index/maildir/maildir-sync.c

	maildir-sync.c revision 305465bb1a4c5d90c4b4e2c2790eb05fa4ebc41e
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch/* Copyright (c) 2004-2009 Dovecot authors, see the included COPYING file */
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch/*
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   Here's a description of how we handle Maildir synchronization and
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   it's problems:
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   We want to be as efficient as we can. The most efficient way to
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   check if changes have occurred is to stat() the new/ and cur/
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   directories and uidlist file - if their mtimes haven't changed,
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   there's no changes and we don't need to do anything.
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   Problem 1: Multiple changes can happen within a single second -
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   nothing guarantees that once we synced it, someone else didn't just
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   then make a modification. Such modifications wouldn't get noticed
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   until a new modification occurred later.
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   Problem 2: Syncing cur/ directory is much more costly than syncing
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   new/. Moving mails from new/ to cur/ will always change mtime of
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   cur/ causing us to sync it as well.
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   Problem 3: We may not be able to move mail from new/ to cur/
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   because we're out of quota, or simply because we're accessing a
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   read-only mailbox.
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   MAILDIR_SYNC_SECS
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   -----------------
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   Several checks below use MAILDIR_SYNC_SECS, which should be maximum
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   clock drift between all computers accessing the maildir (eg. via
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   NFS), rounded up to next second. Our default is 1 second, since
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   everyone should be using NTP.
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch
96fbbe9c6fd04bca25770020bf94eec50888bc9bStephan Bosch   Note that setting it to 0 works only if there's only one computer
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   accessing the maildir. It's practically impossible to make two
659f431cb56f26ec07e308db4d6c563c0eaa5300Stephan Bosch   clocks _exactly_ synchronized.
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   It might be possible to only use file server's clock by looking at
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   the atime field, but I don't know how well that would actually work.
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   cur directory
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   -------------
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch
659f431cb56f26ec07e308db4d6c563c0eaa5300Stephan Bosch   We have dirty_cur_time variable which is set to cur/ directory's
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   mtime when it's >= time() - MAILDIR_SYNC_SECS and we _think_ we have
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   synchronized the directory.
96fbbe9c6fd04bca25770020bf94eec50888bc9bStephan Bosch
620b60321d04006b32a116824d49b88b61be7131Stephan Bosch   When dirty_cur_time is non-zero, we don't synchronize the cur/
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch   directory until
620b60321d04006b32a116824d49b88b61be7131Stephan Bosch
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch      a) cur/'s mtime changes
f9511e684858bf5f6ac77ab12254b85b737beae8Stephan Bosch      b) opening a mail fails with ENOENT
      c) time() > dirty_cur_time + MAILDIR_SYNC_SECS

   This allows us to modify the maildir multiple times without having
   to sync it at every change. The sync will eventually be done to
   make sure we didn't miss any external changes.

   The dirty_cur_time is set when:

      - we change message flags
      - we expunge messages
      - we move mail from new/ to cur/
      - we sync cur/ directory and it's mtime is >= time() - MAILDIR_SYNC_SECS

   It's unset when we do the final syncing, ie. when mtime is
   older than time() - MAILDIR_SYNC_SECS.

   new directory
   -------------

   If new/'s mtime is >= time() - MAILDIR_SYNC_SECS, always synchronize
   it. dirty_cur_time-like feature might save us a few syncs, but
   that might break a client which saves a mail in one connection and
   tries to fetch it in another one. new/ directory is almost always
   empty, so syncing it should be very fast anyway. Actually this can
   still happen if we sync only new/ dir while another client is also
   moving mails from it to cur/ - it takes us a while to see them.
   That's pretty unlikely to happen however, and only way to fix it
   would be to always synchronize cur/ after new/.

   Normally we move all mails from new/ to cur/ whenever we sync it. If
   it's not possible for some reason, we mark the mail with "probably
   exists in new/ directory" flag.

   If rename() still fails because of ENOSPC or EDQUOT, we still save
   the flag changes in index with dirty-flag on. When moving the mail
   to cur/ directory, or when we notice it's already moved there, we
   apply the flag changes to the filename, rename it and remove the
   dirty flag. If there's dirty flags, this should be tried every time
   after expunge or when closing the mailbox.

   uidlist
   -------

   This file contains UID <-> filename mappings. It's updated only when
   new mail arrives, so it may contain filenames that have already been
   deleted. Updating is done by getting uidlist.lock file, writing the
   whole uidlist into it and rename()ing it over the old uidlist. This
   means there's no need to lock the file for reading.

