index/maildir/maildir-sync.c

	maildir-sync.c revision 8e5fedd9ada47735be8ac0f8af2a66e8528bd776
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen/* Copyright (C) 2004 Timo Sirainen */
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen/*
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen   Here's a description of how we handle Maildir synchronization and
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   it's problems:
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   We want to be as efficient as we can. The most efficient way to
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   check if changes have occurred is to stat() the new/ and cur/
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   directories and uidlist file - if their mtimes haven't changed,
daf029d2a627daa39d05507140f385162828172eTimo Sirainen   there's no changes and we don't need to do anything.
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   Problem 1: Multiple changes can happen within a single second -
9b7eeffb5752b500ac62ba1fd01c4a8c4ada14e9Timo Sirainen   nothing guarantees that once we synced it, someone else didn't just
9b7eeffb5752b500ac62ba1fd01c4a8c4ada14e9Timo Sirainen   then make a modification. Such modifications wouldn't get noticed
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen   until a new modification occurred later.
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen
007d354a674fb3ddf49db160cf050cf61270a1a0Timo Sirainen   Problem 2: Syncing cur/ directory is much more costly than syncing
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   new/. Moving mails from new/ to cur/ will always change mtime of
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   cur/ causing us to sync it as well.
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen   Problem 3: We may not be able to move mail from new/ to cur/
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen   because we're out of quota, or simply because we're accessing a
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   read-only mailbox.
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   MAILDIR_SYNC_SECS
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   -----------------
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   Several checks below use MAILDIR_SYNC_SECS, which should be maximum
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   clock drift between all computers accessing the maildir (eg. via
f923659c0e5298263d80622c99f4dc4132b4675bTimo Sirainen   NFS), rounded up to next second. Our default is 1 second, since
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   everyone should be using NTP.
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen
f923659c0e5298263d80622c99f4dc4132b4675bTimo Sirainen   Note that setting it to 0 works only if there's only one computer
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   accessing the maildir. It's practically impossible to make two
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   clocks _exactly_ synchronized.
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen   It might be possible to only use file server's clock by looking at
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   the atime field, but I don't know how well that would actually work.
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen   cur directory
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   -------------
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen   We have dirty_cur_time variable which is set to cur/ directory's
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   mtime when it's >= time() - MAILDIR_SYNC_SECS and we _think_ we have
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   synchronized the directory.
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen   When dirty_cur_time is non-zero, we don't synchronize the cur/
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen   directory until
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen      a) cur/'s mtime changes
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen      b) opening a mail fails with ENOENT
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen      c) time() > dirty_cur_time + MAILDIR_SYNC_SECS
03f5c621d06d6b6d77a145196c9633a7aa64dc78Timo Sirainen
03f5c621d06d6b6d77a145196c9633a7aa64dc78Timo Sirainen   This allows us to modify the maildir multiple times without having
03f5c621d06d6b6d77a145196c9633a7aa64dc78Timo Sirainen   to sync it at every change. The sync will eventually be done to
03f5c621d06d6b6d77a145196c9633a7aa64dc78Timo Sirainen   make sure we didn't miss any external changes.
03f5c621d06d6b6d77a145196c9633a7aa64dc78Timo Sirainen
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen   The dirty_cur_time is set when:
c0435c854a0e7246373b9752d163095cc4fbe985Timo Sirainen
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen      - we change message flags
ecc81625167ed96c04c02aa190a1ea5baa65b474Timo Sirainen      - 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 "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;
    bool partial;

    time_t last_touch, last_notify;

    struct maildir_uidlist_sync_ctx *uidlist_sync_ctx;
        struct maildir_index_sync_context *index_sync_ctx;
};

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) {
        (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)
{
        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;
    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;
    int ret = 0;

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

    t_push();

    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. */
        t_pop();
        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);
            }
        }
        t_pop();
        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);
        ret = -1;
    }
    t_pop();

    return ret;
}

static int
maildir_stat(struct maildir_mailbox *mbox, const char *path, struct stat *st_r)
{
    if (nfs_safe_stat(path, st_r) == 0)
        return 0;
    if (errno == ENOENT) {
        /* if mailbox gets deleted under us, don't log an error */
        mbox->ibox.mailbox_deleted = TRUE;
        return -1;
    }

