debian/rules: Use `git' potty wrapper.

[qmail] / maildir.5

.TH maildir 5
.SH "NAME"
maildir \- directory for incoming mail messages
.SH "INTRODUCTION"
.I maildir
is a structure for
directories of incoming mail messages.
It solves the reliability problems that plague
.I mbox
files and
.I mh
folders.
.SH "RELIABILITY ISSUES"
A machine may crash while it is delivering a message.
For both
.I mbox
files and
.I mh
folders this means that the message will be silently truncated.
Even worse: for
.I mbox
format, if the message is truncated in the middle of a line,
it will be silently joined to the next message.
The mail transport agent will try again later to deliver the message,
but it is unacceptable that a corrupted message should show up at all.
In
.IR maildir ,
every message is guaranteed complete upon delivery.

A machine may have two programs simultaneously delivering mail
to the same user.
The
.I mbox
and
.I mh
formats require the programs to update a single central file.
If the programs do not use some locking mechanism,
the central file will be corrupted.
There are several
.I mbox
and
.I mh
locking mechanisms,
none of which work portably and reliably.
In contrast, in
.IR maildir ,
no locks are ever necessary.
Different delivery processes never touch the same file.

A user may try to delete messages from his mailbox at the same
moment that the machine delivers a new message.
For
.I mbox
and
.I mh
formats, the user's mail-reading program must know
what locking mechanism the mail-delivery programs use.
In contrast, in
.IR maildir ,
any delivered message
can be safely updated or deleted by a mail-reading program.

Many sites use Sun's 
.B Network F\fPa\fBil\fPur\fBe System
(NFS),
presumably because the operating system vendor does not offer
anything else.
NFS exacerbates all of the above problems.
Some NFS implementations don't provide
.B any
reliable locking mechanism.
With 
.I mbox
and
.I mh
formats,
if two machines deliver mail to the same user,
or if a user reads mail anywhere except the delivery machine,
the user's mail is at risk.
.I maildir
works without trouble over NFS.
.SH "THE MAILDIR STRUCTURE"
A directory in
.I maildir
format has three subdirectories,
all on the same filesystem:
.BR tmp ,
.BR new ,
and
.BR cur .

Each file in
.B new
is a newly delivered mail message.
The modification time of the file is the delivery date of the message.
The message is delivered
.I without
an extra UUCP-style
.B From_
line,
.I without
any
.B >From
quoting,
and
.I without
an extra blank line at the end.
The message is normally in RFC 822 format,
starting with a
.B Return-Path
line and a
.B Delivered-To
line,
but it could contain arbitrary binary data.
It might not even end with a newline.

Files in
.B cur
are just like files in
.BR new .
The big difference is that files in
.B cur
are no longer new mail:
they have been seen by the user's mail-reading program.
.SH "HOW A MESSAGE IS DELIVERED"
The
.B tmp
directory is used to ensure reliable delivery,
as discussed here.

A program delivers a mail message in six steps.
First, it
.B chdir()\fPs
to the
.I maildir
directory.
Second, it 
.B stat()s
the name
.BR tmp/\fItime.pid.host ,
where
.I time
is the number of seconds since the beginning of 1970 GMT,
.I pid
is the program's process ID,
and
.I host
is the host name.
Third, if
.B stat()
returned anything other than ENOENT,
the program sleeps for two seconds, updates
.IR time ,
and tries the
.B stat()
again, a limited number of times.
Fourth, the program
creates
.BR tmp/\fItime.pid.host .
Fifth, the program
.I NFS-writes
the message to the file.
Sixth, the program
.BR link() s
the file to
.BR new/\fItime.pid.host .
At that instant the message has been successfully delivered.

The delivery program is required to start a 24-hour timer before
creating
.BR tmp/\fItime.pid.host ,
and to abort the delivery
if the timer expires.
Upon error, timeout, or normal completion,
the delivery program may attempt to
.B unlink()
.BR tmp/\fItime.pid.host .

.I NFS-writing
means
(1) as usual, checking the number of bytes returned from each
.B write()
call;
(2) calling
.B fsync()
and checking its return value;
(3) calling
.B close()
and checking its return value.
(Standard NFS implementations handle
.B fsync()
incorrectly
but make up for it by abusing
.BR close() .)
.SH "HOW A MESSAGE IS READ"
A mail reader operates as follows.

It looks through the
.B new
directory for new messages.
Say there is a new message,
.BR new/\fIunique .
The reader may freely display the contents of
.BR new/\fIunique ,
delete
.BR new/\fIunique ,
or rename
.B new/\fIunique
as
.BR cur/\fIunique:info .
See
.B http://pobox.com/~djb/proto/maildir.html
for the meaning of
.IR info .

The reader is also expected to look through the
.B tmp
directory and to clean up any old files found there.
A file in
.B tmp
may be safely removed if it
has not been accessed in 36 hours.

It is a good idea for readers to skip all filenames in
.B new
and
.B cur
starting with a dot.
Other than this, readers should not attempt to parse filenames.
.SH "ENVIRONMENT VARIABLES"
Mail readers supporting
.I maildir
use the
.B MAILDIR
environment variable
as the name of the user's primary mail directory.
.SH "SEE ALSO"
mbox(5),
qmail-local(8)