assert(f != NULL);

#ifdef HAVE_LIBZ
    if (dir_data->compression > 0)
        r = gzclose(df->gzfp);
    else
#endif
        r = close(df->fd);

    if (r == 0)
    {
        /* Build path to the current version of the file */
        if (method == CLOSE_NORMAL && df->temp_suffix)
        {
            char       *filename;
            char       *filename2;

            /*
             * If we have a temp prefix, normal operation is to rename the
             * file.
             */
            filename = dir_get_file_name(df->pathname, df->temp_suffix);
            snprintf(tmppath, sizeof(tmppath), "%s/%s",
                     dir_data->basedir, filename);
            pg_free(filename);

            /* permanent name, so no need for the prefix */
            filename2 = dir_get_file_name(df->pathname, NULL);
            snprintf(tmppath2, sizeof(tmppath2), "%s/%s",
                     dir_data->basedir, filename2);
            pg_free(filename2);
            r = durable_rename(tmppath, tmppath2);
        }
        else if (method == CLOSE_UNLINK)
        {
            char       *filename;

            /* Unlink the file once it's closed */
            filename = dir_get_file_name(df->pathname, df->temp_suffix);
            snprintf(tmppath, sizeof(tmppath), "%s/%s",
                     dir_data->basedir, filename);
            pg_free(filename);
            r = unlink(tmppath);
        }
        else
        {
            /*
             * Else either CLOSE_NORMAL and no temp suffix, or
             * CLOSE_NO_RENAME. In this case, fsync the file and containing
             * directory if sync mode is requested.
             */
            if (dir_data->sync)
            {
                r = fsync_fname(df->fullpath, false);
                if (r == 0)
                    r = fsync_parent_path(df->fullpath);
            }
        }
    }

    pg_free(df->pathname);
    pg_free(df->fullpath);
    if (df->temp_suffix)
        pg_free(df->temp_suffix);
    pg_free(df);

    return r;
}

static int
dir_sync(Walfile f)
{
    Assert(f != NULL);

    if (!dir_data->sync)
        return 0;

#ifdef HAVE_LIBZ
    if (dir_data->compression > 0)
    {
        if (gzflush(((DirectoryMethodFile *) f)->gzfp, Z_SYNC_FLUSH) != Z_OK)
            return -1;
    }
#endif

    return fsync(((DirectoryMethodFile *) f)->fd);
}

static ssize_t
dir_get_file_size(const char *pathname)
{
    struct stat statbuf;
    static char tmppath[MAXPGPATH];

    snprintf(tmppath, sizeof(tmppath), "%s/%s",
             dir_data->basedir, pathname);

    if (stat(tmppath, &statbuf) != 0)
        return -1;

    return statbuf.st_size;
}

static int
dir_compression(void)
{
    return dir_data->compression;
}

static bool
dir_existsfile(const char *pathname)
{
    static char tmppath[MAXPGPATH];
    int         fd;

    snprintf(tmppath, sizeof(tmppath), "%s/%s",
             dir_data->basedir, pathname);

    fd = open(tmppath, O_RDONLY | PG_BINARY, 0);
    if (fd < 0)
        return false;
    close(fd);
    return true;
}

static bool
dir_finish(void)
{
    if (dir_data->sync)
    {
        /*
         * Files are fsynced when they are closed, but we need to fsync the
         * directory entry here as well.
         */
        if (fsync_fname(dir_data->basedir, true) != 0)
            return false;
    }
    return true;
}


WalWriteMethod *
CreateWalDirectoryMethod(const char *basedir, int compression, bool sync)
{
    WalWriteMethod *method;

    method = pg_malloc0(sizeof(WalWriteMethod));
    method->open_for_write = dir_open_for_write;
    method->write = dir_write;
    method->get_current_pos = dir_get_current_pos;
    method->get_file_size = dir_get_file_size;
    method->get_file_name = dir_get_file_name;
    method->compression = dir_compression;
    method->close = dir_close;
    method->sync = dir_sync;
    method->existsfile = dir_existsfile;
    method->finish = dir_finish;
    method->getlasterror = dir_getlasterror;

    dir_data = pg_malloc0(sizeof(DirectoryMethodData));
    dir_data->compression = compression;
    dir_data->basedir = pg_strdup(basedir);
    dir_data->sync = sync;

    return method;
}

void
FreeWalDirectoryMethod(void)
{
    pg_free(dir_data->basedir);
    pg_free(dir_data);
}


