Bug 1284232 - Guard against ill-behaved objects on error check; r=automatedtester

It turns out that certain objects such as a DOMRectList have peculiarities
that cause string comparison to throw. This is normally not a problem
as error.isError is usually passed JSON serialisable data. But when it
is not, this try condition helps diagnose problems.

MozReview-Commit-ID: BLNSziwfxXs

--HG--
extra : rebase_source : 8bad973a20d8b69fa27f5de66e4ea287d4bcddcd

testing/marionette/error.js

@@ -29,21 +29,21 @@ const ERRORS = [
   "WebDriverError",
 ];
 
+const BUILTIN_ERRORS = new Set([
+  "Error",
+  "EvalError",
+  "InternalError",
+  "RangeError",
+  "ReferenceError",
+  "SyntaxError",
+  "TypeError",
+  "URIError",
+]);
+
 this.EXPORTED_SYMBOLS = ["error"].concat(ERRORS);
 
 this.error = {};
 
-error.BuiltinErrors = {
-  Error: 0,
-  EvalError: 1,
-  InternalError: 2,
-  RangeError: 3,
-  ReferenceError: 4,
-  SyntaxError: 5,
-  TypeError: 6,
-  URIError: 7,
-};
-
 /**
  * Checks if obj is an instance of the Error prototype in a safe manner.
  * Prefer using this over using instanceof since the Error prototype
@@ -61,7 +61,13 @@ error.isError = function(val) {
   } else if (val instanceof Ci.nsIException) {
     return true;
   } else {
-    return Object.getPrototypeOf(val) in error.BuiltinErrors;
+    // DOMRectList errors on string comparison
+   try {
+      let proto = Object.getPrototypeOf(val);
+      return BUILTIN_ERRORS.has(proto.toString());
+    } catch (e) {
+      return false;
+    }
   }
 };
 

testing/marionette/test_error.js

@@ -10,19 +10,6 @@ function notok(condition) {
   ok(!(condition));
 }
 
-add_test(function test_BuiltinErrors() {
-  ok("Error" in error.BuiltinErrors);
-  ok("EvalError" in error.BuiltinErrors);
-  ok("InternalError" in error.BuiltinErrors);
-  ok("RangeError" in error.BuiltinErrors);
-  ok("ReferenceError" in error.BuiltinErrors);
-  ok("SyntaxError" in error.BuiltinErrors);
-  ok("TypeError" in error.BuiltinErrors);
-  ok("URIError" in error.BuiltinErrors);
-
-  run_next_test();
-});
-
 add_test(function test_isError() {
   notok(error.isError(null));
   notok(error.isError([]));