So, taking jgh@??? URL:
https://exim.org/exim-html-current/doc/html/spec_html/ch-the_appendfile_transport.html
I see the text: (I've added **highlights**)
...Before appending to a file, a number of security checks are made, and
the file
is locked. A detailed description is given below, after the list of private
options.
... The file option specifies a single file, to which the message is
appended. One or more of use_fcntl_lock, use_flock_lock, or
use_lockfile must
be set with file
...
use_fcntl_lock Use: appendfile Type: boolean Default: see below
This option controls the use of the fcntl() function to lock a file for
exclusive use when a message is being appended. **It is set by default
unless
use_flock_lock is set.**
So this is set by default UNLESS ...one of the other locking options is
chosen:
If I do:
root@ybox:/var/lib/exim4# grep -i use_ /var/lib/exim4/config.autogenerated
root@ybox:/var/lib/exim4# grep -i lock /var/lib/exim4/config.autogenerated
<no output>
And I note, I have in that same file:
mail_spool:
debug_print = "T: appendfile for $local_part@$domain"
driver = appendfile
file = /var/mail/$local_part_data
delivery_date_add
envelope_to_add
return_path_add
group = mail
mode = 0660
mode_fail_narrower = false
So I take these together to mean:
exim will use "apendfile" to append mail to /var/mail/<myuserid>
and it will use fcntl(2) (I'm assuming advisory locking?)
So I (may need to) just ensure maildrop(1) also uses fcntl(2) locking
FYI, my concern is the end of one of my maildrop configs, which ends:
if ("$ADDRLIST" ne "")
{
log "@ in maildrop.manual $NOW, will send (using
/etc/maildroprc.manual) copies like this: $ADDRLIST [SENDMAIL=$SENDMAIL]"
# Spaces at the start really mess it up
ADDRLIST=substr($ADDRLIST,1)
to '!'"$ADDRLIST"
}
else
{
log "@ in maildrop.manual $NOW, no routing found, will drop through
to DEFAULT=$DEFAULT"
}
So if it uses [ to '!'"$ADDRLIST" ] then it will pass the message to
SMTP (aka
exim4) and exim4 will write to e.g. /var/mail/myuserid ... or if it drops
through, then maildrop will write to (the file) $DEFAULT, which could
well be
/var/mail/myuserid. Now I can't actually work out a case where 2
instances of
this could be running simultaneously, but I'm just nervous of two
different bits
of code writing to the same file. I'm reasonably confident exim4 would
protect
itself from another exim4 instance and likewise maildrop.
I think I'll go back to maildrop and see why I saw flock(2) getting used
an if
it can use fcntl(2) instead.
From maildrop:
--enable-use-flock=flag - if this option is set to 1, maildrop will use
either
the flock(), the lockf(), or the fcntl() system call to lock a
mailbox file
when delivering a message. **On most systems, all three use
compatible locking
mechanisms.** In some very isolated cases, flock(), lockf(), and
fcntl(), are
different, incompatible, locking mechanisms. maildrop must use the same
locking mechanism as any mail reading programs. The configuration
script will
run some tests to determine what locking function calls are
available, and
will choose one by itself. The --with-locking-method can be used to
manually
choose the locking function call to use.
--with-locking-method=name - manually select a locking function call.
name is
either "fcntl", "flock", or "lockf". Otherwise the configuration
script will
pick one by itself.
The comment there is somewhat at odds, to the flock(2) man page:
NOTES
Since kernel 2.0, flock() is implemented as a system call in
its own
right rather than being emulated in the GNU C library as a call
to fc‐
ntl(2). With this implementation, **there is no interaction
between the
types of lock placed by flock() and fcntl(2),** and flock()
does not de‐
tect deadlock.
... I'm probably worrying about nothing, this is a standard Debian
install and this same issue exists for Thunderbird, pine, mutt etc
so I suspect it was "sorted out" during the packaging.
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