On Thu, Jun 30, 2022 at 10:19 AM Rowan Tommins <rowan.coll...@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. -- PHP Internals - PHP Runtime Development Mailing List To unsubscribe, visit: https://www.php.net/unsub.php