On 04/27/2018 12:14 AM, Ben Kelly wrote:
Hi all,
I just pushed another helper class that I thought others might find useful.
CopyableErrorResult is a specialized form of ErrorResult. Its intended to
allow slightly more rich error values to be passed through things like ipdl
structure and MozPromise. This is useful when implementing web exposed
features that need to execute code in the parent process and surface useful
TypeError messages back to the page. A simple nsresult loses the developer
friendly messages. If you are building something like this, then you might
want to consider CopyableErrorResult.
If in doubt, though, please use ErrorResult itself. It has stricter
checking and should be preferred. Particularly if there is any chance the
error might contain a js exception.
The CopyableErrorResult has a few properties:
1. Asserts if you try to store a js exception. It may only contain simple
errors or errors with messages. In release builds it converts the js
exception to an NS_ERROR_FAILURE.
2. Has a copy constructor, assignment operator, etc.
3. May be safely transferred across threads.
4. Automatically suppresses errors instead of asserting if its not consumed.
Hmm, (4) is worrisome. The name of the class doesn't hint this kind of behavior,
and we have IgnoredErrorResult.
Why does CopyableErrorResult have this behavior? I understand not asserting if
the error was copied from some other ErrorResult to a CopyableErrorResult
instance,
but shouldn't we assert if that didn't happen?
These are significant because they mean you can create an ipdl structure or
union type that contains a CopyableErrorResult. This is not possible with
an ordinary ErrorResult because there is no copy constructor, etc.
It is safer to use CopyableErrorResult in a MozPromise reaction because its
thread safe and does not assert if its destroyed before consumed. These
are important because MozPromise reactions can cross thread boundaries and
a disconnected promise may not consume its reaction value, etc.
Using CopyableErrorResult allows you to provide error messages along with
TypeError, etc. Passing just an nsresult provides a worse developer
experience for webdevs.
One quirk you may run into is that CopyableErrorResult may not be used as a
value argument to a method. This is because it contains a js MOZ_NON_PARAM
value in a union type even through it does runtime checking to avoid ever
actually using that type. To work around this you do this:
DoFoo(const CopyableErrorResult& aRv) {
ConsumeResult(CopyableErrorResult(aRv));
}
This is often needed in MozPromise reaction handlers.
Please let me know if you run into any problems.
Thanks.
Ben
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