    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 *dir;
    DIR *dirp;
    string_t *src, *dest;
    struct dirent *dp;
    enum maildir_uidlist_rec_flag flags;
    unsigned int i = 0, move_count = 0;
    int ret = 1;
    bool move_new, check_touch;

    dir = new_dir ? ctx->new_dir : ctx->cur_dir;
    dirp = opendir(dir);
    if (dirp == NULL) {
        if (errno == ENOENT) {
            ctx->mbox->ibox.mailbox_deleted = TRUE;
            return -1;
        }
        mail_storage_set_critical(storage,
                      "opendir(%s) failed: %m", dir);
        return -1;
    }

    t_push();
    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;

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

        ret = maildir_uidlist_sync_next_pre(ctx->uidlist_sync_ctx,
                            dp->d_name);
        if (ret == 0) {
            /* new file and we couldn't lock uidlist, check this
               later in next sync. */
            if (new_dir)
                ctx->mbox->last_new_mtime = 0;
            else
                ctx->mbox->dirty_cur_time = ioloop_time;
            continue;
        }
        if (ret < 0)
            break;

        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 */
                move_count++;
                ctx->mbox->dirty_cur_time = ioloop_time;
                flags |= MAILDIR_UIDLIST_REC_FLAG_MOVED |
                    MAILDIR_UIDLIST_REC_FLAG_RECENT;
            } else if (ENOTFOUND(errno)) {
                /* someone else moved it already */
                move_count++;
                flags |= MAILDIR_UIDLIST_REC_FLAG_MOVED;
            } else if (ENOSPACE(errno) || errno == EACCES) {
                /* not enough disk space / read-only maildir,
                   leave here */
                flags |= MAILDIR_UIDLIST_REC_FLAG_NEW_DIR |
                    MAILDIR_UIDLIST_REC_FLAG_RECENT;
                move_new = FALSE;
            } else {
                flags |= MAILDIR_UIDLIST_REC_FLAG_NEW_DIR |
                    MAILDIR_UIDLIST_REC_FLAG_RECENT;
                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 */
            if (maildir_fix_duplicate(ctx, dir, dp->d_name) < 0) {
                ret = -1;
                break;
            }
        }
    }

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

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

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

static void
maildir_sync_update_from_header(struct maildir_mailbox *mbox,
                struct mail_index_header *hdr_r)
{
    struct mail_index_view *view;
    const struct mail_index_header *hdr;

    /* open a new view so we get the latest header */
    view = mail_index_view_open(mbox->ibox.index);
    hdr = mail_index_get_header(view);

    /* FIXME: ugly, replace with extension header */
    mbox->last_new_mtime = hdr->sync_size & 0xffffffff;
    mbox->last_dirty_flags = (hdr->sync_size >> 32) &
        (MAILDIR_DIRTY_NEW | MAILDIR_DIRTY_CUR);

    mbox->last_cur_mtime = hdr->sync_stamp;

    if ((mbox->last_dirty_flags & MAILDIR_DIRTY_CUR) != 0 &&
        mbox->dirty_cur_time < mbox->last_cur_mtime)
        mbox->dirty_cur_time = mbox->last_cur_mtime;

    *hdr_r = *hdr;
    mail_index_view_close(&view);
}

static int
maildir_sync_quick_check(struct maildir_mailbox *mbox,
             const char *new_dir, const char *cur_dir,
             bool *new_changed_r, bool *cur_changed_r)
{
    struct index_mailbox *ibox = &mbox->ibox;
    struct mail_index_header hdr;
    struct stat st;
    time_t new_mtime, cur_mtime;

    *new_changed_r = *cur_changed_r = FALSE;

    if (maildir_stat(mbox, new_dir, &st) < 0)
        return -1;
    new_mtime = st.st_mtime;

    if (maildir_stat(mbox, cur_dir, &st) < 0)
        return -1;
    cur_mtime = st.st_mtime;

    /* cur stamp is kept in index, we don't have to sync if
       someone else has done it and updated the index.

       FIXME: For now we're using sync_size field as the new/ dir's stamp.
       Pretty ugly.. */
        maildir_sync_update_from_header(mbox, &hdr);
    if ((mbox->dirty_cur_time == 0 && cur_mtime != mbox->last_cur_mtime) ||
        (new_mtime != mbox->last_new_mtime)) {
        /* check if the index has been updated.. */
        if (mail_index_refresh(ibox->index) < 0) {
            mail_storage_set_index_error(ibox);
            return -1;
        }