   Whenever uidlist is rewritten, it's mtime must be larger than the old
   one's. Use utime() before rename() if needed. Note that inode checking
   wouldn't have been sufficient as inode numbers can be reused.

   This file is usually read the first time you need to know filename for
   given UID. After that it's not re-read unless new mails come that we
   don't know about.

   broken clients
   --------------

   Originally the middle identifier in Maildir filename was specified
   only as <process id>_<delivery counter>. That however created a
   problem with randomized PIDs which made it possible that the same
   PID was reused within one second.

   So if within one second a mail was delivered, MUA moved it to cur/
   and another mail was delivered by a new process using same PID as
   the first one, we likely ended up overwriting the first mail when
   the second mail was moved over it.

   Nowadays everyone should be giving a bit more specific identifier,
   for example include microseconds in it which Dovecot does.

   There's a simple way to prevent this from happening in some cases:
   Don't move the mail from new/ to cur/ if it's mtime is >= time() -
   MAILDIR_SYNC_SECS. The second delivery's link() call then fails
   because the file is already in new/, and it will then use a
   different filename. There's a few problems with this however:

      - it requires extra stat() call which is unneeded extra I/O
      - another MUA might still move the mail to cur/
      - if first file's flags are modified by either Dovecot or another
        MUA, it's moved to cur/ (you _could_ just do the dirty-flagging
    but that'd be ugly)

   Because this is useful only for very few people and it requires
   extra I/O, I decided not to implement this. It should be however
   quite easy to do since we need to be able to deal with files in new/
   in any case.

   It's also possible to never accidentally overwrite a mail by using
   link() + unlink() rather than rename(). This however isn't very
   good idea as it introduces potential race conditions when multiple
   clients are accessing the mailbox:

   Trying to move the same mail from new/ to cur/ at the same time:

      a) Client 1 uses slightly different filename than client 2,
         for example one sets read-flag on but the other doesn't.
     You have the same mail duplicated now.

      b) Client 3 sees the mail between Client 1's and 2's link() calls
         and changes it's flag. You have the same mail duplicated now.

   And it gets worse when they're unlink()ing in cur/ directory:

      c) Client 1 changes mails's flag and client 2 changes it back
         between 1's link() and unlink(). The mail is now expunged.

      d) If you try to deal with the duplicates by unlink()ing another
         one of them, you might end up unlinking both of them.

   So, what should we do then if we notice a duplicate? First of all,
   it might not be a duplicate at all, readdir() might have just
   returned it twice because it was just renamed. What we should do is
   create a completely new base name for it and rename() it to that.
   If the call fails with ENOENT, it only means that it wasn't a
   duplicate after all.
*/

#include "lib.h"
#include "ioloop.h"
#include "array.h"
#include "buffer.h"
#include "hash.h"
#include "str.h"
#include "eacces-error.h"
#include "nfs-workarounds.h"
#include "maildir-storage.h"
#include "maildir-uidlist.h"
#include "maildir-filename.h"
#include "maildir-sync.h"

#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>
#include <unistd.h>
#include <dirent.h>
#include <sys/stat.h>

#define MAILDIR_FILENAME_FLAG_FOUND 128

/* When rename()ing many files from new/ to cur/, it's possible that next
   readdir() skips some files. we don't of course wish to lose them, so we
   go and rescan the new/ directory again from beginning until no files are
   left. This value is just an optimization to avoid checking the directory
   twice unneededly. usually only NFS is the problem case. 1 is the safest
   bet here, but I guess 5 will do just fine too. */
#define MAILDIR_RENAME_RESCAN_COUNT 5

/* This is mostly to avoid infinite looping when rename() destination already
   exists as the hard link of the file itself. */
#define MAILDIR_SCAN_DIR_MAX_COUNT 5

#define DUPE_LINKS_DELETE_SECS 30

struct maildir_sync_context {
        struct maildir_mailbox *mbox;
    const char *new_dir, *cur_dir;

    enum mailbox_sync_flags flags;
    time_t last_touch, last_notify;

    struct maildir_uidlist_sync_ctx *uidlist_sync_ctx;
    struct maildir_index_sync_context *index_sync_ctx;

    unsigned int partial:1;
    unsigned int locked:1;
};

void maildir_sync_notify(struct maildir_sync_context *ctx)
{
    time_t now;

    if (ctx == NULL) {
        /* we got here from maildir-save.c. it has no
           maildir_sync_context,  */
        return;
    }

