servo: Merge #16884 - Create higher-level APIs for manipulating custom structured clones (from jdm:structuredclone); r=jdm

Rebase of #16321.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #16031
- [X] There are tests for these changes

Source-Repo: https://github.com/servo/servo
Source-Revision: a7b77306f985b79d3b8b4d9b2cb51fa38f45bd39

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 95dcc98b1fcc1fe4fd238bf676125b14c211bf2c

servo/components/script/dom/bindings/structuredclone.rs

@@ -67,16 +67,44 @@ unsafe fn read_length(r: *mut JSStructuredCloneReader)
   return length as usize;
 }
 
+struct StructuredCloneWriter {
+    w: *mut JSStructuredCloneWriter,
+}
+
+impl StructuredCloneWriter {
+    unsafe fn write_slice(&self, v: &[u8]) {
+        let type_length = v.len();
+        write_length(self.w, type_length);
+        assert!(JS_WriteBytes(self.w, v.as_ptr() as *const raw::c_void, type_length));
+    }
+    unsafe fn write_str(&self, s: &str) {
+        self.write_slice(s.as_bytes());
+    }
+}
+
+struct StructuredCloneReader {
+    r: *mut JSStructuredCloneReader,
+}
+
+impl StructuredCloneReader {
+    unsafe fn read_bytes(&self) -> Vec<u8> {
+        let mut bytes = vec![0u8; read_length(self.r)];
+        let blob_length = bytes.len();
+        assert!(JS_ReadBytes(self.r, bytes.as_mut_ptr() as *mut raw::c_void, blob_length));
+        return bytes;
+    }
+    unsafe fn read_str(&self) -> String {
+        let str_buffer = self.read_bytes();
+        return String::from_utf8_unchecked(str_buffer);
+    }
+}
+
 unsafe fn read_blob(cx: *mut JSContext,
                     r: *mut JSStructuredCloneReader)
                     -> *mut JSObject {
-    let blob_length = read_length(r);
-    let type_str_length = read_length(r);
-    let mut blob_buffer = vec![0u8; blob_length];
-    assert!(JS_ReadBytes(r, blob_buffer.as_mut_ptr() as *mut raw::c_void, blob_length));
-    let mut type_str_buffer = vec![0u8; type_str_length];
-    assert!(JS_ReadBytes(r, type_str_buffer.as_mut_ptr() as *mut raw::c_void, type_str_length));
-    let type_str = String::from_utf8_unchecked(type_str_buffer);
+    let structured_reader = StructuredCloneReader { r: r };
+    let blob_buffer = structured_reader.read_bytes();
+    let type_str = structured_reader.read_str();
     let target_global = GlobalScope::from_context(cx);
     let blob = Blob::new(&target_global, BlobImpl::new_from_bytes(blob_buffer), type_str);
     return blob.reflector().get_jsobject().get()
@@ -85,15 +113,11 @@ unsafe fn read_blob(cx: *mut JSContext,
 unsafe fn write_blob(blob: Root<Blob>,
                      w: *mut JSStructuredCloneWriter)
                      -> Result<(), ()> {
+    let structured_writer = StructuredCloneWriter { w: w };
     let blob_vec = try!(blob.get_bytes());
-    let blob_length = blob_vec.len();
-    let type_string_bytes = blob.type_string().as_bytes().to_vec();
-    let type_string_length = type_string_bytes.len();
     assert!(JS_WriteUint32Pair(w, StructuredCloneTags::DomBlob as u32, 0));
-    write_length(w, blob_length);
-    write_length(w, type_string_length);
-    assert!(JS_WriteBytes(w, blob_vec.as_ptr() as *const raw::c_void, blob_length));
-    assert!(JS_WriteBytes(w, type_string_bytes.as_ptr() as *const raw::c_void, type_string_length));
+    structured_writer.write_slice(&blob_vec);
+    structured_writer.write_str(&blob.type_string());
     return Ok(())
 }