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Date: Wed, 11 Mar 2020 11:09:13 +0100
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To: Marcio Almada <marcio.web2@gmail.com>
Cc: PHP internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] exit() via exception
From: nikita.ppv@gmail.com (Nikita Popov)

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On Fri, Oct 11, 2019 at 4:11 PM Nikita Popov <nikita.ppv@gmail.com> wrote:

> On Fri, Oct 11, 2019 at 3:47 PM Marcio Almada <marcio.web2@gmail.com>
> wrote:
>
>> Em sex, 11 de out de 2019 =C3=A0s 08:05, Nikita Popov
>> <nikita.ppv@gmail.com> escreveu:
>> >
>> > Hi,
>> >
>>
>> Hello :)
>>
>> > Currently exit() is implemented using bailout and unclean shutdown,
>> which
>> > means that we're going to perform a longjmp back to the top-level scop=
e
>> and
>> > let the memory manager clean up all the memory it knows about. Anythin=
g
>> not
>> > allocated using ZMM is going to leak persistently.
>> >
>> > For me, one of the most annoying things about this is that we can't
>> perform
>> > proper leak checks on code using PhpUnit, because it will always exit(=
)
>> at
>> > the end, which will result in "expected" memory leaks.
>> >
>> > I think it would be good to switch exit() to work by throwing a magic
>> > exception, similar to what Python does. This would allow us to properl=
y
>> > unwind the stack, executing finally blocks (which are currently skippe=
d)
>> > and perform a clean engine shutdown.
>> >
>> > Depending on the implementation, we could also allow code to actually
>> catch
>> > this exception, which may be useful for testing scenarios, as well as
>> > long-running daemons.
>> >
>> > I'm mainly wondering how exactly we'd go about integrating this in the
>> > existing exception hierarchy.
>>
>> > Assuming that it is desirable to allow people
>> > to actually catch this exception
>> > my first thought would be along these
>> > lines:
>> >
>> > Throwable (convert to abstract class)
>> > \-> Exception
>> > \-> Error
>> > \-> ExitThrowable
>> >
>> > This does mean though that existing code using catch(Throwable) is
>> going to
>> > catch exit()s as well. This can be avoided by introducing *yet another=
*
>> > super-class/interface above Throwable, which is something I'd rather
>> avoid.
>> >
>>
>> Since you brought python as inspiration, I believe the hierarchy goes
>> like this on their land:
>>
>> BaseException
>>  +-- SystemExit
>>  +-- KeyboardInterrupt
>>  +-- GeneratorExit
>>  +-- Exception
>>      +-- [kitchen sink]
>>
>> Being `BaseException` the base class for all built-in exceptions. It
>> is not meant to be directly
>> inherited by user-defined classes. It 's the equivalent to our
>> `Throwable` situation. In this context
>> `ExitThrowable -> Throwable ` appears legit.
>>
>> >
>> > Anyone have thoughts on this matter?
>> >
>>
>> Yes. There is an obvious can of worms if I've got this right: `exit()`
>> and `die()` would no longer guarantee a
>> program to actually terminate in case catching `ExitThrowable` is
>> allowed. Python solves this by actually
>> having two patterns:
>>
>> 1. `quit()`, `exit()`, `sys.exit()` are the equivalent to `raise
>> SystemExit`, can be caught / interrupted
>> 2. `os._exit()`, can't be caught but has a callback mechanism like our
>> `register_shutdown_function`,
>> see https://docs.python.org/3/library/atexit.html
>
>
> I don't believe atexit applies to os._exit(). In any case, I agree that
> this is something we're currently missing -- we should probably add a
> pcntl_exit() for this purpose. It should be noted though that this is
> really very different from exit(), which is still quite graceful and usab=
le
> in a webserver context, while a hypothetical pcntl_exit() would bring dow=
n
> the server process. As the Python docs mention, the primary use-case woul=
d
> be exiting from forked processes without going through shutdown, which ha=
s
> also recently come up in https://github.com/php/php-src/pull/4712.
>
>
>> If we bind `exit()` and `die()` to a catchable exception how would we
>> still have the scenario 2 available
>> on PHP land without a BCB? :)
>>
>
>> I have one simple suggestion: Introduce `EngineShutdown -> Throwable`,
>> bind `exit|die` to it but disallow
>> `catch(\EngineShutdown $e)` at compile time. This would allow keeping
>> backwards compatibility to
>> scenario 2 without messing with our current exception hierarchy.
>>
>
> I think the options are basically:
>
> 1. Making EngineShutdown implement Throwable, which would make existing
> catch(Throwable) catch it -- probably a no-go.
>
> 2. Making EngineShutdown not implement Throwable, which means that not al=
l
> "exceptions" implement the interface, which is rather odd. It still allow=
s
> explicitly catching the exit.
>
> 3. Introducing a function like catch_exit(function() { ... }). This would
> still allow catching exits (for phpunit + daemon use cases), but the fact
> that this is actually implemented based on an exception would be hidden a=
nd
> the only way to catch the exit is through this function.
>
> 4. Don't allow catching exits at all. In this case the exception is just
> an implementation detail.
>

I've started implementing variant 4 in
https://github.com/php/php-src/pull/5243. It uses an internal exception
type to implement exit() that cannot be caught.

Finally blocks do get executed, as they should be. This does mean that it
is possibly to discard the exit through finally, because control flow
performed in finally always wins:

try {
    exit;
} finally {
    (null)->method(); // throw Error
}

This snippet will throw an Error exception, with the original exit being
discarded.

try {
    try {
        exit;
    } finally {
        (null)->method(); // throw Error
    }
} catch (Error $e) {
    // Ignore
}

This snippet will catch the Error exception and thus stops unwinding.

I believe this behavior is correct and expected (by which I mean:
consistent with finally semantics), but it does mean that exits can be
discarded even without having an explicit "catch_exit" function or similar.

Nikita

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