maildir-sync.c revision ead79af955bccc360de08c150918474c1809748e
/* Copyright (C) 2004 Timo Sirainen */
/*
Here's a description of how we handle Maildir synchronization and
it's problems:
We want to be as efficient as we can. The most efficient way to
check if changes have occured is to stat() the new/ and cur/
directories and uidlist file - if their mtimes haven't changed,
there's no changes and we don't need to do anything.
Problem 1: Multiple changes can happen within a single second -
nothing guarantees that once we synced it, someone else didn't just
then make a modification. Such modifications wouldn't get noticed
until a new modification occured later.
Problem 2: Syncing cur/ directory is much more costly than syncing
new/. Moving mails from new/ to cur/ will always change mtime of
cur/ causing us to sync it as well.
Problem 3: We may not be able to move mail from new/ to cur/
because we're out of quota, or simply because we're accessing a
read-only mailbox.
MAILDIR_SYNC_SECS
-----------------
Several checks below use MAILDIR_SYNC_SECS, which should be maximum
clock drift between all computers accessing the maildir (eg. via
NFS), rounded up to next second. Our default is 1 second, since
everyone should be using NTP.
Note that setting it to 0 works only if there's only one computer
accessing the maildir. It's practically impossible to make two
clocks _exactly_ synchronized.
It might be possible to only use file server's clock by looking at
the atime field, but I don't know how well that would actually work.
cur directory
-------------
We have dirty_cur_time variable which is set to cur/ directory's
mtime when it's >= time() - MAILDIR_SYNC_SECS and we _think_ we have
synchronized the directory.
When dirty_cur_time is non-zero, we don't synchronize the cur/
directory until
a) cur/'s mtime changes
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 "buffer.h"
#include "hash.h"
#include "str.h"
#include "maildir-storage.h"
#include "maildir-uidlist.h"
#include <stdio.h>
#include <unistd.h>
#include <dirent.h>
#define MAILDIR_SYNC_SECS 1
#define MAILDIR_FILENAME_FLAG_FOUND 128
struct maildir_sync_context {
struct index_mailbox *ibox;
int partial;
struct maildir_uidlist_sync_ctx *uidlist_sync_ctx;
};
struct maildir_index_sync_context {
struct index_mailbox *ibox;
struct mail_index_view *view;
struct mail_index_sync_ctx *sync_ctx;
struct mail_index_sync_rec sync_rec;
};
void *context __attr_unused__)
{
return 1;
}
return 0;
"unlink(%s) failed: %m", path);
return -1;
}
void *context)
{
const char *newpath;
enum mail_flags flags;
return 1;
}
return 0;
return -1;
return 1;
}
return -1;
}
struct maildir_index_sync_context *ctx)
{
break;
return -1;
NULL) < 0)
return -1;
}
break;
return -1;
ctx) < 0)
return -1;
}
break;
}
return 0;
}
{
struct maildir_index_sync_context ctx;
int ret;
if (ibox->commit_log_file_seq == 0)
return 0;
if (ret > 0) {
ret = -1;
break;
}
}
ret = -1;
}
if (ret == 0) {
ibox->commit_log_file_seq = 0;
ibox->commit_log_file_offset = 0;
} else {
}
return ret;
}
static struct maildir_sync_context *
{
struct maildir_sync_context *ctx;
return ctx;
}
{
}
const char *old_fname)
{
int ret = 0;
t_push();
i_warning("Fixed duplicate in %s: %s -> %s",
ret = -1;
}
t_pop();
return ret;
}
{
const char *dir;
"opendir(%s) failed: %m", dir);
return -1;
}
continue;
flags = 0;
if (move_new) {
str_truncate(src, 0);
str_truncate(dest, 0);
/* we moved it - it's \Recent for use */
/* someone else moved it already */
/* not enough disk space, leave here */
} else {
"rename(%s, %s) failed: %m",
}
} else if (new_dir) {
}
if (ret <= 0) {
if (ret < 0)
break;
/* possibly duplicate - try fixing it */
ret = -1;
break;
}
}
}
"closedir(%s) failed: %m", dir);
}
return ret < 0 ? -1 : 0;
}
int *new_changed_r, int *cur_changed_r)
{
const struct mail_index_header *hdr;
return -1;
}
return -1;
}
if (ibox->last_cur_mtime == 0) {
/* first sync in this session, get cur stamp from index */
}
*new_changed_r = TRUE;
}
(ibox->dirty_cur_time != 0 &&
/* cur/ changed, or delayed cur/ check */
*cur_changed_r = TRUE;
}
return 0;
}
{
struct maildir_index_sync_context sync_ctx;
struct maildir_uidlist_iter_ctx *iter;
struct mail_index_transaction *trans;
struct mail_index_view *view;
const struct mail_index_header *hdr;
const struct mail_index_record *rec;
const char *filename;
enum mail_flags flags;
int ret;
return -1;
}
seq = 0;
seq++;
if ((uflags & MAILDIR_UIDLIST_REC_FLAG_NONSYNCED) != 0) {
/* partial syncing */
continue;
}
continue;
}
ret = -1;
break;
}
/* expunged */
goto __again;
}
/* new UID in the middle of the mailbox -
shouldn't happen */
"Maildir sync: UID inserted in the middle "
ret = -1;
break;
}
/* we haven't been able to update maildir with this
record's flag changes. don't sync them. */
continue;
}
INDEX_KEYWORDS_BYTE_COUNT) != 0) {
}
}
/* expunge the rest */
}
if (ret < 0)
else {
else {
}
}
/* now, sync the index */
ret = -1;
break;
}
}
ret = -1;
if (ret == 0) {
ibox->commit_log_file_seq = 0;
ibox->commit_log_file_offset = 0;
} else {
}
return ret;
}
{
return -1;
if (!new_changed && !cur_changed)
return 0;
return -1;
if (cur_changed) {
return -1;
}
if (ret == 0)
return ret;
}
{
int ret;
return -1;
return -1;
return ret;
}
{
struct maildir_sync_context *ctx;
int ret;
return ret;
}
{
struct maildir_sync_context *ctx;
int ret;
if ((flags & MAILBOX_SYNC_FLAG_FAST) == 0 ||
if (ret < 0)
return -1;
}
}