Fix crash when printing RGENGC_DEBUG=5 output from GC

I was trying to debug an (unrelated) issue in the GC, and wanted to turn
on the trace-level GC output by compiling it with -DRGENGC_DEBUG=5.
Unfortunately, this actually causes a crash in newobj_init() because the
code there tries to log the obj_info() of the newly created object.
However, the object is not actually sufficiently set up for some of the
things that obj_info() tries to do:

* The instance variable table for a class is not yet initialized, and
  when using variable-length RVALUES, said ivar table is embedded in
  as-yet unitialized memory after the struct RValue. Attempting to read
  this, as obj_info() does, causes a crash.
* T_DATA variables need to dereference their ->type field to print out
  the underlying C type name, which is not set up until newobj_fill() is
  called.

To fix this, create a new method `obj_info_basic`, which dumps out only
the parts of the object that are valid before the object is fully
initialized.

[Fixes #18795]

1 files changed, 1 insertions, 0 deletions

diff --git a/internal/gc.h b/internal/gc.h
index 34a6043e8a..2537671855 100644
--- a/internal/gc.h
+++ b/internal/gc.h
@@ -122,6 +122,7 @@ int ruby_get_stack_grow_direction(volatile VALUE *addr);
 
 const char *rb_obj_info(VALUE obj);
 const char *rb_raw_obj_info(char *const buff, const size_t buff_size, VALUE obj);
+const char *rb_raw_obj_info_basic(char *const buff, const size_t buff_size, VALUE obj);
 
 size_t rb_size_pool_slot_size(unsigned char pool_id);