/*-------------------------------------------------------------------------
 * WalTarMethod - write wal to a tar file containing pg_wal contents
 *-------------------------------------------------------------------------
 */

typedef struct TarMethodFile
{
    off_t       ofs_start;      /* Where does the *header* for this file start */
    off_t       currpos;
    char        header[TAR_BLOCK_SIZE];
    char       *pathname;
    size_t      pad_to_size;
} TarMethodFile;

typedef struct TarMethodData
{
    char       *tarfilename;
    int         fd;
    int         compression;
    bool        sync;
    TarMethodFile *currentfile;
    char        lasterror[1024];
#ifdef HAVE_LIBZ
    z_streamp   zp;
    void       *zlibOut;
#endif
} TarMethodData;
static TarMethodData *tar_data = NULL;

#define tar_clear_error() tar_data->lasterror[0] = '\0'
#define tar_set_error(msg) strlcpy(tar_data->lasterror, _(msg), sizeof(tar_data->lasterror))

static const char *
tar_getlasterror(void)
{
    /*
     * If a custom error is set, return that one. Otherwise, assume errno is
     * set and return that one.
     */
    if (tar_data->lasterror[0])
        return tar_data->lasterror;
    return strerror(errno);
}

#ifdef HAVE_LIBZ
static bool
tar_write_compressed_data(void *buf, size_t count, bool flush)
{
    tar_data->zp->next_in = buf;
    tar_data->zp->avail_in = count;

    while (tar_data->zp->avail_in || flush)
    {
        int         r;

        r = deflate(tar_data->zp, flush ? Z_FINISH : Z_NO_FLUSH);
        if (r == Z_STREAM_ERROR)
        {
            tar_set_error("could not compress data");
            return false;
        }

        if (tar_data->zp->avail_out < ZLIB_OUT_SIZE)
        {
            size_t      len = ZLIB_OUT_SIZE - tar_data->zp->avail_out;

            errno = 0;
            if (write(tar_data->fd, tar_data->zlibOut, len) != len)
            {
                /*
                 * If write didn't set errno, assume problem is no disk space.
                 */
                if (errno == 0)
                    errno = ENOSPC;
                return false;
            }

            tar_data->zp->next_out = tar_data->zlibOut;
            tar_data->zp->avail_out = ZLIB_OUT_SIZE;
        }

        if (r == Z_STREAM_END)
            break;
    }

    if (flush)
    {
        /* Reset the stream for writing */
        if (deflateReset(tar_data->zp) != Z_OK)
        {
            tar_set_error("could not reset compression stream");
            return false;
        }
    }

    return true;
}
#endif

static ssize_t
tar_write(Walfile f, const void *buf, size_t count)
{
    ssize_t     r;

    Assert(f != NULL);
    tar_clear_error();

    /* Tarfile will always be positioned at the end */
    if (!tar_data->compression)
    {
        r = write(tar_data->fd, buf, count);
        if (r > 0)
            ((TarMethodFile *) f)->currpos += r;
        return r;
    }
#ifdef HAVE_LIBZ
    else
    {
        if (!tar_write_compressed_data(unconstify(void *, buf), count, false))
            return -1;
        ((TarMethodFile *) f)->currpos += count;
        return count;
    }
#else
    else
        /* Can't happen - compression enabled with no libz */
        return -1;
#endif
}

static bool
tar_write_padding_data(TarMethodFile *f, size_t bytes)
{
    PGAlignedXLogBlock zerobuf;
    size_t      bytesleft = bytes;

    memset(zerobuf.data, 0, XLOG_BLCKSZ);
    while (bytesleft)
    {
        size_t      bytestowrite = Min(bytesleft, XLOG_BLCKSZ);
        ssize_t     r = tar_write(f, zerobuf.data, bytestowrite);

        if (r < 0)
            return false;
        bytesleft -= r;
    }

    return true;
}

static char *
tar_get_file_name(const char *pathname, const char *temp_suffix)
{
    char       *filename = pg_malloc0(MAXPGPATH * sizeof(char));

    snprintf(filename, MAXPGPATH, "%s%s",
             pathname, temp_suffix ? temp_suffix : "");

    return filename;
}

static Walfile
tar_open_for_write(const char *pathname, const char *temp_suffix, size_t pad_to_size)
{
    int         save_errno;
    char       *tmppath;

    tar_clear_error();

    if (tar_data->fd < 0)
    {
        /*
         * We open the tar file only when we first try to write to it.
         */
        tar_data->fd = open(tar_data->tarfilename,
                            O_WRONLY | O_CREAT | PG_BINARY,
                            pg_file_create_mode);
        if (tar_data->fd < 0)
            return NULL;

