Bug 942795 - [email] autoconfig XHR callbacks seem to be firing multi…

…ple times on nightly, may produce duplicate accounts; investigate XHR event changes, switch to promises

data/lib/mailapi/accountcommon.js

@@ -306,11 +306,15 @@ Autoconfigurator.prototype = {
       });
     };
 
-    xhr.ontimeout = xhr.onerror = function() {
+    // Caution: don't overwrite ".onerror" twice here. Just be careful
+    // to only assign that once until <http://bugzil.la/949722> is fixed.
+
+    xhr.ontimeout = function() {
       // The effective result is a failure to get configuration info, but make
       // sure the status conveys that a timeout occurred.
       callback('no-config-info', null, { status: 'timeout' });
     };
+
     xhr.onerror = function() {
       // The effective result is a failure to get configuration info, but make
       // sure the status conveys that a timeout occurred.

test/unit/resources/fake_xhr.js

@@ -17,7 +17,6 @@ function FakeXHR() {
   };
 
   this.onload = null;
-  this.onerror = null;
   this.ontimeout = null;
 
   this.timeout = null;
@@ -37,6 +36,21 @@ FakeXHR.prototype = {
     this._args.async = async;
   },
 
+  // To prevent regressions related to <https://bugzil.la/949722> in
+  // which XHRs treat .onerror like .addEventListener('error'), ensure
+  // that we only assign to this property once for safety:
+
+  get onerror() {
+    return this._onerror;
+  },
+
+  set onerror(fn) {
+    if (fn && this._onerror) {
+      throw new Error("onerror already set on XHR! Don't. See bug 949722.");
+    }
+    this._onerror = fn;
+  },
+
   send: function() {
     this._args.timeout = this.timeout;
     if (window.gFakeXHRListener) {