flock with timeout support

[glensc]

I like FlockMutex because it work on kernel level and is reliable.

however there's the problem that acquiring the lock makes program wait forever. I wish to get error "unable to obtain lock withing timeout"

for flock there's LOCK_NB flag which can be used.

ps: SemaphoreMutex has similar issue

[hidenorigoto]

+1 for SemaphoreMutex

[malkusch]

Having a timeout is actually a nice feature. LOCK_NB would turn the implementation into a busy waiting lock and suffer from being inacurate. I will consider this option as a last resort. But first I will see if pcntl_alarm might be a better option. However:

Process Control support in PHP is not enabled by default. […] Currently, this module will not function on non-Unix platforms (Windows).

This won't be a low hanging fruit.

[glensc]

i don't see how pcntl_alarm helps here, it's just weird way of saying sleep(2).

i'd love to see the timeout being implemented, as for inaccuracy and race conditions, i think the sleep and retry should be somehow randomized and exponential.

and as LOCK_NB would move the locking to "userspace" from "kernel", it should be enabled only when timeout > 0 so previous behavior could be used.

[malkusch]

i don't see how pcntl_alarm helps here

Compared to a busy waiting, it's a simple, cheap and precise way to put a timeout on flock():

pcntl_signal(SIGALRM, function() {});
pcntl_alarm(3);
try {
    if (!flock($handle, LOCK_EX)) {
        throw new \Exception("Timeout after 3 seconds");
    }
} finally {
    pcntl_alarm(0);
    pcntl_signal_dispatch();
    pcntl_signal(SIGALRM, SIG_DFL);
}

Unfortunately it doesn't seem to be portable, which is the whole point about flock(). I guess I'll chose a hybrid implementation. That is: scheduling the signal and falling back to busy waiting if pcntl is not available.

[glensc]

ah, it makes flock "timeout" without LOCK_NB, i see now

[glensc]

https://symfony.com/blog/new-in-symfony-3-3-lock-component

symfony/lock seems to have timeout support, it has flock backend. will test if it suits.

[glensc]

update: symfony/lock: blocking doesn't support timeout and for nonblocking mode one would need to implement timeout themselves: symfony/symfony#25418

[glensc]

update2: seems flock doesn't support timeout: https://symfony.com/doc/3.4/components/lock.html#available-stores and nonblocking mode i misread the implementation (because i looked at internal code which were void: FlockStore::save, FlockStore::waitAndSave)

[TheLevti]

The pcntl extension should not be used in a web server environment.

http://php.net/manual/en/intro.pcntl.php

Process Control should not be enabled within a web server environment and unexpected results may happen if any Process Control functions are used within a web server environment.