Differenciate WeakMaps from bare HashTables used as weak maps for GC purposes

[arnaud-lb]

Possible fix for GH-19543.

Since cbf67e4, the GC needs to find all WeakMaps referencing a weakly referenced object. Doing so, it treats all ZEND_WEAKREF_TAG_MAP as WeakMap instances.

However, a ZEND_WEAKREF_TAG_MAP reference may be a bare HashTable when zend_weakrefs_hash_add() is used.

Introduce a new tag, ZEND_WEAKREF_TAG_BARE_HT, and use this tag when weakly referencing an object from a bare HashTable. Ignore such references in GC.

cc @bwoebi @TimWolla

Edit: should probably target 8.3.

[TimWolla]

Would it make sense to add explicit handling to zend_weakmap_get_*_gc()? Something like:

	else if (tag == ZEND_WEAKREF_TAG_BARE_HT) {
		/* Bare HashTables are intentionally ignored, since they are intended for internal usage by extensions and might be allocated in globals. */
	}

Basically to serve as documentation for the future reader.

[TimWolla]

For proper attribution:

The test case is Co-authored-by: Tim Düsterhus <tim@tideways-gmbh.com> (different email address, this was done on company time).

[TimWolla]

Edit: should probably target 8.3.

Given that this does not touch any header files, it should be safe to fix for 8.3. Nevertheless extensions would need to include workarounds either way, since they can't necessarily guarantee they are running on a fixed version of PHP.

[TimWolla]

Soft LGTM. I'll double-check this against my actual use-case (incl. my work-around) to verify that this doesn't break anything before approving properly.

[TimWolla]

For my use case I can confirm that:

• 8.3 without this PR crashes.
• 8.3 with this PR works.
• 8.3 without this PR and my workaround works.
• 8.3 with this PR and my workaround works.

Thus this PR improves the situation without the workaround and doesn't break the workaround.

[bwoebi]

Looks good to me, thank you!