Implement a good enough version of rb_gc_register_address.

Author: aardvark179

lib/truffle/truffle/cext.rb

@@ -1945,4 +1945,15 @@ def rb_exception_set_message(e, mesg)
   def rb_global_variable(obj)
     GLOBALLY_PRESERVED_VALUES << obj
   end
+
+  GC_REGISTERED_ADDRESSES = {}
+  def rb_gc_register_address(address, obj)
+    Truffle::Interop.to_native(address) unless Truffle::Interop.pointer?(address)
+    GC_REGISTERED_ADDRESSES[address] = obj
+  end
+
+  def rb_gc_unregister_address(address)
+    Truffle::Interop.to_native(address) unless Truffle::Interop.pointer?(address)
+    GC_REGISTERED_ADDRESSES.delete(address)
+  end
 end

src/main/c/cext/gc.c

@@ -13,10 +13,14 @@
 // GC, rb_gc_*
 
 void rb_gc_register_address(VALUE *address) {
+  /* TODO: This should guard the address of the pointer, not the
+     object pointed to, but we haven't yet found a good way to implement
+     that, or a real world use case where it is required. */
+  polyglot_invoke(RUBY_CEXT, "rb_gc_register_address", address, rb_tr_unwrap(*address));
 }
 
 void rb_gc_unregister_address(VALUE *address) {
-  // VALUE is only ever in managed memory. So, it is already garbage collected.
+  polyglot_invoke(RUBY_CEXT, "rb_gc_unregister_address", address);
 }
 
 void rb_gc_mark(VALUE ptr) {