#ifdef HAVE_LIBZ
        if (tar_data->compression)
        {
            tar_data->zp = (z_streamp) pg_malloc(sizeof(z_stream));
            tar_data->zp->zalloc = Z_NULL;
            tar_data->zp->zfree = Z_NULL;
            tar_data->zp->opaque = Z_NULL;
            tar_data->zp->next_out = tar_data->zlibOut;
            tar_data->zp->avail_out = ZLIB_OUT_SIZE;

            /*
             * Initialize deflation library. Adding the magic value 16 to the
             * default 15 for the windowBits parameter makes the output be
             * gzip instead of zlib.
             */
            if (deflateInit2(tar_data->zp, tar_data->compression, Z_DEFLATED, 15 + 16, 8, Z_DEFAULT_STRATEGY) != Z_OK)
            {
                pg_free(tar_data->zp);
                tar_data->zp = NULL;
                tar_set_error("could not initialize compression library");
                return NULL;
            }
        }
#endif

        /* There's no tar header itself, the file starts with regular files */
    }

    Assert(tar_data->currentfile == NULL);
    if (tar_data->currentfile != NULL)
    {
        tar_set_error("implementation error: tar files can't have more than one open file");
        return NULL;
    }

    tar_data->currentfile = pg_malloc0(sizeof(TarMethodFile));

    tmppath = tar_get_file_name(pathname, temp_suffix);

    /* Create a header with size set to 0 - we will fill out the size on close */
    if (tarCreateHeader(tar_data->currentfile->header, tmppath, NULL, 0, S_IRUSR | S_IWUSR, 0, 0, time(NULL)) != TAR_OK)
    {
        pg_free(tar_data->currentfile);
        pg_free(tmppath);
        tar_data->currentfile = NULL;
        tar_set_error("could not create tar header");
        return NULL;
    }

    pg_free(tmppath);

#ifdef HAVE_LIBZ
    if (tar_data->compression)
    {
        /* Flush existing data */
        if (!tar_write_compressed_data(NULL, 0, true))
            return NULL;

        /* Turn off compression for header */
        if (deflateParams(tar_data->zp, 0, 0) != Z_OK)
        {
            tar_set_error("could not change compression parameters");
            return NULL;
        }
    }
#endif

    tar_data->currentfile->ofs_start = lseek(tar_data->fd, 0, SEEK_CUR);
    if (tar_data->currentfile->ofs_start == -1)
    {
        save_errno = errno;
        pg_free(tar_data->currentfile);
        tar_data->currentfile = NULL;
        errno = save_errno;
        return NULL;
    }
    tar_data->currentfile->currpos = 0;

    if (!tar_data->compression)
    {
        errno = 0;
        if (write(tar_data->fd, tar_data->currentfile->header,
                  TAR_BLOCK_SIZE) != TAR_BLOCK_SIZE)
        {
            save_errno = errno;
            pg_free(tar_data->currentfile);
            tar_data->currentfile = NULL;
            /* if write didn't set errno, assume problem is no disk space */
            errno = save_errno ? save_errno : ENOSPC;
            return NULL;
        }
    }
#ifdef HAVE_LIBZ
    else
    {
        /* Write header through the zlib APIs but with no compression */
        if (!tar_write_compressed_data(tar_data->currentfile->header,
                                       TAR_BLOCK_SIZE, true))
            return NULL;

        /* Re-enable compression for the rest of the file */
        if (deflateParams(tar_data->zp, tar_data->compression, 0) != Z_OK)
        {
            tar_set_error("could not change compression parameters");
            return NULL;
        }
    }
#endif

    tar_data->currentfile->pathname = pg_strdup(pathname);

    /*
     * Uncompressed files are padded on creation, but for compression we can't
     * do that
     */
    if (pad_to_size)
    {
        tar_data->currentfile->pad_to_size = pad_to_size;
        if (!tar_data->compression)
        {
            /* Uncompressed, so pad now */
            tar_write_padding_data(tar_data->currentfile, pad_to_size);
            /* Seek back to start */
            if (lseek(tar_data->fd,
                      tar_data->currentfile->ofs_start + TAR_BLOCK_SIZE,
                      SEEK_SET) != tar_data->currentfile->ofs_start + TAR_BLOCK_SIZE)
                return NULL;

            tar_data->currentfile->currpos = 0;
        }
    }

    return tar_data->currentfile;
}

static ssize_t
tar_get_file_size(const char *pathname)
{
    tar_clear_error();

