Expose GC.active_gc_name, to query the currently active GC

And internal GC API function rb_gc_active_gc_name

gc.c

@@ -642,6 +642,8 @@ typedef struct gc_function_map {
     bool (*garbage_object_p)(void *objspace_ptr, VALUE obj);
     void (*set_event_hook)(void *objspace_ptr, const rb_event_flag_t event);
     void (*copy_attributes)(void *objspace_ptr, VALUE dest, VALUE obj);
+    // GC Identification
+    const char *(*active_gc_name)(void);
 } rb_gc_function_map_t;
 
 static rb_gc_function_map_t rb_gc_functions;
@@ -775,6 +777,8 @@ ruby_external_gc_init(void)
     load_external_gc_func(garbage_object_p);
     load_external_gc_func(set_event_hook);
     load_external_gc_func(copy_attributes);
+    //GC Identification
+    load_external_gc_func(active_gc_name);
 
 # undef load_external_gc_func
 
@@ -854,6 +858,8 @@ ruby_external_gc_init(void)
 # define rb_gc_impl_garbage_object_p rb_gc_functions.garbage_object_p
 # define rb_gc_impl_set_event_hook rb_gc_functions.set_event_hook
 # define rb_gc_impl_copy_attributes rb_gc_functions.copy_attributes
+// GC Identification
+# define rb_gc_impl_active_gc_name rb_gc_functions.active_gc_name
 #endif
 
 static VALUE initial_stress = Qfalse;
@@ -2752,6 +2758,12 @@ rb_gc_copy_attributes(VALUE dest, VALUE obj)
     rb_gc_impl_copy_attributes(rb_gc_get_objspace(), dest, obj);
 }
 
+const char *
+rb_gc_active_gc_name(void)
+{
+    return rb_gc_impl_active_gc_name();
+}
+
 // TODO: rearchitect this function to work for a generic GC
 size_t
 rb_obj_gc_flags(VALUE obj, ID* flags, size_t max)
@@ -3515,7 +3527,16 @@ gc_disable(rb_execution_context_t *ec, VALUE _)
     return rb_gc_disable();
 }
 
-
+static VALUE
+active_gc_name(rb_execution_context_t *ec, VALUE _) {
+    if (getenv("RUBY_GC_LIBRARY")) {
+        const char *name = rb_gc_active_gc_name();
+        return rb_fstring_new(name, strlen(name));
+    }
+    else {
+        return Qnil;
+    }
+}
 
 // TODO: think about moving ruby_gc_set_params into Init_heap or Init_gc
 void

gc.rb

@@ -368,6 +368,18 @@ def self.total_time
       ULL2NUM(rb_gc_impl_get_total_time(rb_gc_get_objspace()))
     }
   end
+
+  # call-seq:
+  #    GC.active_gc_name -> string
+  #
+  # Return the configured name for the active GC module
+  def self.active_gc_name
+    # shared_gc_dir is mandatory when configuring `with-shared-gc`.
+    # When shared GC is not enabled it will be blank
+    if RbConfig::CONFIG['shared_gc_dir'].length > 0
+      Primitive.active_gc_name
+    end
+  end
 end
 
 module ObjectSpace

gc/default.c

@@ -6162,6 +6162,12 @@ rb_gc_impl_copy_attributes(void *objspace_ptr, VALUE dest, VALUE obj)
     rb_gc_copy_finalizer(dest, obj);
 }
 
+const char *
+rb_gc_impl_active_gc_name(void)
+{
+    return "default";
+}
+
 void
 rb_gc_impl_writebarrier_remember(void *objspace_ptr, VALUE obj)
 {

test/ruby/test_gc.rb

@@ -872,4 +872,16 @@ def test_old_to_young_reference
       assert_include ObjectSpace.dump(young_obj), '"old":true'
     end
   end
+
+  def test_active_gc_name
+    omit unless /darwin|linux/.match(RUBY_PLATFORM)
+
+    gc_name = (ENV['RUBY_GC_LIBRARY'] || "").split('.')[1]
+
+    if RbConfig::CONFIG['shared_gc_dir'].length > 0
+      assert_equal gc_name, GC.active_gc_name
+    else
+      assert_nil GC.active_gc_name
+    end
+  end
 end