Profiling: Skip class/module memsize on Ruby 4 to avoid heap profiling SIGSEGV

[navidemad]

What does this PR do?

Skips computing rb_obj_memsize_of for T_CLASS / T_MODULE / T_ICLASS objects on Ruby 4.0+, where it can crash the VM with a SIGSEGV during heap profiling.

ruby_obj_memsize_of already keeps a denylist of rb_obj_memsize_of paths that crash the VM (e.g. T_NODE). This adds the class-like types to that denylist on Ruby 4.0+, gated by a new NO_SAFE_CLASS_MEMSIZE compile-time define. On older Rubies the define is absent, so the preprocessed code is byte-for-byte unchanged.

Motivation:

With experimental heap profiling enabled on Ruby 4.0, we saw recurring SIGSEGV crashes on production Sidekiq workers (#5936). During heap profile serialization, the recorder resurrects each tracked object via ObjectSpace._id2ref and calls rb_obj_memsize_of on it. For class objects, rb_obj_memsize_of walks Ruby 4.0's per-namespace class extensions (rb_class_classext_foreach → classext_memsize → rb_id_table_memsize) and dereferences invalid classext memory, killing the process. Class objects contribute negligibly to a heap size profile, so skipping them removes the crash with no meaningful loss of signal. The full root-cause analysis is in #5936.

Change log entry

Yes. Profiling: Fix a SIGSEGV crash that could happen with experimental heap profiling enabled on Ruby 4.0.

Additional Notes:

The gate is a new NO_SAFE_CLASS_MEMSIZE define, set for RUBY_VERSION >= "4", following the existing extconf.rb convention for Ruby-4-specific behavior. A complete fix would also need Ruby-side hardening of classext_memsize / rb_class_classext_foreach, but skipping class memsize is enough to stop the crash.

How to test the change?

Added a regression test in stack_recorder_spec.rb: it tracks a Class as a live heap object and checks that the profiler reports it without crashing, with a heap-live-size of 0 on Ruby 4.0+ (and the real ObjectSpace.memsize_of on older Rubies). Verified on Linux + Ruby 4.0 (tracer-4.0): the full stack_recorder_spec passes (64 examples, 0 failures). On Ruby < 4 the change is a no-op at the preprocessor level.