file_utils.c

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/*-------------------------------------------------------------------------
 *
 * File-processing utility routines.
 *
 * Assorted utility functions to work on files.
 *
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/common/file_utils.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres_fe.h"

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

#include "common/file_utils.h"
#include "common/logging.h"


/* Define PG_FLUSH_DATA_WORKS if we have an implementation for pg_flush_data */
#if defined(HAVE_SYNC_FILE_RANGE)
#define PG_FLUSH_DATA_WORKS 1
#elif defined(USE_POSIX_FADVISE) && defined(POSIX_FADV_DONTNEED)
#define PG_FLUSH_DATA_WORKS 1
#endif

/*
 * pg_xlog has been renamed to pg_wal in version 10.
 */
#define MINIMUM_VERSION_FOR_PG_WAL  100000

#ifdef PG_FLUSH_DATA_WORKS
static int  pre_sync_fname(const char *fname, bool isdir);
#endif
static void walkdir(const char *path,
                    int (*action) (const char *fname, bool isdir),
                    bool process_symlinks);

/*
 * Issue fsync recursively on PGDATA and all its contents.
 *
 * We fsync regular files and directories wherever they are, but we follow
 * symlinks only for pg_wal (or pg_xlog) and immediately under pg_tblspc.
 * Other symlinks are presumed to point at files we're not responsible for
 * fsyncing, and might not have privileges to write at all.
 *
 * serverVersion indicates the version of the server to be fsync'd.
 *
 * Errors are reported but not considered fatal.
 */
void
fsync_pgdata(const char *pg_data,
             int serverVersion)
{
    bool        xlog_is_symlink;
    char        pg_wal[MAXPGPATH];
    char        pg_tblspc[MAXPGPATH];

    /* handle renaming of pg_xlog to pg_wal in post-10 clusters */
    snprintf(pg_wal, MAXPGPATH, "%s/%s", pg_data,
             serverVersion < MINIMUM_VERSION_FOR_PG_WAL ? "pg_xlog" : "pg_wal");
    snprintf(pg_tblspc, MAXPGPATH, "%s/pg_tblspc", pg_data);

    /*
     * If pg_wal is a symlink, we'll need to recurse into it separately,
     * because the first walkdir below will ignore it.
     */
    xlog_is_symlink = false;

#ifndef WIN32
    {
        struct stat st;

        if (lstat(pg_wal, &st) < 0)
            pg_log_error("could not stat file \"%s\": %m", pg_wal);
        else if (S_ISLNK(st.st_mode))
            xlog_is_symlink = true;
    }
#else
    if (pgwin32_is_junction(pg_wal))
        xlog_is_symlink = true;
#endif

    /*
     * If possible, hint to the kernel that we're soon going to fsync the data
     * directory and its contents.
     */
#ifdef PG_FLUSH_DATA_WORKS
    walkdir(pg_data, pre_sync_fname, false);
    if (xlog_is_symlink)
        walkdir(pg_wal, pre_sync_fname, false);
    walkdir(pg_tblspc, pre_sync_fname, true);
#endif

    /*
     * Now we do the fsync()s in the same order.
     *
     * The main call ignores symlinks, so in addition to specially processing
     * pg_wal if it's a symlink, pg_tblspc has to be visited separately with
     * process_symlinks = true.  Note that if there are any plain directories
     * in pg_tblspc, they'll get fsync'd twice.  That's not an expected case
     * so we don't worry about optimizing it.
     */
    walkdir(pg_data, fsync_fname, false);
    if (xlog_is_symlink)
        walkdir(pg_wal, fsync_fname, false);
    walkdir(pg_tblspc, fsync_fname, true);
}

/*
 * Issue fsync recursively on the given directory and all its contents.
 *
 * This is a convenient wrapper on top of walkdir().
 */
void
fsync_dir_recurse(const char *dir)
{
    /*
     * If possible, hint to the kernel that we're soon going to fsync the data
     * directory and its contents.
     */
#ifdef PG_FLUSH_DATA_WORKS
    walkdir(dir, pre_sync_fname, false);
#endif

    walkdir(dir, fsync_fname, false);
}

/*
 * walkdir: recursively walk a directory, applying the action to each
 * regular file and directory (including the named directory itself).
 *
 * If process_symlinks is true, the action and recursion are also applied
 * to regular files and directories that are pointed to by symlinks in the
 * given directory; otherwise symlinks are ignored.  Symlinks are always
 * ignored in subdirectories, ie we intentionally don't pass down the
 * process_symlinks flag to recursive calls.
 *
 * Errors are reported but not considered fatal.
 *
 * See also walkdir in fd.c, which is a backend version of this logic.
 */
static void
walkdir(const char *path,
        int (*action) (const char *fname, bool isdir),
        bool process_symlinks)
{
    DIR        *dir;
    struct dirent *de;

    dir = opendir(path);
    if (dir == NULL)
    {
        pg_log_error("could not open directory \"%s\": %m", path);
        return;
    }

    while (errno = 0, (de = readdir(dir)) != NULL)
    {
        char        subpath[MAXPGPATH * 2];
        struct stat fst;
        int         sret;

        if (strcmp(de->d_name, ".") == 0 ||
            strcmp(de->d_name, "..") == 0)
            continue;

        snprintf(subpath, sizeof(subpath), "%s/%s", path, de->d_name);

        if (process_symlinks)
            sret = stat(subpath, &fst);
        else
            sret = lstat(subpath, &fst);

        if (sret < 0)
        {
            pg_log_error("could not stat file \"%s\": %m", subpath);
            continue;
        }

        if (S_ISREG(fst.st_mode))
            (*action) (subpath, false);
        else if (S_ISDIR(fst.st_mode))
            walkdir(subpath, action, false);
    }

    if (errno)
        pg_log_error("could not read directory \"%s\": %m", path);

