[RFC PATCH v2 2/5] server: Add a new server request "notify_async_direct_result."

[Jinoh Kang]

On 1/28/22 08:32, Zebediah Figura (she/her) wrote:
> On 1/27/22 13:22, Jinoh Kang wrote:
>> On 1/28/22 04:13, Jinoh Kang wrote:
>>> On 1/28/22 04:05, Jinoh Kang wrote:
>>>> +
>>>> +/* Notify direct completion of async and close the wait handle */
>>>> +DECL_HANDLER(notify_async_direct_result)
>>>> +{
>>>> +    struct async *async = (struct async *)get_handle_obj( current->process, req->handle, 0, &async_ops );
>>>> +
>>>> +    if (!async) return;
>>>> +
>>>> +    if (async->iosb && async->unknown_status && !async->pending && async->terminated)
>>>> +    {
>>>> +        /* Reactivate async.  We call async_reselect() later. */
>>>> +        async->terminated = 0;
>>>> +
>>>> +        /* Set result for async_handoff(). */
>>>> +        set_error( req->status );
>>>> +        async->iosb->result = req->information;
>>>> +
>>>> +        /* The async_handoff() call prior to the current server request was
>>>> +         * effectively a no-op since async->unknown_status is 1.  Calling it
>>>> +         * again with async->unknown_status = 0 will do the remaining steps.
>>>> +         */
>>>> +        async->unknown_status = 0;
>>>> +        async_handoff( async, NULL, 0 );
>>>> +
>>>> +        if (get_error() == STATUS_PENDING)
>>>> +        {
>>>> +            async_reselect( async );
>>>> +        }
>>>> +        else if (NT_ERROR( get_error() ))
>>>> +        {
>>>> +            /* synchronous I/O failure: don't invoke callbacks, only dequeue it. */
>>>> +            async_dequeue( async );
>>>
>>> Note: we can't use async_set_result() here.  async_handoff() leaves async->terminated as 0, and async_set_result() has "assert( async->terminated )".
>>>
>>> Thus, reusing async_set_result() here requires either:
>>>
>>> 1. Setting async->pending to 0 so that async_handoff() will call async_terminate().
>>>     This is obviously incorrect since unwanted IOCP packets and APCs will fire on synchronous failure.
>>>
>>> 2. Not using async_handoff().  We have to copy a lot of code (particulary setting pending and direct_result flags) out of async_handoff() to do this.
>>
>> Maybe just merge both branches (error/success) and force async_terminate() before calling async_set_result()?
> 
> Actually, I believe the right thing to do is just to set async->terminated = 1. In fact, I believe you should set async->terminated = 1 in the recv_socket request,

Yes, it's already done in async_handoff().  Note the precondition above:

> +    if (async->iosb && async->unknown_status && !async->pending && async->terminated)
> +    {
> +        /* Reactivate async.  We call async_reselect() later. */
> +        async->terminated = 0;

> otherwise async_waiting() will return 1 and the server will end up spinning and/or trying to wake the async. (Granted, that could be protected against by checking unknown_status, but...)
> 
> There are a few different ways to look at this, but one way (and what I think makes the most sense to me) is that we're completing the async as if it had been alerted, much as we would from a kernel APC, except that we're also setting the initial status, which wasn't known yet. Hence we want to go through async_set_result().
> 
> Along these lines, if you also set async->alerted = 1, you get the STATUS_PENDING "restart the wait" behaviour for free. Which makes conceptual sense as well.

I'm honestly split off as to which side should take the responsibility of setting "async->alerted = 1", async_handoff() or notify_async_direct_result.
I suppose I'm going with the former, since that's where STATUS_ALERTED is received.  But then, shall the latter have "assert( async->alerted );"...?

> 
> I suppose this is why you mentioned direct_result in [1]—i.e. we could just async_terminate(STATUS_ALERTED) in recv_socket. I can see both approaches working, but I think that open-coding the relevant parts of async_terminate would be better than trying to get direct_result manipulation exactly right.

I agree.  I hope we don't also have to also open code "async_handoff()", though.

> Of course, I also haven't tried...
> 
> [1] https://www.winehq.org/pipermail/wine-devel/2022-January/205774.html

That said, the more I work with the async code, the more I learn to hate all those status bitfields...
I think it's much better off merging as many states as possible into a single enum, so that the lifecycle is explicit (e.g. ASYNC_INITIAL -> ASYNC_QUEUED -> ASYNC_ALERTED -> ...)
(I'm aware it was much worse before that, making the status field serve the dual role of async state and I/O status.)

-- 
Sincerely,
Jinoh Kang