Breaking Change: set_exception_handler no longer returns the exception handler within itself

[greg0ire]

Description

The following code:

<?php
set_exception_handler(function ($e): void {
    var_dump(set_exception_handler('is_int'));
});
throw new \Exception;

Resulted in this output:

NULL

But I expected this output instead:

object(Closure)#1 (1) {
  ["parameter"]=>
  array(1) {
    ["$e"]=>
    string(10) "<required>"
  }
}

PHP Version

8.3.0 and up

Operating System

No response

[nielsdos]

Reverting the change in zend_user_exception_handler from b3e33be seems to restore the old behaviour.
cc @iluuu1994 is this behaviour expected, based on what I remember and read it seems unintended but I could be wrong?

[iluuu1994]

Yes, this is expected behavior. Exception handlers are not called repeatedly, so they are unregistered before they are called. If you want to catch errors within an error handler, install a new one. You should probably avoid registering the same one, as this will lead to infinite recursion if there's a bug in your error handler. This was previously not done because errors in your error handler were never handled.

[nielsdos]

Ok thanks for checking. In that case I'll close this.

[greg0ire]

Note that this also affects shutdown functions: https://3v4l.org/fXeHc#v8.3.3, and I think that's what really causes symfony/symfony#53946: my understanding is that with the shutdown function, Symfony is trying to reuse its exception handler to handle fatal errors.

[nielsdos]

FTR, the behaviour change was to implement #10695, but I see how this causes issues.
Not sure what to do with this, if the existing code could still be supported, I'll defer to @iluuu1994 as they're more familiar

[greg0ire]

I think it makes sense to unregister the exception handler when you are inside it, but maybe it should be restored after exiting it, and before entering the shutdown function. Doesn't make a lot of sense though… when I said Symfony is reusing the exception handler, it's not like they are throwing an exception and letting the handler handle it… instead, they are invoking it manually. So I'd say it's not a super legitimate use case. This looks like a breaking change though.

[nicolas-grekas]

Hi, this is a significant BC break, please reconsider. See symfony/symfony#53946, which is nasty.