Add support for the cleanup attribute in GCC/Clang

The new json_auto_t macro allows easy declaration of json_t types that
automatically decrement at the end of their scope.

doc/apiref.rst

@@ -253,6 +253,24 @@ other. Moreover, trying to encode the values with any of the encoding
 functions will fail. The encoder detects circular references and
 returns an error status.
 
+Scope Dereferencing
+-------------------
+
+It is possible to use the ``json_auto_t`` type to automatically
+dereference a value at the end of a scope. For example::
+
+  void function(void) {
+    json_auto_t *value = NULL;
+    value = json_string("foo");
+    /* json_decref(value) is automatically called. */
+  }
+
+This feature is only available on GCC and Clang. So if your project
+has a portability requirement for other compilers, you should avoid
+this feature.
+
+Additionally, as always, care should be taken when passing values to
+functions that steal references.
 
 True, False and Null
 ====================

src/jansson.h

@@ -112,6 +112,19 @@ void json_decref(json_t *json)
         json_delete(json);
 }
 
+#if defined(__GNUC__) || defined(__clang__)
+static JSON_INLINE
+void json_decrefp(json_t **json)
+{
+    if(json) {
+        json_decref(*json);
+	*json = NULL;
+    }
+}
+
+#define json_auto_t json_t __attribute__((cleanup(json_decrefp)))
+#endif
+
 
 /* error reporting */
 

test/suites/api/test_simple.c

@@ -224,4 +224,17 @@ static void run_tests()
     json_incref(value);
     if(value->refcount != (size_t)-1)
       fail("refcounting null works incorrectly");
+
+#ifdef json_auto_t
+    value = json_string("foo");
+    {
+        json_auto_t *test = json_incref(value);
+        /* Use test so GCC doesn't complain it is unused. */
+        if(!json_is_string(test))
+            fail("value type check failed");
+    }
+    if(value->refcount != 1)
+	fail("automatic decrement failed");
+    json_decref(value);
+#endif
 }