Bug 701060 - Fix race condition with pref observers causing test orange. r=ehsan.

layout/style/test/test_bug389464.html

@@ -31,7 +31,7 @@ SimpleTest.waitForExplicitFinish();
 var cs1 = getComputedStyle(document.getElementById("one"), "");
 var cs2 = getComputedStyle(document.getElementById("two"), "");
 
-SpecialPowers.pushPrefEnv({'set': [['variable.x-western', 25], ['fixed.x-western', 20]]}, function() setTimeout(part1, 0));
+SpecialPowers.pushPrefEnv({'set': [['variable.x-western', 25], ['fixed.x-western', 20]]}, part1);
 
 function part1()
 {

layout/style/test/test_bug401046.html

@@ -56,7 +56,7 @@ function fs(idx) {
   return getComputedStyle(elts[idx], "").marginBottom;
 }
 
-SpecialPowers.pushPrefEnv({'clear': [['font.minimum-size.x-western']]}, function() setTimeout(step1, 0));
+SpecialPowers.pushPrefEnv({'clear': [['font.minimum-size.x-western']]}, step1);
 
 function step1() {
     is(fs(0), "0px", "at min font size 0, 0px should compute to 0px");
@@ -64,7 +64,7 @@ function step1() {
     is(fs(2), "12px", "at min font size 0, 12px should compute to 12px");
     is(fs(3), "28px", "at min font size 0, 28px should compute to 28px");
 
-    SpecialPowers.pushPrefEnv({'set': [['font.minimum-size.x-western', 7]]}, function() setTimeout(step2, 0));
+    SpecialPowers.pushPrefEnv({'set': [['font.minimum-size.x-western', 7]]}, step2);
 }
 
 function step2() {
@@ -73,7 +73,7 @@ function step2() {
     is(fs(2), "12px", "at min font size 7, 12px should compute to 12px");
     is(fs(3), "28px", "at min font size 7, 28px should compute to 28px");
 
-    SpecialPowers.pushPrefEnv({'set': [['font.minimum-size.x-western', 18]]}, function() setTimeout(step3, 0));
+    SpecialPowers.pushPrefEnv({'set': [['font.minimum-size.x-western', 18]]}, step3);
 }
 
 function step3() {

testing/mochitest/tests/SimpleTest/specialpowersAPI.js

@@ -546,8 +546,21 @@ SpecialPowersAPI.prototype = {
     }
 
     if (pendingActions.length > 0) {
+      // The callback needs to be delayed twice. One delay is because the pref
+      // service doesn't guarantee the order it calls its observers in, so it
+      // may notify the observer holding the callback before the other
+      // observers have been notified and given a chance to make the changes
+      // that the callback checks for. The second delay is because pref
+      // observers often defer making their changes by posting an event to the
+      // event loop.
+      function delayedCallback() {
+        function delayAgain() {
+          content.window.setTimeout(callback, 0);
+        }
+        content.window.setTimeout(delayAgain, 0);
+      }
       this._prefEnvUndoStack.push(cleanupActions);
-      this._pendingPrefs.push([pendingActions, callback]);
+      this._pendingPrefs.push([pendingActions, delayedCallback]);
       this._applyPrefs();
     } else {
       content.window.setTimeout(callback, 0);
@@ -556,8 +569,16 @@ SpecialPowersAPI.prototype = {
 
   popPrefEnv: function(callback) {
     if (this._prefEnvUndoStack.length > 0) {
+      // See pushPrefEnv comment regarding delay.
+      function delayedCallback() {
+        function delayAgain() {
+          content.window.setTimeout(callback, 0);
+        }
+        content.window.setTimeout(delayAgain, 0);
+      }
+      let cb = callback ? delayedCallback : null; 
       /* Each pop will have a valid block of preferences */
-      this._pendingPrefs.push([this._prefEnvUndoStack.pop(), callback]);
+      this._pendingPrefs.push([this._prefEnvUndoStack.pop(), cb]);
       this._applyPrefs();
     } else {
       content.window.setTimeout(callback, 0);