Bug 1431194 - avoid timeouts when clearing prefs set via pushPrefEnv;r=jmaher

MozReview-Commit-ID: 1EYsxYnj2GE

--HG--
extra : rebase_source : dfa5d822e46387d9d473b5b744289f15f4410dbe

testing/mochitest/tests/Harness_sanity/test_SpecialPowersPushPrefEnv.html

@@ -21,6 +21,7 @@ function starttest() {
     SpecialPowers.setIntPref("test.int", 1);
   }
   SpecialPowers.setCharPref("test.char", 'test');
+  SpecialPowers.setBoolPref("test.cleanup", false);
 
   setTimeout(test1, 0, 0);
 }
@@ -200,9 +201,29 @@ function test11(aCount) {
     SpecialPowers.getCharPref('test.char');
     setTimeout(test11, 0, ++aCount);
   } catch(e) {
-    SimpleTest.finish();
+    test12();
   }
 }
+
+function test12() {
+  // Set test.cleanup to true via pushPrefEnv, while its default value is false.
+  SpecialPowers.pushPrefEnv({"set": [["test.cleanup", true]]}, () => {
+    // Update the preference manually back to its original value.
+    SpecialPowers.setBoolPref("test.cleanup", false);
+    setTimeout(test13, 0);
+  });
+}
+
+function test13() {
+  // Try to flush preferences. Since test.cleanup has manually been set to false, there
+  // will not be any visible update. Check that the flush does not timeout.
+  SpecialPowers.flushPrefEnv(() => {
+    ok(true, 'flushPrefEnv did not time out');
+    is(SpecialPowers.getBoolPref('test.cleanup'), false, 'test.cleanup should be false');
+    SimpleTest.finish();
+  });
+}
+
 // todo - test non-changing values, test complex values, test mixing of pushprefEnv 'set' and 'clear'
 // When bug 776424 gets fixed, getPref doesn't throw anymore, so this test would have to be changed accordingly
 </script>

testing/specialpowers/content/specialpowersAPI.js

@@ -1191,6 +1191,22 @@ SpecialPowersAPI.prototype = {
     });
   },
 
+  _isPrefActionNeeded(prefAction) {
+    if (prefAction.action === "clear") {
+      return Services.prefs.prefHasUserValue(prefAction.name);
+    } else if (prefAction.action === "set") {
+      try {
+        let currentValue  = this._getPref(prefAction.name, prefAction.type, {});
+        return currentValue != prefAction.value;
+      } catch (e) {
+        // If the preference is not defined yet, setting the value will have an effect.
+        return true;
+      }
+    }
+    // Only "clear" and "set" actions are supported.
+    return false;
+  },
+
   /*
     Iterate through one atomic set of pref actions and perform sets/clears as appropriate.
     All actions performed must modify the relevant pref.
@@ -1206,19 +1222,33 @@ SpecialPowersAPI.prototype = {
     var pendingActions = transaction[0];
     var callback = transaction[1];
 
-    var lastPref = pendingActions[pendingActions.length - 1];
+    // Filter out all the pending actions that will not have any effect.
+    pendingActions = pendingActions.filter(action => {
+      return this._isPrefActionNeeded(action);
+    });
 
-    var pb = Services.prefs;
     var self = this;
-    pb.addObserver(lastPref.name, function prefObs(subject, topic, data) {
-      pb.removeObserver(lastPref.name, prefObs);
-
+    let onPrefActionsApplied = function() {
       self._setTimeout(callback);
       self._setTimeout(function() {
         self._applyingPrefs = false;
         // Now apply any prefs that may have been queued while we were applying
         self._applyPrefs();
       });
+    };
+
+    // If no valid action remains, call onPrefActionsApplied directly and bail out.
+    if (pendingActions.length === 0) {
+      onPrefActionsApplied();
+      return;
+    }
+
+    var lastPref = pendingActions[pendingActions.length - 1];
+
+    var pb = Services.prefs;
+    pb.addObserver(lastPref.name, function prefObs(subject, topic, data) {
+      pb.removeObserver(lastPref.name, prefObs);
+      onPrefActionsApplied();
     });
 
     for (var idx in pendingActions) {