Auto merge of #9621 - KiChjang:xhr-response-json, r=Ms2ger

Cache XHR JSON responses

Fixes #3919.

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.svg" height="40" alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/9621)
<!-- Reviewable:end -->

components/script/dom/xmlhttprequest.rs

@@ -17,7 +17,7 @@ use dom::bindings::conversions::{ToJSValConvertible};
 use dom::bindings::error::{Error, ErrorResult, Fallible};
 use dom::bindings::global::{GlobalRef, GlobalRoot};
 use dom::bindings::inheritance::Castable;
-use dom::bindings::js::{JS, MutNullableHeap};
+use dom::bindings::js::{JS, MutHeapJSVal, MutNullableHeap};
 use dom::bindings::js::{Root, RootedReference};
 use dom::bindings::refcounted::Trusted;
 use dom::bindings::reflector::{Reflectable, reflect_dom_object};
@@ -125,6 +125,8 @@ pub struct XMLHttpRequest {
     response_type: Cell<XMLHttpRequestResponseType>,
     response_xml: MutNullableHeap<JS<Document>>,
     response_blob: MutNullableHeap<JS<Blob>>,
+    #[ignore_heap_size_of = "Defined in rust-mozjs"]
+    response_json: MutHeapJSVal,
     #[ignore_heap_size_of = "Defined in hyper"]
     response_headers: DOMRefCell<Headers>,
     #[ignore_heap_size_of = "Defined in hyper"]
@@ -165,6 +167,7 @@ impl XMLHttpRequest {
             response_type: Cell::new(XMLHttpRequestResponseType::_empty),
             response_xml: Default::default(),
             response_blob: Default::default(),
+            response_json: MutHeapJSVal::new(),
             response_headers: DOMRefCell::new(Headers::new()),
             override_mime_type: DOMRefCell::new(None),
             override_charset: DOMRefCell::new(None),
@@ -720,15 +723,7 @@ impl XMLHttpRequestMethods for XMLHttpRequest {
                     }
                 },
                 XMLHttpRequestResponseType::Json => {
-                    let decoded = UTF_8.decode(&self.response.borrow(), DecoderTrap::Replace).unwrap().to_owned();
+                    self.json_response(cx).to_jsval(cx, rval.handle_mut());
-                    let decoded: Vec<u16> = decoded.utf16_units().collect();
-                    if !JS_ParseJSON(cx,
-                                     decoded.as_ptr(),
-                                     decoded.len() as u32,
-                                     rval.handle_mut()) {
-                        JS_ClearPendingException(cx);
-                        return NullValue();
-                    }
                 },
                 XMLHttpRequestResponseType::Blob => {
                     self.blob_response().to_jsval(cx, rval.handle_mut());
@@ -1093,6 +1088,39 @@ impl XMLHttpRequest {
         Some(temp_doc)
     }
 
+    #[allow(unsafe_code)]
+    // https://xhr.spec.whatwg.org/#json-response
+    fn json_response(&self, cx: *mut JSContext) -> JSVal {
+        // Step 1
+        let response_json = self.response_json.get();
+        if !response_json.is_null_or_undefined() {
+            return response_json;
+        }
+        // Step 2
+        let bytes = self.response.borrow();
+        // Step 3
+        if bytes.len() == 0 {
+            return NullValue();
+        }
+        // Step 4
+        let json_text = UTF_8.decode(&bytes, DecoderTrap::Replace).unwrap().to_owned();
+        let json_text: Vec<u16> = json_text.utf16_units().collect();
+        // Step 5
+        let mut rval = RootedValue::new(cx, UndefinedValue());
+        unsafe {
+            if !JS_ParseJSON(cx,
+                             json_text.as_ptr(),
+                             json_text.len() as u32,
+                             rval.handle_mut()) {
+                JS_ClearPendingException(cx);
+                return NullValue();
+            }
+        }
+        // Step 6
+        self.response_json.set(rval.ptr);
+        self.response_json.get()
+    }
+
     fn document_text_html(&self) -> Root<Document>{
         let charset = self.final_charset().unwrap_or(UTF_8);
         let wr = self.global();

tests/wpt/metadata/XMLHttpRequest/response-json.htm.ini

@@ -1,8 +0,0 @@
-[response-json.htm]
-  type: testharness
-  [JSON object roundtrip]
-    expected: FAIL
-
-  [JSON roundtrip with Japanese text]
-    expected: FAIL
-