Drop support for multiple generators yielding from one generator

[nikic]

This drops support for having multiple generators yield from the same generator in parallel. Support for this requires the famous "doubly-linked refcounted generator tree" that is very hard to understand and serves a non-existent use-case.

A new use-after-free in this area has recently surfaced (oss-fuzz #25344), as triggered by the test case Zend/tests/generators/yield_from_multi_child_destroyed.phpt. I tried to find a fix, but it's really not simple. The code didn't really consider the possibility of a "child" generator (the one calling yield from) being destroyed before the "parent" generator (the one being yielded from) and the parent generator being used afterwards.

This PR only adds an exception. I would clean up the implementation in a separate commit.

[nikic]

cc @bwoebi @dstogov @sgolemon I would like to make this change for PHP 8. I don't think anyone uses this functionality and it's a big maintenance headache.

[morrisonlevi]

Is there a known library or framework that makes heavy use of yield from that we could double-check with for any legitimate use cases?

[nikic]

I did check that https://github.com/amphp/amp doesn't break.

[dstogov]

I'm not an expert in this area.
As I understand, yield from implements behavior similar to async/await.
I afraid, this PR may introduce a limitation that would break async frameworks.

[staabm]

//cc @kelunik @trowski from amphp

[trowski]

Amp never uses yield from on the same generator in parallel. yield from is only used with private methods as single-use coroutines. Here's an example in amphp/postgres. +1 for this change.

[staabm]

//cc @clue from reactphp

[kelunik]

I also don't think the BC break potential is very large. I'm pretty sure Amp isn't affected, which is probably the heaviest user of Generators in PHP.

AFAIK yield from is just syntactic sugar and the same could be achieved without it, no? If so, do we have similar issues if it's implemented without yield from?

[clue]

No objections from my end, to the best of my knowledge this feature isn't used in any ReactPHP-related projects.

[nikic]

#6344 has at least partially addressed the implementation complexity issues here. And the ship has sailed for PHP 8.0 in either case...