Bug 633062 p1 - Introduce AsyncQueueCaller and AsyncObserver. r=markh

MozReview-Commit-ID: GGuSkA5DZUQ

--HG--
extra : rebase_source : 496d90945ec2e6d41f5751d26cf87a14458961bb

services/common/async.js

@@ -14,6 +14,7 @@ const CB_FAIL = {};
 const REASON_ERROR = Ci.mozIStorageStatementCallback.REASON_ERROR;
 
 ChromeUtils.import("resource://gre/modules/Services.jsm");
+ChromeUtils.import("resource://gre/modules/XPCOMUtils.jsm");
 
 /*
  * Helpers for various async operations.
@@ -203,5 +204,65 @@ this.Async = {
         await Async.promiseYield();
       }
     };
+  },
+
+  asyncQueueCaller(log) {
+    return new AsyncQueueCaller(log);
+  },
+
+  asyncObserver(log, obj) {
+    return new AsyncObserver(log, obj);
   }
 };
+
+/**
+ * Allows consumers to enqueue asynchronous callbacks to be called in order.
+ * Typically this is used when providing a callback to a caller that doesn't
+ * await on promises.
+ */
+class AsyncQueueCaller {
+  constructor(log) {
+    this._log = log;
+    this._queue = Promise.resolve();
+    this.QueryInterface = XPCOMUtils.generateQI([Ci.nsIObserver, Ci.nsISupportsWeakReference]);
+  }
+
+  /**
+   * /!\ Never await on another function that calls enqueueCall
+   *     on the same queue or we will deadlock. /!\
+   */
+  enqueueCall(func) {
+    this._queue = (async () => {
+      await this._queue;
+      try {
+        await func();
+      } catch (e) {
+        this._log.error(e);
+      }
+    })();
+  }
+
+  promiseCallsComplete() {
+    return this._queue;
+  }
+}
+
+/*
+ * Subclass of AsyncQueueCaller that can be used with Services.obs directly.
+ * When this observe() is called, it will enqueue a call to the consumers's
+ * observe().
+ */
+class AsyncObserver extends AsyncQueueCaller {
+  constructor(obj, log) {
+    super(log);
+    this.obj = obj;
+  }
+
+  observe(subject, topic, data) {
+    this.enqueueCall(() => this.obj.observe(subject, topic, data));
+  }
+
+  promiseObserversComplete() {
+    return this.promiseCallsComplete();
+  }
+}