        maildir_sync_update_from_header(mbox, &hdr);
    }

    /* If we're removing recent flags, always sync new/ directory if
       it has mails. */
    if (new_mtime != mbox->last_new_mtime ||
        ((mbox->last_dirty_flags & MAILDIR_DIRTY_NEW) != 0 &&
         new_mtime < ioloop_time - MAILDIR_SYNC_SECS) ||
        (!ibox->keep_recent && hdr.recent_messages_count > 0)) {
        *new_changed_r = TRUE;
        mbox->last_new_mtime = new_mtime;

        if (new_mtime < ioloop_time - MAILDIR_SYNC_SECS)
            mbox->last_dirty_flags &= ~MAILDIR_DIRTY_NEW;
        else
            mbox->last_dirty_flags |= MAILDIR_DIRTY_NEW;
    }

    if (cur_mtime != mbox->last_cur_mtime ||
        (mbox->dirty_cur_time != 0 &&
         ioloop_time - mbox->dirty_cur_time > MAILDIR_SYNC_SECS)) {
        /* cur/ changed, or delayed cur/ check */
        *cur_changed_r = TRUE;
        mbox->last_cur_mtime = cur_mtime;

        if (cur_mtime < ioloop_time - MAILDIR_SYNC_SECS) {
            mbox->last_dirty_flags &= ~MAILDIR_DIRTY_CUR;
            mbox->dirty_cur_time = 0;
        } else {
            mbox->last_dirty_flags |= MAILDIR_DIRTY_CUR;
            mbox->dirty_cur_time = cur_mtime;
        }
    }

    return 0;
}

static int maildir_sync_context(struct maildir_sync_context *ctx, bool forced,
                bool sync_last_commit)
{
    bool new_changed, cur_changed, full_rescan = FALSE;
    int ret;

    if (sync_last_commit) {
        new_changed = cur_changed = FALSE;
    } else if (!forced) {
        if (maildir_sync_quick_check(ctx->mbox,
                         ctx->new_dir, ctx->cur_dir,
                         &new_changed, &cur_changed) < 0)
            return -1;

        if (!new_changed && !cur_changed)
            return 1;
    } else {
        new_changed = cur_changed = TRUE;
    }

    /*
       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.
    */

    ctx->partial = !cur_changed;
    ret = maildir_uidlist_sync_init(ctx->mbox->uidlist, ctx->partial,
                    &ctx->uidlist_sync_ctx);
    if (ret <= 0) {
        /* failure / timeout. if forced is TRUE, we could still go
           forward and check only for renamed files, but is it worth
           the trouble? .. */
        return ret;
    }

    if (!ctx->mbox->syncing_commit) {
        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;
        }

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

    if (!ctx->mbox->syncing_commit) {
        /* NOTE: index syncing here might cause a re-sync due to
           files getting lost, so this function might be called
           re-entrantly. FIXME: and that breaks in
           maildir_uidlist_sync_deinit() */
        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)
            full_rescan = TRUE;

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

    ret = maildir_uidlist_sync_deinit(&ctx->uidlist_sync_ctx);
    return ret < 0 ? -1 : (full_rescan ? 0 : 1);
}

int maildir_storage_sync_force(struct maildir_mailbox *mbox)
{
        struct maildir_sync_context *ctx;
    int ret;

    ctx = maildir_sync_context_new(mbox);
    ret = maildir_sync_context(ctx, TRUE, FALSE);
    maildir_sync_deinit(ctx);
    return ret < 0 ? -1 : 0;
}

int maildir_sync_last_commit(struct maildir_mailbox *mbox)
{
        struct maildir_sync_context *ctx;
    int ret;

    if (mbox->ibox.commit_log_file_seq == 0)
        return 0;

    ctx = maildir_sync_context_new(mbox);
    ret = maildir_sync_context(ctx, FALSE, TRUE);
    maildir_sync_deinit(ctx);
    return ret < 0 ? -1 : 0;
}

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;
    int ret = 0;

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

    if ((flags & MAILBOX_SYNC_FLAG_FAST) == 0 ||
        mbox->ibox.sync_last_check + MAILBOX_FULL_SYNC_INTERVAL <=
        ioloop_time) {
        mbox->ibox.sync_last_check = ioloop_time;

        ctx = maildir_sync_context_new(mbox);
        ret = maildir_sync_context(ctx, FALSE, FALSE);
        maildir_sync_deinit(ctx);

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

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

    return index_mailbox_sync_init(box, flags, ret < 0);
}

int maildir_sync_is_synced(struct maildir_mailbox *mbox)
{
    const char *new_dir, *cur_dir;
    bool new_changed, cur_changed;
    int ret;

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

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