    now = time(NULL);
    if (now - ctx->last_touch > MAILDIR_LOCK_TOUCH_SECS && ctx->locked) {
        (void)maildir_uidlist_lock_touch(ctx->mbox->uidlist);
        ctx->last_touch = now;
    }
    if (now - ctx->last_notify > MAIL_STORAGE_STAYALIVE_SECS) {
        struct mailbox *box = &ctx->mbox->ibox.box;

        if (box->storage->callbacks->notify_ok != NULL) {
            box->storage->callbacks->
                notify_ok(box, "Hang in there..",
                      box->storage->callback_context);
        }
        ctx->last_notify = now;
    }
}

static struct maildir_sync_context *
maildir_sync_context_new(struct maildir_mailbox *mbox,
             enum mailbox_sync_flags flags)
{
        struct maildir_sync_context *ctx;

    ctx = t_new(struct maildir_sync_context, 1);
    ctx->mbox = mbox;
    ctx->new_dir = t_strconcat(mbox->path, "/new", NULL);
    ctx->cur_dir = t_strconcat(mbox->path, "/cur", NULL);
    ctx->last_touch = ioloop_time;
    ctx->last_notify = ioloop_time;
    ctx->flags = flags;
    return ctx;
}

static void maildir_sync_deinit(struct maildir_sync_context *ctx)
{
    if (ctx->uidlist_sync_ctx != NULL)
        (void)maildir_uidlist_sync_deinit(&ctx->uidlist_sync_ctx);
    if (ctx->index_sync_ctx != NULL) {
        (void)maildir_sync_index_finish(&ctx->index_sync_ctx,
                        TRUE, FALSE);
    }
}

static int maildir_fix_duplicate(struct maildir_sync_context *ctx,
                 const char *dir, const char *fname2)
{
    const char *fname1, *path1, *path2;
    const char *new_fname, *new_path;
    struct stat st1, st2;

    fname1 = maildir_uidlist_sync_get_full_filename(ctx->uidlist_sync_ctx,
                            fname2);
    i_assert(fname1 != NULL);

    path1 = t_strconcat(dir, "/", fname1, NULL);
    path2 = t_strconcat(dir, "/", fname2, NULL);

    if (stat(path1, &st1) < 0 || stat(path2, &st2) < 0) {
        /* most likely the files just don't exist anymore.
           don't really care about other errors much. */
        return 0;
    }
    if (st1.st_ino == st2.st_ino &&
        CMP_DEV_T(st1.st_dev, st2.st_dev)) {
        /* Files are the same. this means either a race condition
           between stat() calls, or that the files were link()ed. */
        if (st1.st_nlink > 1 && st2.st_nlink == st1.st_nlink &&
            st1.st_ctime == st2.st_ctime &&
            st1.st_ctime < ioloop_time - DUPE_LINKS_DELETE_SECS) {
            /* The file has hard links and it hasn't had any
               changes (such as renames) for a while, so this
               isn't a race condition.

               rename()ing one file on top of the other would fix
               this safely, except POSIX decided that rename()
               doesn't work that way. So we'll have unlink() one
               and hope that another process didn't just decide to
               unlink() the other (uidlist lock prevents this from
               happening) */
            if (unlink(path2) == 0)
                i_warning("Unlinked a duplicate: %s", path2);
            else {
                mail_storage_set_critical(
                    &ctx->mbox->storage->storage,
                    "unlink(%s) failed: %m", path2);
            }
        }
        return 0;
    }

    new_fname = maildir_filename_generate();
    new_path = t_strconcat(ctx->mbox->path, "/new/", new_fname, NULL);

    if (rename(path2, new_path) == 0)
        i_warning("Fixed a duplicate: %s -> %s", path2, new_fname);
    else if (errno != ENOENT) {
        mail_storage_set_critical(&ctx->mbox->storage->storage,
            "Couldn't fix a duplicate: rename(%s, %s) failed: %m",
            path2, new_path);
        return -1;
    }
    return 0;
}

static int
maildir_stat(struct maildir_mailbox *mbox, const char *path, struct stat *st_r)
{
    int i;

    for (i = 0;; i++) {
        if (nfs_safe_stat(path, st_r) == 0)
            return 0;
        if (errno != ENOENT || i == MAILDIR_DELETE_RETRY_COUNT)
            break;

        if (!maildir_set_deleted(mbox))
            return -1;
        /* try again */
    }

    mail_storage_set_critical(mbox->ibox.box.storage,
                  "stat(%s) failed: %m", path);
    return -1;
}

