[ruby/fiddle] closure: add support for const char *

GitHub: fix GH-62

Reported by Cody Krieger. Thanks!!!

https://github.com/ruby/fiddle/commit/284b820f2d

ext/fiddle/closure.c

@@ -130,6 +130,10 @@ with_gvl_callback(void *ptr)
 	    rb_ary_push(params, ULL2NUM(*(unsigned LONG_LONG *)x->args[i]));
 	    break;
 #endif
+	  case TYPE_CONST_STRING:
+	    rb_ary_push(params,
+                        rb_str_new_cstr(*((const char **)(x->args[i]))));
+	    break;
 	  default:
 	    rb_raise(rb_eRuntimeError, "closure args: %d", type);
         }
@@ -175,6 +179,10 @@ with_gvl_callback(void *ptr)
 	*(unsigned LONG_LONG *)x->resp = NUM2ULL(ret);
 	break;
 #endif
+      case TYPE_CONST_STRING:
+        /* Dangerous. Callback must keep reference of the String. */
+        *((const char **)(x->resp)) = StringValueCStr(ret);
+        break;
       default:
 	rb_raise(rb_eRuntimeError, "closure retval: %d", type);
     }

test/fiddle/test_closure.rb

@@ -54,6 +54,19 @@ module Fiddle
       assert_equal 10, func.call(10)
     end
 
+    def test_const_string
+      closure_class = Class.new(Closure) do
+        def call(string)
+          @return_string = "Hello! #{string}"
+          @return_string
+        end
+      end
+      closure = closure_class.new(:const_string, [:const_string])
+
+      func = Function.new(closure, [:const_string], :const_string)
+      assert_equal("Hello! World!", func.call("World!"))
+    end
+
     def test_block_caller
       cb = Closure::BlockCaller.new(TYPE_INT, [TYPE_INT]) do |one|
         one