notmuch's idea of concurrency / failing an invocation

[Austin Clements]

Actually, this is trivial to play with.  Here's a stop-gap wrapper
script for people having trouble with Xapian locking,

#!/bin/bash

NOTMUCH_BIN="/path/to/notmuch"
MAIL_DIR="/path/to/mailroot"

(
    case "$1" in
        setup|help)
            ;;
        search|show|count|reply|dump|search-tags|part)
            flock -s 200;;
        *)
            flock -x 200;;
    esac
    "$NOTMUCH_BIN" "$@" 200>&-
) 200>"$MAIL_DIR"/.notmuch/lock

On Fri, Jan 28, 2011 at 11:57 AM, Austin Clements <amdragon at mit.edu> wrote:
>
> On Fri, Jan 28, 2011 at 10:36 AM, Mike Kelly <pioto at pioto.org> wrote:
>>
>> On Fri, 28 Jan 2011 10:45:19 +0100, Thomas Schwinge <thomas at schwinge.name> wrote:
>> > > It would definitely be nice to avoid the complexity inherent in having a
>> > > daemon, but how do you imagine "queue on a lock" to work? We don't have
>> > > anything like that in place now.
>> >
>> > I suppose what he means is trying to get the lock, and if that fails wait
>> > a bit / wait until it is available again.
>> >
>> > Actually, as a next step, wouldn't it also be possible to add some
>> > heuristic to avoid ``notmuch new'' (being a low-priority task) blocking
>> > some interactive user (UI; high-priority task)?  But we can pursue such
>> > schemes as soon as the basic infrastructure is in place.
>>
>> Couldn't we pretty much get the desired behavior by using flock(2)?
>> Basically, take out a LOCK_EX when we need to write, and a LOCK_SH when
>> we only need to read. Using the blocking form, things should pretty much
>> just queue up and take their turn, right?
>>
>> I'm not familiar with Xapian, but if it doesn't give us something we
>> could use this sort of locking on, couldn't we just add some
>> /path/to/mail/.notmuch.lock file that we open to hold a lock on?
>
> Yes, exactly.  All of this.  Unfortunately, Xapian doesn't expose the ability to block on the lock (see the fcntl call in backends/flint_lock.cc, which is hard-coded to the non-blocking F_SETLK instead of F_SETLKW), so we'd either need a new Xapian option, or we would just have to wrap our own flock/fcntl lock around things as you suggest.