static int maildir_scan_dir(struct maildir_sync_context *ctx, bool new_dir)
{
    struct mail_storage *storage = &ctx->mbox->storage->storage;
    const char *path;
    DIR *dirp;
    string_t *src, *dest;
    struct dirent *dp;
    struct stat st;
    enum maildir_uidlist_rec_flag flags;
    unsigned int i = 0, move_count = 0;
    time_t now;
    int ret = 1;
    bool move_new, check_touch, dir_changed = FALSE;

    path = new_dir ? ctx->new_dir : ctx->cur_dir;
    for (i = 0;; i++) {
        dirp = opendir(path);
        if (dirp != NULL)
            break;

        if (errno != ENOENT || i == MAILDIR_DELETE_RETRY_COUNT) {
            if (errno == EACCES) {
                mail_storage_set_critical(storage, "%s",
                    eacces_error_get("opendir", path));
            } else {
                mail_storage_set_critical(storage,
                    "opendir(%s) failed: %m", path);
            }
            return -1;
        }

        if (!maildir_set_deleted(ctx->mbox))
            return -1;
        /* try again */
    }

#ifdef HAVE_DIRFD
    if (fstat(dirfd(dirp), &st) < 0) {
        mail_storage_set_critical(storage,
            "fstat(%s) failed: %m", path);
        (void)closedir(dirp);
        return -1;
    }
#else
    if (maildir_stat(ctx->mbox, path, &st) < 0) {
        (void)closedir(dirp);
        return -1;
    }
#endif

    now = time(NULL);
    if (new_dir) {
        ctx->mbox->maildir_hdr.new_check_time = now;
        ctx->mbox->maildir_hdr.new_mtime = st.st_mtime;
        ctx->mbox->maildir_hdr.new_mtime_nsecs = ST_MTIME_NSEC(st);
    } else {
        ctx->mbox->maildir_hdr.cur_check_time = now;
        ctx->mbox->maildir_hdr.cur_mtime = st.st_mtime;
        ctx->mbox->maildir_hdr.cur_mtime_nsecs = ST_MTIME_NSEC(st);
    }

    src = t_str_new(1024);
    dest = t_str_new(1024);

    move_new = new_dir && !mailbox_is_readonly(&ctx->mbox->ibox.box) &&
        !ctx->mbox->ibox.keep_recent && ctx->locked;

    errno = 0;
    for (; (dp = readdir(dirp)) != NULL; errno = 0) {
        if (dp->d_name[0] == '.')
            continue;

        check_touch = FALSE;
        flags = 0;
        if (move_new) {
            str_truncate(src, 0);
            str_truncate(dest, 0);
            str_printfa(src, "%s/%s", ctx->new_dir, dp->d_name);
            str_printfa(dest, "%s/%s", ctx->cur_dir, dp->d_name);
            if (strchr(dp->d_name, MAILDIR_INFO_SEP) == NULL) {
                str_append(dest, MAILDIR_FLAGS_FULL_SEP);
            }
            if (rename(str_c(src), str_c(dest)) == 0) {
                /* we moved it - it's \Recent for us */
                dir_changed = TRUE;
                move_count++;
                flags |= MAILDIR_UIDLIST_REC_FLAG_MOVED |
                    MAILDIR_UIDLIST_REC_FLAG_RECENT;
            } else if (ENOTFOUND(errno)) {
                /* someone else moved it already */
                dir_changed = TRUE;
                move_count++;
                flags |= MAILDIR_UIDLIST_REC_FLAG_MOVED |
                    MAILDIR_UIDLIST_REC_FLAG_RECENT;
            } else if (ENOSPACE(errno) || errno == EACCES) {
                /* not enough disk space / read-only maildir,
                   leave here */
                flags |= MAILDIR_UIDLIST_REC_FLAG_NEW_DIR;
                move_new = FALSE;
            } else {
                flags |= MAILDIR_UIDLIST_REC_FLAG_NEW_DIR;
                mail_storage_set_critical(storage,
                    "rename(%s, %s) failed: %m",
                    str_c(src), str_c(dest));
            }
            if ((move_count % MAILDIR_SLOW_MOVE_COUNT) == 0)
                maildir_sync_notify(ctx);
        } else if (new_dir) {
            flags |= MAILDIR_UIDLIST_REC_FLAG_NEW_DIR |
                MAILDIR_UIDLIST_REC_FLAG_RECENT;
        }

        i++;
        if ((i % MAILDIR_SLOW_CHECK_COUNT) == 0)
            maildir_sync_notify(ctx);

        ret = maildir_uidlist_sync_next(ctx->uidlist_sync_ctx,
                        dp->d_name, flags);
        if (ret <= 0) {
            if (ret < 0)
                break;

