Bug 1370752: Part 2 - Allow fallback serializer when JSON.serialize fails. r=aswan

Currently, we need to be able to handle serializing non-JSON-compatible
objects without catastrophically failing to save the storage file. Ideally, we
would ensure this in the ordinary toJSON method. However, that would require
a unnecessary extra calls to JSON.stringify for each object that needs to be
sanitized before returning a JSON-safe value, which is more expensive than we
can afford.

The fallback toJSONSafe method allows us to do this only when necessary, due
to an initial failed JSON serialization.

MozReview-Commit-ID: JXQ001dOGtW

--HG--
extra : rebase_source : 0b7b388316fdc464b47cdd4f7d8c70bc906a9c27

toolkit/modules/JSONFile.jsm

@@ -286,8 +286,20 @@ JSONFile.prototype = {
    * @rejects JavaScript exception.
    */
   async _save() {
+    let json;
+    try {
+      json = JSON.stringify(this._data);
+    } catch (e) {
+      // If serialization fails, try fallback safe JSON converter.
+      if (typeof this._data.toJSONSafe == "function") {
+        json = JSON.stringify(this._data.toJSONSafe());
+      } else {
+        throw e;
+      }
+    }
+
     // Create or overwrite the file.
-    let bytes = gTextEncoder.encode(JSON.stringify(this._data));
+    let bytes = gTextEncoder.encode(json);
     if (this._beforeSave) {
       await Promise.resolve(this._beforeSave());
     }