    (void) closedir(dir);

    /*
     * It's important to fsync the destination directory itself as individual
     * file fsyncs don't guarantee that the directory entry for the file is
     * synced.  Recent versions of ext4 have made the window much wider but
     * it's been an issue for ext3 and other filesystems in the past.
     */
    (*action) (path, true);
}

/*
 * Hint to the OS that it should get ready to fsync() this file.
 *
 * Ignores errors trying to open unreadable files, and reports other errors
 * non-fatally.
 */
#ifdef PG_FLUSH_DATA_WORKS

static int
pre_sync_fname(const char *fname, bool isdir)
{
    int         fd;

    fd = open(fname, O_RDONLY | PG_BINARY, 0);

    if (fd < 0)
    {
        if (errno == EACCES || (isdir && errno == EISDIR))
            return 0;
        pg_log_error("could not open file \"%s\": %m", fname);
        return -1;
    }

    /*
     * We do what pg_flush_data() would do in the backend: prefer to use
     * sync_file_range, but fall back to posix_fadvise.  We ignore errors
     * because this is only a hint.
     */
#if defined(HAVE_SYNC_FILE_RANGE)
    (void) sync_file_range(fd, 0, 0, SYNC_FILE_RANGE_WRITE);
#elif defined(USE_POSIX_FADVISE) && defined(POSIX_FADV_DONTNEED)
    (void) posix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED);
#else
#error PG_FLUSH_DATA_WORKS should not have been defined
#endif

    (void) close(fd);
    return 0;
}

#endif                          /* PG_FLUSH_DATA_WORKS */

/*
 * fsync_fname -- Try to fsync a file or directory
 *
 * Ignores errors trying to open unreadable files, or trying to fsync
 * directories on systems where that isn't allowed/required.  Reports
 * other errors non-fatally.
 */
int
fsync_fname(const char *fname, bool isdir)
{
    int         fd;
    int         flags;
    int         returncode;

    /*
     * Some OSs require directories to be opened read-only whereas other
     * systems don't allow us to fsync files opened read-only; so we need both
     * cases here.  Using O_RDWR will cause us to fail to fsync files that are
     * not writable by our userid, but we assume that's OK.
     */
    flags = PG_BINARY;
    if (!isdir)
        flags |= O_RDWR;
    else
        flags |= O_RDONLY;

    /*
     * Open the file, silently ignoring errors about unreadable files (or
     * unsupported operations, e.g. opening a directory under Windows), and
     * logging others.
     */
    fd = open(fname, flags, 0);
    if (fd < 0)
    {
        if (errno == EACCES || (isdir && errno == EISDIR))
            return 0;
        pg_log_error("could not open file \"%s\": %m", fname);
        return -1;
    }

    returncode = fsync(fd);

    /*
     * Some OSes don't allow us to fsync directories at all, so we can ignore
     * those errors. Anything else needs to be reported.
     */
    if (returncode != 0 && !(isdir && (errno == EBADF || errno == EINVAL)))
    {
        pg_log_error("could not fsync file \"%s\": %m", fname);
        (void) close(fd);
        return -1;
    }

    (void) close(fd);
    return 0;
}

/*
 * fsync_parent_path -- fsync the parent path of a file or directory
 *
 * This is aimed at making file operations persistent on disk in case of
 * an OS crash or power failure.
 */
int
fsync_parent_path(const char *fname)
{
    char        parentpath[MAXPGPATH];

    strlcpy(parentpath, fname, MAXPGPATH);
    get_parent_directory(parentpath);

    /*
     * get_parent_directory() returns an empty string if the input argument is
     * just a file name (see comments in path.c), so handle that as being the
     * current directory.
     */
    if (strlen(parentpath) == 0)
        strlcpy(parentpath, ".", MAXPGPATH);

    if (fsync_fname(parentpath, true) != 0)
        return -1;

    return 0;
}

/*
 * durable_rename -- rename(2) wrapper, issuing fsyncs required for durability
 *
 * Wrapper around rename, similar to the backend version.
 */
int
durable_rename(const char *oldfile, const char *newfile)
{
    int         fd;

    /*
     * First fsync the old and target path (if it exists), to ensure that they
     * are properly persistent on disk. Syncing the target file is not
     * strictly necessary, but it makes it easier to reason about crashes;
     * because it's then guaranteed that either source or target file exists
     * after a crash.
     */
    if (fsync_fname(oldfile, false) != 0)
        return -1;

    fd = open(newfile, PG_BINARY | O_RDWR, 0);
    if (fd < 0)
    {
        if (errno != ENOENT)
        {
            pg_log_error("could not open file \"%s\": %m", newfile);
            return -1;
        }
    }
    else
    {
        if (fsync(fd) != 0)
        {
            pg_log_error("could not fsync file \"%s\": %m", newfile);
            close(fd);
            return -1;
        }
        close(fd);
    }

    /* Time to do the real deal... */
    if (rename(oldfile, newfile) != 0)
    {
        pg_log_error("could not rename file \"%s\" to \"%s\": %m",
                     oldfile, newfile);
        return -1;
    }

    /*
     * To guarantee renaming the file is persistent, fsync the file with its
     * new name, and its containing directory.
     */
    if (fsync_fname(newfile, false) != 0)
        return -1;

    if (fsync_parent_path(newfile) != 0)
        return -1;

    return 0;
}