            /* possibly duplicate - try fixing it */
            T_BEGIN {
                ret = maildir_fix_duplicate(ctx, path,
                                dp->d_name);
            } T_END;
            if (ret < 0)
                break;
        }
    }

    if (errno != 0) {
        mail_storage_set_critical(storage,
                      "readdir(%s) failed: %m", path);
        ret = -1;
    }

    if (closedir(dirp) < 0) {
        mail_storage_set_critical(storage,
                      "closedir(%s) failed: %m", path);
        ret = -1;
    }

    if (dir_changed) {
        /* save the exact new times. the new mtimes should be >=
           "now", but just in case something weird happens and mtime
           doesn't update, use "now". */
        if (stat(ctx->new_dir, &st) == 0) {
            ctx->mbox->maildir_hdr.new_check_time =
                I_MAX(st.st_mtime, now);
            ctx->mbox->maildir_hdr.new_mtime = st.st_mtime;
            ctx->mbox->maildir_hdr.new_mtime_nsecs =
                ST_MTIME_NSEC(st);
        }
        if (stat(ctx->cur_dir, &st) == 0) {
            ctx->mbox->maildir_hdr.new_check_time =
                I_MAX(st.st_mtime, now);
            ctx->mbox->maildir_hdr.cur_mtime = st.st_mtime;
            ctx->mbox->maildir_hdr.cur_mtime_nsecs =
                ST_MTIME_NSEC(st);
        }
    }

    return ret < 0 ? -1 :
        (move_count <= MAILDIR_RENAME_RESCAN_COUNT ? 0 : 1);
}

int maildir_sync_header_refresh(struct maildir_mailbox *mbox)
{
    const void *data;
    size_t data_size;

    if (mail_index_refresh(mbox->ibox.index) < 0) {
        mail_storage_set_index_error(&mbox->ibox);
        return -1;
    }

    mail_index_get_header_ext(mbox->ibox.view, mbox->maildir_ext_id,
                  &data, &data_size);
    if (data_size == 0) {
        /* doesn't exist */
        return 0;
    }

    memcpy(&mbox->maildir_hdr, data,
           I_MIN(sizeof(mbox->maildir_hdr), data_size));
    return 0;
}

static int maildir_sync_quick_check(struct maildir_mailbox *mbox, bool undirty,
                    const char *new_dir, const char *cur_dir,
                    bool *new_changed_r, bool *cur_changed_r)
{
#define DIR_DELAYED_REFRESH(hdr, name) \
    ((hdr)->name ## _check_time <= \
        (hdr)->name ## _mtime + MAILDIR_SYNC_SECS && \
     (undirty || \
      (time_t)(hdr)->name ## _check_time < ioloop_time - MAILDIR_SYNC_SECS))

#define DIR_MTIME_CHANGED(st, hdr, name) \
    ((st).st_mtime != (time_t)(hdr)->name ## _mtime || \
     !ST_NTIMES_EQUAL(ST_MTIME_NSEC(st), (hdr)->name ## _mtime_nsecs))

    struct maildir_index_header *hdr = &mbox->maildir_hdr;
    struct stat new_st, cur_st;
    bool refreshed = FALSE, check_new = FALSE, check_cur = FALSE;

    if (mbox->maildir_hdr.new_mtime == 0) {
        if (maildir_sync_header_refresh(mbox) < 0)
            return -1;
        if (mbox->maildir_hdr.new_mtime == 0) {
            /* first sync */
            *new_changed_r = *cur_changed_r = TRUE;
            return 0;
        }
    }

    *new_changed_r = *cur_changed_r = FALSE;

    /* try to avoid stat()ing by first checking delayed changes */
    if (DIR_DELAYED_REFRESH(hdr, new) ||
        (DIR_DELAYED_REFRESH(hdr, cur) &&
         !mbox->storage->set->maildir_very_dirty_syncs)) {
        /* refresh index and try again */
        if (maildir_sync_header_refresh(mbox) < 0)
            return -1;
        refreshed = TRUE;

        if (DIR_DELAYED_REFRESH(hdr, new))
            *new_changed_r = TRUE;
        if (DIR_DELAYED_REFRESH(hdr, cur) &&
            !mbox->storage->set->maildir_very_dirty_syncs)
            *cur_changed_r = TRUE;
        if (*new_changed_r && *cur_changed_r)
            return 0;
    }

    if (!*new_changed_r) {
        if (maildir_stat(mbox, new_dir, &new_st) < 0)
            return -1;
        check_new = TRUE;
    }
    if (!*cur_changed_r) {
        if (maildir_stat(mbox, cur_dir, &cur_st) < 0)
            return -1;
        check_cur = TRUE;
    }

    for (;;) {
        if (check_new)
            *new_changed_r = DIR_MTIME_CHANGED(new_st, hdr, new);
        if (check_cur)
            *cur_changed_r = DIR_MTIME_CHANGED(cur_st, hdr, cur);

        if ((!*new_changed_r && !*cur_changed_r) || refreshed)
            break;

