Applied changes from emailed code review

lib/hooks.js

+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+function hook(obj, name, func, cleanup)
+{
+  let orig = obj[name];
+  let origGet = obj.__lookupGetter__(name);
+  let origSet = obj.__lookupSetter__(name);
+  let dumbOverrideAttempt = false;
+
+  let newFunc = function()
+  {
+    let params = arguments;
+    try
+    {
+      let result = func.apply(this, params);
+      if (typeof result == "object")
+        params = result;
+    }
+    catch(e)
+    {
+      Cu.reportError(e);
+    }
+
+    try
+    {
+      return orig.apply(this, params);
+    }
+    finally
+    {
+      if (typeof cleanup == "function")
+        cleanup();
+    }
+  };
+  newFunc.toString = function()
+  {
+    dumbOverrideAttempt = true;
+    return orig.toString();
+  };
+
+  obj.__defineGetter__(name, function()
+  {
+    dumbOverrideAttempt = false;
+    return newFunc;
+  });
+
+  obj.__defineSetter__(name, function(value)
+  {
+    if (dumbOverrideAttempt)
+    {
+      orig = value;
+    }
+    else
+    {
+      delete obj[name];
+      obj[name] = value;
+    }
+  });
+
+  return function()
+  {
+    delete obj[name];
+    obj[name] = orig;
+    if (origGet)
+    {
+      obj.__defineGetter__(name, origGet);
+    }
+    if (origSet)
+    {
+      obj.__defineSetter__(name, origSet);
+    }
+  };
+}
+exports.hook = hook;