Fix GH-23301: nested "yield from" repeats a value after "yield from []"

[lazerg]

GH-15375's fix made the DO_INIT re-advance guard in zend_generator_resume() read the flag from the delegating generator rather than from orig_generator. That flag is set by zend_generator_yield_from() and only ever cleared on orig_generator, so on a middle generator it stays set for the rest of its life.

When such a middle generator then delegates to a non-generator iterable (yield from []), it still sits on a ZEND_YIELD_FROM opline, so it is picked as the delegator even though no new generator link was established, and its stale DO_INIT suppresses the resume. The value it yielded last is presented a second time. Twig hits this on every template, since doDisplay() always ends with yield from [];.

Only treat the generator as the delegator when it actually delegated to another generator (node.parent is set); otherwise keep orig_generator as before. The GH-15375 tests still pass.

Fixes GH-23301

[LamentXU123]

I'd say this is an extremely crafted case that basically don't happens read world. But code-wise, this looks good.
@arnaud-lb Could you please take a look at this?

[kocsismate]

@LamentXU123 no it's actually a widespread problem, twig doesn't work basically: #22640 (comment)

[iliaal]

Fix looks right to me. node.parent is the correct discriminator: zend_generator_yield_from() sets it alongside DO_INIT and is only reached for a generator operand, so it separates a live delegation from the stale flag a middle generator keeps afterwards.

The scope is wider than the title suggests, though. Diffing against a pre-22640 build, current 8.4 also repeats the value for yield from new ArrayIterator([]) and for a non-empty array tail, triples it in a four-level chain, and yields a duplicate key: k0=A,k1=B,k0=B where 8.3 gives k0=A,k1=B. Every one of those matches the pre-22640 output again with this patch, and gh15375 including its shared-primed section still passes.

One thing for whoever merges it: merging into current PHP-8.4 auto-merges NEWS and silently drops the GH-23301 line, so it needs adding back by hand.