        /* refresh index and try again */
        if (maildir_sync_header_refresh(mbox) < 0)
            return -1;
        refreshed = TRUE;
    }

    return 0;
}

static void maildir_sync_update_next_uid(struct maildir_mailbox *mbox)
{
    const struct mail_index_header *hdr;
    uint32_t uid_validity, next_uid;

    hdr = mail_index_get_header(mbox->ibox.view);
    if (hdr->uid_validity == 0)
        return;

    uid_validity = maildir_uidlist_get_uid_validity(mbox->uidlist);
    next_uid = maildir_uidlist_get_next_uid(mbox->uidlist);

    if (uid_validity == hdr->uid_validity || uid_validity == 0) {
        /* make sure uidlist's next_uid is at least as large as
           index file's. typically this happens only if uidlist gets
           deleted. */
        maildir_uidlist_set_uid_validity(mbox->uidlist,
                         hdr->uid_validity);
        maildir_uidlist_set_next_uid(mbox->uidlist,
                         hdr->next_uid, FALSE);
    }
}

static bool move_recent_messages(struct maildir_sync_context *ctx)
{
    const struct mail_index_header *hdr;

    if (ctx->mbox->ibox.keep_recent)
        return FALSE;

    (void)maildir_uidlist_refresh(ctx->mbox->uidlist);

    /* if there are files in new/, we'll need to move them. we'll check
       this by checking if we have any recent messages */
    hdr = mail_index_get_header(ctx->mbox->ibox.view);
    return hdr->first_recent_uid <
        maildir_uidlist_get_next_uid(ctx->mbox->uidlist);
}

static int maildir_sync_get_changes(struct maildir_sync_context *ctx,
                    bool *new_changed_r, bool *cur_changed_r)
{
    enum mail_index_sync_flags flags = 0;
    bool undirty = (ctx->flags & MAILBOX_SYNC_FLAG_FULL_READ) != 0;

    if (maildir_sync_quick_check(ctx->mbox, undirty,
                     ctx->new_dir, ctx->cur_dir,
                     new_changed_r, cur_changed_r) < 0)
        return -1;

    if (*new_changed_r || *cur_changed_r)
        return 1;

    if (move_recent_messages(ctx)) {
        *new_changed_r = TRUE;
        return 1;
    }

    if (!ctx->mbox->ibox.keep_recent)
        flags |= MAIL_INDEX_SYNC_FLAG_DROP_RECENT;

    return mail_index_sync_have_any(ctx->mbox->ibox.index, flags) ? 1 : 0;
}

static int maildir_sync_context(struct maildir_sync_context *ctx, bool forced,
                uint32_t *find_uid, bool *lost_files_r)
{
    enum maildir_uidlist_sync_flags sync_flags;
    enum maildir_uidlist_rec_flag flags;
    bool new_changed, cur_changed, lock_failure;
    int ret;

    *lost_files_r = FALSE;

    if (forced)
        new_changed = cur_changed = TRUE;
    else {
        ret = maildir_sync_get_changes(ctx, &new_changed, &cur_changed);
        if (ret <= 0)
            return ret;
    }

    /*
       Locking, locking, locking.. Wasn't maildir supposed to be lockless?

       We can get here either as beginning a real maildir sync, or when
       committing changes to maildir but a file was lost (maybe renamed).

       So, we're going to need two locks. One for index and one for
       uidlist. To avoid deadlocking do the uidlist lock always first.

       uidlist is needed only for figuring out UIDs for newly seen files,
       so theoretically we wouldn't need to lock it unless there are new
       files. It has a few problems though, assuming the index lock didn't
       already protect it (eg. in-memory indexes):

       1. Just because you see a new file which doesn't exist in uidlist
       file, doesn't mean that the file really exists anymore, or that
       your readdir() lists all new files. Meaning that this is possible:

         A: opendir(), readdir() -> new file ...
         -- new files are written to the maildir --
         B: opendir(), readdir() -> new file, lock uidlist,
        readdir() -> another new file, rewrite uidlist, unlock
         A: ... lock uidlist, readdir() -> nothing left, rewrite uidlist,
        unlock

       The second time running A didn't see the two new files. To handle
       this correctly, it must not remove the new unseen files from
       uidlist. This is possible to do, but adds extra complexity.

