As originally reported on GitHub as #1124, ReadPreference.c may cause a segfault during shutdown/GC. The crash can be reproduced using the sample project that was attached to the original issue.

PHPC-850 introduced get_properties handlers for ReadConcern, ReadPreference, and WriteConcern classes, which return an internally cached HashTable with properties for debugging and serialization. This was previously done for BSON classes in PHPC-460 (revised in PHPC-894 and PHPC-939). No other driver classes define get_properties handlers, per PHPC-1023.

In the BSON patches, we implemented get_gc handler that returns a pointer to the internally cached HashTable (note: this could be null if get_properties was never called previously). I don't believe that implementation was correct, as the data in get_properties originates from internal BSON structures instead of zvals, and thus isn't relevant to GC. Prior art in core PHP extensions suggests that get_gc should return zend_std_get_properties(object).

Delegating to zend_std_get_properties also ensures that any userland-assigned (public) properties would get passed on to the garbage collector for examination – whereas they are currently not in our present implementation. Granted, this is probably an uncommon case.

The suggested changes for this ticket are as follows:

• Create regression tests for segfaults during GC for all classes implementing get_properties handlers. This covers RC, RP, WC, and BSON classes, although we don't expect the BSON classes to fail as-is.
• Modify the BSON get_gc handlers to return zend_std_get_properties(object) instead of the internally cached HashTable.
• Define get_gc handlers for the RC, RP, and WC objects.