    /* Currently not used, so not supported */
    errno = ENOSYS;
    return -1;
}

static int
tar_compression(void)
{
    return tar_data->compression;
}

static off_t
tar_get_current_pos(Walfile f)
{
    Assert(f != NULL);
    tar_clear_error();

    return ((TarMethodFile *) f)->currpos;
}

static int
tar_sync(Walfile f)
{
    Assert(f != NULL);
    tar_clear_error();

    if (!tar_data->sync)
        return 0;

    /*
     * Always sync the whole tarfile, because that's all we can do. This makes
     * no sense on compressed files, so just ignore those.
     */
    if (tar_data->compression)
        return 0;

    return fsync(tar_data->fd);
}

static int
tar_close(Walfile f, WalCloseMethod method)
{
    ssize_t     filesize;
    int         padding;
    TarMethodFile *tf = (TarMethodFile *) f;

    Assert(f != NULL);
    tar_clear_error();

    if (method == CLOSE_UNLINK)
    {
        if (tar_data->compression)
        {
            tar_set_error("unlink not supported with compression");
            return -1;
        }

        /*
         * Unlink the file that we just wrote to the tar. We do this by
         * truncating it to the start of the header. This is safe as we only
         * allow writing of the very last file.
         */
        if (ftruncate(tar_data->fd, tf->ofs_start) != 0)
            return -1;

        pg_free(tf->pathname);
        pg_free(tf);
        tar_data->currentfile = NULL;

        return 0;
    }

    /*
     * Pad the file itself with zeroes if necessary. Note that this is
     * different from the tar format padding -- this is the padding we asked
     * for when the file was opened.
     */
    if (tf->pad_to_size)
    {
        if (tar_data->compression)
        {
            /*
             * A compressed tarfile is padded on close since we cannot know
             * the size of the compressed output until the end.
             */
            size_t      sizeleft = tf->pad_to_size - tf->currpos;

            if (sizeleft)
            {
                if (!tar_write_padding_data(tf, sizeleft))
                    return -1;
            }
        }
        else
        {
            /*
             * An uncompressed tarfile was padded on creation, so just adjust
             * the current position as if we seeked to the end.
             */
            tf->currpos = tf->pad_to_size;
        }
    }

    /*
     * Get the size of the file, and pad out to a multiple of the tar block
     * size.
     */
    filesize = tar_get_current_pos(f);
    padding = tarPaddingBytesRequired(filesize);
    if (padding)
    {
        char        zerobuf[TAR_BLOCK_SIZE];

        MemSet(zerobuf, 0, padding);
        if (tar_write(f, zerobuf, padding) != padding)
            return -1;
    }


#ifdef HAVE_LIBZ
    if (tar_data->compression)
    {
        /* Flush the current buffer */
        if (!tar_write_compressed_data(NULL, 0, true))
        {
            errno = EINVAL;
            return -1;
        }
    }
#endif

    /*
     * Now go back and update the header with the correct filesize and
     * possibly also renaming the file. We overwrite the entire current header
     * when done, including the checksum.
     */
    print_tar_number(&(tf->header[124]), 12, filesize);

    if (method == CLOSE_NORMAL)

        /*
         * We overwrite it with what it was before if we have no tempname,
         * since we're going to write the buffer anyway.
         */
        strlcpy(&(tf->header[0]), tf->pathname, 100);

    print_tar_number(&(tf->header[148]), 8, tarChecksum(((TarMethodFile *) f)->header));
    if (lseek(tar_data->fd, tf->ofs_start, SEEK_SET) != ((TarMethodFile *) f)->ofs_start)
        return -1;
    if (!tar_data->compression)
    {
        errno = 0;
        if (write(tar_data->fd, tf->header, TAR_BLOCK_SIZE) != TAR_BLOCK_SIZE)
        {
            /* if write didn't set errno, assume problem is no disk space */
            if (errno == 0)
                errno = ENOSPC;
            return -1;
        }
    }
#ifdef HAVE_LIBZ
    else
    {
        /* Turn off compression */
        if (deflateParams(tar_data->zp, 0, 0) != Z_OK)
        {
            tar_set_error("could not change compression parameters");
            return -1;
        }

        /* Overwrite the header, assuming the size will be the same */
        if (!tar_write_compressed_data(tar_data->currentfile->header,
                                       TAR_BLOCK_SIZE, true))
            return -1;

        /* Turn compression back on */
        if (deflateParams(tar_data->zp, tar_data->compression, 0) != Z_OK)
        {
            tar_set_error("could not change compression parameters");
            return -1;
        }
    }
#endif