       2. If another process is rename()ing files while we are
       readdir()ing, it's possible that readdir() never lists some files,
       causing Dovecot to assume they were expunged. In next sync they
       would show up again, but client could have already been notified of
       that and they would show up under new UIDs, so the damage is
       already done.

       Both of the problems can be avoided if we simply lock the uidlist
       before syncing and keep it until sync is finished. Typically this
       would happen in any case, as there is the index lock..

       The second case is still a problem with external changes though,
       because maildir doesn't require any kind of locking. Luckily this
       problem rarely happens except under high amount of modifications.
    */

    if (!cur_changed) {
        ctx->partial = TRUE;
        sync_flags = MAILDIR_UIDLIST_SYNC_PARTIAL;
    } else {
        ctx->partial = FALSE;
        sync_flags = 0;
        if (forced)
            sync_flags |= MAILDIR_UIDLIST_SYNC_FORCE;
        if ((ctx->flags & MAILBOX_SYNC_FLAG_FAST) != 0)
            sync_flags |= MAILDIR_UIDLIST_SYNC_TRYLOCK;
    }
    ret = maildir_uidlist_sync_init(ctx->mbox->uidlist, sync_flags,
                    &ctx->uidlist_sync_ctx);
    lock_failure = ret <= 0;
    if (ret <= 0) {
        struct mail_storage *storage = ctx->mbox->ibox.box.storage;

        if (ret == 0) {
            /* timeout */
            return 0;
        }
        /* locking failed. sync anyway without locking so that it's
           possible to expunge messages when out of quota. */
        if (forced) {
            /* we're already forcing a sync, we're trying to find
               a message that was probably already expunged, don't
               loop for a long time trying to find it. */
            return -1;
        }
        ret = maildir_uidlist_sync_init(ctx->mbox->uidlist, sync_flags |
                        MAILDIR_UIDLIST_SYNC_NOLOCK,
                        &ctx->uidlist_sync_ctx);
        if (ret <= 0) {
            i_assert(ret != 0);
            return -1;
        }

        if (storage->callbacks->notify_no != NULL) {
            storage->callbacks->notify_no(&ctx->mbox->ibox.box,
                "Internal mailbox synchronization failure, "
                "showing only old mails.",
                storage->callback_context);
        }
    }
    ctx->locked = maildir_uidlist_is_locked(ctx->mbox->uidlist);
    if (!ctx->locked)
        ctx->partial = TRUE;

    if (!ctx->mbox->syncing_commit && (ctx->locked || lock_failure)) {
        if (maildir_sync_index_begin(ctx->mbox, ctx,
                         &ctx->index_sync_ctx) < 0)
            return -1;
    }

    if (new_changed || cur_changed) {
        /* if we're going to check cur/ dir our current logic requires
           that new/ dir is checked as well. it's a good idea anyway. */
        unsigned int count = 0;

        while ((ret = maildir_scan_dir(ctx, TRUE)) > 0) {
            /* rename()d at least some files, which might have
               caused some other files to be missed. check again
               (see MAILDIR_RENAME_RESCAN_COUNT). */
            if (++count > MAILDIR_SCAN_DIR_MAX_COUNT)
                break;
        }
        if (ret < 0)
            return -1;

        if (cur_changed) {
            if (maildir_scan_dir(ctx, FALSE) < 0)
                return -1;
        }

        maildir_sync_update_next_uid(ctx->mbox);

        /* finish uidlist syncing, but keep it still locked */
        maildir_uidlist_sync_finish(ctx->uidlist_sync_ctx);
    }

    if (!ctx->locked) {
        /* make sure we sync the maildir later */
        ctx->mbox->maildir_hdr.new_mtime = 0;
        ctx->mbox->maildir_hdr.cur_mtime = 0;
    }

    if (ctx->index_sync_ctx != NULL) {
        /* NOTE: index syncing here might cause a re-sync due to
           files getting lost, so this function might be called
           re-entrantly. */
        ret = maildir_sync_index(ctx->index_sync_ctx, ctx->partial);
        if (maildir_sync_index_finish(&ctx->index_sync_ctx,
                          ret < 0, FALSE) < 0)
            return -1;

