Add a way to "catch" an ErrorResult, and a way to safely convert an ErrorResult to an nsresult.

dom/bindings/BindingUtils.cpp

@@ -331,6 +331,24 @@ ErrorResult::ReportNotEnoughArgsError(JSContext* cx,
   ThrowErrorMessage(cx, dom::MSG_MISSING_ARGUMENTS, errorMessage.get());
 }
 
+void
+ErrorResult::SuppressException()
+{
+  WouldReportJSException();
+  if (IsErrorWithMessage()) {
+    ClearMessage();
+  } else if (IsJSException()) {
+    JSContext* cx = nsContentUtils::GetDefaultJSContextForThread();
+    // Just steal it into a stack value (unrooting it in the process)
+    // that we then allow to die.
+    JS::Rooted<JS::Value> temp(cx);
+    StealJSException(cx, &temp);
+  }
+  // We don't use AssignErrorCode, because we want to override existing error
+  // states, which AssignErrorCode is not allowed to do.
+  mResult = NS_OK;
+}
+
 namespace dom {
 
 bool

dom/bindings/ErrorResult.h

@@ -64,6 +64,20 @@ public:
     mResult = rv;
   }
 
+  // Use SuppressException when you want to suppress any exception that might be
+  // on the ErrorResult.  After this call, the ErrorResult will be back a "no
+  // exception thrown" state.
+  void SuppressException();
+
+  // Use StealNSResult() when you want to safely convert the ErrorResult to an
+  // nsresult that you will then return to a caller.  This will
+  // SuppressException(), since there will no longer be a way to report it.
+  nsresult StealNSResult() {
+    nsresult rv = ErrorCode();
+    SuppressException();
+    return rv;
+  }
+
   void ThrowTypeError(const dom::ErrNum errorNumber, ...);
   void ThrowRangeError(const dom::ErrNum errorNumber, ...);
   void ReportErrorWithMessage(JSContext* cx);