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]

gc.c

@@ -1427,6 +1427,7 @@ static inline void gc_prof_set_heap_info(rb_objspace_t *);
 #endif
 PRINTF_ARGS(static void gc_report_body(int level, rb_objspace_t *objspace, const char *fmt, ...), 3, 4);
 static const char *obj_info(VALUE obj);
+static const char *obj_info_basic(VALUE obj);
 static const char *obj_type_name(VALUE obj);
 
 static void gc_finalize_deferred(void *dmy);
@@ -2638,7 +2639,7 @@ newobj_init(VALUE klass, VALUE flags, int wb_protected, rb_objspace_t *objspace,
     GC_ASSERT(!SPECIAL_CONST_P(obj)); /* check alignment */
 #endif
 
-    gc_report(5, objspace, "newobj: %s\n", obj_info(obj));
+    gc_report(5, objspace, "newobj: %s\n", obj_info_basic(obj));
 
     // RUBY_DEBUG_LOG("obj:%p (%s)", (void *)obj, obj_type_name(obj));
     return obj;
@@ -14099,6 +14100,17 @@ rb_raw_obj_info(char *const buff, const size_t buff_size, VALUE obj)
     return buff;
 }
 
+const char *
+rb_raw_obj_info_basic(char *const buff, const size_t buff_size, VALUE obj)
+{
+    asan_unpoisoning_object(obj) {
+        size_t pos = rb_raw_obj_info_common(buff, buff_size, obj);
+        if (pos >= buff_size) {} // truncated
+    }
+
+    return buff;
+}
+
 #undef APPEND_S
 #undef APPEND_F
 #undef BUFF_ARGS
@@ -14130,12 +14142,27 @@ obj_info(VALUE obj)
     char *const buff = obj_info_buffers[index];
     return rb_raw_obj_info(buff, OBJ_INFO_BUFFERS_SIZE, obj);
 }
+
+static const char *
+obj_info_basic(VALUE obj)
+{
+    rb_atomic_t index = atomic_inc_wraparound(&obj_info_buffers_index, OBJ_INFO_BUFFERS_NUM);
+    char *const buff = obj_info_buffers[index];
+    return rb_raw_obj_info_basic(buff, OBJ_INFO_BUFFERS_SIZE, obj);
+}
 #else
 static const char *
 obj_info(VALUE obj)
 {
     return obj_type_name(obj);
 }
+
+static const char *
+obj_info_basic(VALUE obj)
+{
+    return obj_type_name(obj);
+}
+
 #endif
 
 const char *

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);