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Date: Thu, 30 Jun 2022 11:20:24 +0200
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To: Rowan Tommins <rowan.collins@gmail.com>
Cc: internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] [RFC] Short Closures 2, aka auto-capture take 3
From: landers.robert@gmail.com (Robert Landers)

On Thu, Jun 30, 2022 at 10:19 AM Rowan Tommins <rowan.collins@gmail.com> wrote:
>
> On 29/06/2022 23:31, Dan Ackroyd wrote:
> > Imagine some code that looks like this:
> >
> > // Acquire some resource e.g. an exclusive lock.
> > $some_resource = acquire_some_resource();
> >
> > $fn = fn () {
> >      // Free that resource
> >      $some_resource = null;
> > }
> >
> > // do some stuff that assumes the exclusive
> > // lock is still active.
> >
> > // call the callback that we 'know' frees the resource
> > $fn();
> >
> > That's a not unreasonable piece of code to write
>
>
> For that to work, it would require the variable to be captured by
> reference, not value. Writing to a variable captured by value, like
> writing to a parameter passed by value, is just writing to a local variable.
>
>
> In fact, the "optimisation" is in my opinion a critical part of the
> semantics, to avoid the opposite problem:
>
> // Acquire some resource e.g. an exclusive lock.
> $some_resource = acquire_some_resource();
>
> $fn = fn () {
>      // Use a variable that happens to have the same name
>      // A naive implementation would see $some_resource mentioned, and
> capture it
>      // Over-writing the local variable here makes no difference; the
> closure still holds the value for next time
>      $some_resource = 'hello';
> }
>
> // Free what we believe is the last pointer, to trigger the destructor
> unset($some_resource);
>
> // If $some_resource gets captured, it can only be released by
> destroying the closure
> unset($fn);
>
>
> Regards,
>
> --
> Rowan Tommins
> [IMSoP]
>
> --
> PHP Internals - PHP Runtime Development Mailing List
> To unsubscribe, visit: https://www.php.net/unsub.php
>

> For that to work, it would require the variable to be captured by
> reference, not value.

I think their suggested code would work (at least currently in PHP) by
the simple fact they would increase the reference count on that
object/resource until they set it as null. However, with the
"optimization," the reference count will never be incremented and thus
fail to work as defined.