        if (ret < 0)
            return -1;
        if (ret == 0)
            *lost_files_r = TRUE;

        i_assert(maildir_uidlist_is_locked(ctx->mbox->uidlist) ||
             lock_failure);
    }

    if (find_uid != NULL && *find_uid != 0) {
        if (maildir_uidlist_lookup_nosync(ctx->mbox->uidlist, *find_uid,
                          &flags) == NULL) {
            /* UID is expunged */
            *find_uid = 0;
        } else if ((flags & MAILDIR_UIDLIST_REC_FLAG_NONSYNCED) == 0) {
            /* we didn't find it, possibly expunged? */
            *find_uid = 0;
        }
    }

    return maildir_uidlist_sync_deinit(&ctx->uidlist_sync_ctx);
}

int maildir_storage_sync_force(struct maildir_mailbox *mbox, uint32_t uid)
{
        struct maildir_sync_context *ctx;
    bool lost_files;
    int ret;

    T_BEGIN {
        ctx = maildir_sync_context_new(mbox, MAILBOX_SYNC_FLAG_FAST);
        ret = maildir_sync_context(ctx, TRUE, &uid, &lost_files);
        maildir_sync_deinit(ctx);
    } T_END;

    if (uid != 0) {
        /* maybe it's expunged. check again. */
        T_BEGIN {
            ctx = maildir_sync_context_new(mbox, 0);
            ret = maildir_sync_context(ctx, TRUE, NULL,
                           &lost_files);
            maildir_sync_deinit(ctx);
        } T_END;
    }
    return ret;
}

struct mailbox_sync_context *
maildir_storage_sync_init(struct mailbox *box, enum mailbox_sync_flags flags)
{
    struct maildir_mailbox *mbox = (struct maildir_mailbox *)box;
    struct maildir_sync_context *ctx;
    bool lost_files, force_resync;
    int ret = 0;

    if (!box->opened)
        index_storage_mailbox_open(&mbox->ibox);

    force_resync = (flags & MAILBOX_SYNC_FLAG_FORCE_RESYNC) != 0;
    if (index_mailbox_want_full_sync(&mbox->ibox, flags)) {
        T_BEGIN {
            ctx = maildir_sync_context_new(mbox, flags);
            ret = maildir_sync_context(ctx, force_resync, NULL,
                           &lost_files);
            maildir_sync_deinit(ctx);
        } T_END;

        i_assert(!maildir_uidlist_is_locked(mbox->uidlist) ||
             mbox->ibox.keep_locked);

        if (lost_files) {
            /* lost some files from new/, see if thery're in cur/ */
            ret = maildir_storage_sync_force(mbox, 0);
        }
    }

    if (mbox->storage->set->maildir_very_dirty_syncs) {
        struct mail_index_view_sync_ctx *sync_ctx;
        bool b;

        if (mbox->flags_view == NULL) {
            mbox->flags_view =
                mail_index_view_open(mbox->ibox.index);
        }
        sync_ctx = mail_index_view_sync_begin(mbox->flags_view,
                MAIL_INDEX_VIEW_SYNC_FLAG_FIX_INCONSISTENT);
        if (mail_index_view_sync_commit(&sync_ctx, &b) < 0) {
            mail_storage_set_index_error(&mbox->ibox);
            ret = -1;
        }
        /* make sure the map stays in private memory */
        if (mbox->flags_view->map->refcount > 1) {
            struct mail_index_map *map;

            map = mail_index_map_clone(mbox->flags_view->map);
            mail_index_unmap(&mbox->flags_view->map);
            mbox->flags_view->map = map;
        }
        mail_index_record_map_move_to_private(mbox->flags_view->map);
        mail_index_map_move_to_memory(mbox->flags_view->map);
        maildir_uidlist_set_all_nonsynced(mbox->uidlist);
    }
    return index_mailbox_sync_init(box, flags, ret < 0);
}

int maildir_sync_is_synced(struct maildir_mailbox *mbox)
{
    bool new_changed, cur_changed;
    int ret;

    T_BEGIN {
        const char *new_dir, *cur_dir;

        new_dir = t_strconcat(mbox->path, "/new", NULL);
        cur_dir = t_strconcat(mbox->path, "/cur", NULL);

        ret = maildir_sync_quick_check(mbox, FALSE, new_dir, cur_dir,
                           &new_changed, &cur_changed);
    } T_END;
    return ret < 0 ? -1 : (!new_changed && !cur_changed);
}