    /* Move file pointer back down to end, so we can write the next file */
    if (lseek(tar_data->fd, 0, SEEK_END) < 0)
        return -1;

    /* Always fsync on close, so the padding gets fsynced */
    if (tar_sync(f) < 0)
        exit(1);

    /* Clean up and done */
    pg_free(tf->pathname);
    pg_free(tf);
    tar_data->currentfile = NULL;

    return 0;
}

static bool
tar_existsfile(const char *pathname)
{
    tar_clear_error();
    /* We only deal with new tarfiles, so nothing externally created exists */
    return false;
}

static bool
tar_finish(void)
{
    char        zerobuf[1024];

    tar_clear_error();

    if (tar_data->currentfile)
    {
        if (tar_close(tar_data->currentfile, CLOSE_NORMAL) != 0)
            return false;
    }

    /* A tarfile always ends with two empty blocks */
    MemSet(zerobuf, 0, sizeof(zerobuf));
    if (!tar_data->compression)
    {
        errno = 0;
        if (write(tar_data->fd, zerobuf, sizeof(zerobuf)) != sizeof(zerobuf))
        {
            /* if write didn't set errno, assume problem is no disk space */
            if (errno == 0)
                errno = ENOSPC;
            return false;
        }
    }
#ifdef HAVE_LIBZ
    else
    {
        if (!tar_write_compressed_data(zerobuf, sizeof(zerobuf), false))
            return false;

        /* Also flush all data to make sure the gzip stream is finished */
        tar_data->zp->next_in = NULL;
        tar_data->zp->avail_in = 0;
        while (true)
        {
            int         r;

            r = deflate(tar_data->zp, Z_FINISH);

            if (r == Z_STREAM_ERROR)
            {
                tar_set_error("could not compress data");
                return false;
            }
            if (tar_data->zp->avail_out < ZLIB_OUT_SIZE)
            {
                size_t      len = ZLIB_OUT_SIZE - tar_data->zp->avail_out;

                errno = 0;
                if (write(tar_data->fd, tar_data->zlibOut, len) != len)
                {
                    /*
                     * If write didn't set errno, assume problem is no disk
                     * space.
                     */
                    if (errno == 0)
                        errno = ENOSPC;
                    return false;
                }
            }
            if (r == Z_STREAM_END)
                break;
        }

        if (deflateEnd(tar_data->zp) != Z_OK)
        {
            tar_set_error("could not close compression stream");
            return false;
        }
    }
#endif

    /* sync the empty blocks as well, since they're after the last file */
    if (tar_data->sync)
    {
        if (fsync(tar_data->fd) != 0)
            return false;
    }

    if (close(tar_data->fd) != 0)
        return false;

    tar_data->fd = -1;

    if (tar_data->sync)
    {
        if (fsync_fname(tar_data->tarfilename, false) != 0)
            return false;
        if (fsync_parent_path(tar_data->tarfilename) != 0)
            return false;
    }

    return true;
}

WalWriteMethod *
CreateWalTarMethod(const char *tarbase, int compression, bool sync)
{
    WalWriteMethod *method;
    const char *suffix = (compression != 0) ? ".tar.gz" : ".tar";

    method = pg_malloc0(sizeof(WalWriteMethod));
    method->open_for_write = tar_open_for_write;
    method->write = tar_write;
    method->get_current_pos = tar_get_current_pos;
    method->get_file_size = tar_get_file_size;
    method->get_file_name = tar_get_file_name;
    method->compression = tar_compression;
    method->close = tar_close;
    method->sync = tar_sync;
    method->existsfile = tar_existsfile;
    method->finish = tar_finish;
    method->getlasterror = tar_getlasterror;

    tar_data = pg_malloc0(sizeof(TarMethodData));
    tar_data->tarfilename = pg_malloc0(strlen(tarbase) + strlen(suffix) + 1);
    sprintf(tar_data->tarfilename, "%s%s", tarbase, suffix);
    tar_data->fd = -1;
    tar_data->compression = compression;
    tar_data->sync = sync;
#ifdef HAVE_LIBZ
    if (compression)
        tar_data->zlibOut = (char *) pg_malloc(ZLIB_OUT_SIZE + 1);
#endif

    return method;
}

void
FreeWalTarMethod(void)
{
    pg_free(tar_data->tarfilename);
#ifdef HAVE_LIBZ
    if (tar_data->compression)
        pg_free(tar_data->zlibOut);
#endif
    pg_free(tar_data);
}