[PATCH xlib] Add support for a display-specific error handler

[Jasper St. Pierre]

Now do this:

static void Foo()
{
    Request1();
    Request2();
}

static void Bar()
{
    Request1();
    Request3();
}

static void Thing(int x)
{
    SetErrorHandler();
    if (x == 0)
        Foo();
    else
        Bar();
    UnsetErrorHandlerAndCheck();
}

We have a similar codepath in our driver and this is where I gave up
and started on the Xlib solution.


On Mon, Nov 16, 2015 at 12:39 PM, Uli Schlachter <psychon at znc.in> wrote:
> Am 16.11.2015 um 18:06 schrieb Jasper St. Pierre:
> [...]
>> We currently use a behavior like:
>>
>>     SetErrorHandler();
>>     Request1();
>>     Request2();
>>     Request3();
>>     if (UnsetErrorHandlerAndCheck())
>>         goto error;
>>
>> UnsetErrorHandlerAndCheck will XSync, and look at any newly incoming
>> errors, check their serials so that they match up, and if they are
>> inside those bounds, we return the error to the user and return that
>> no error happened. Fairly standard trick.
>>
>> With xcb, I can do:
>>
>>     if (check_request(request_1_checked(), &err) {
>>         free(err);
>>         goto error;
>>     }
>>     if (check_request(request_2_checked(), &err) {
>>         free(err);
>>         goto error;
>>     }
>>     if (check_request(request_3_checked(), &err) {
>>         free(err);
>>         goto error;
>>     }
>>
>> So that's clearly unusable.
>
> Well, why?
>
> How about this version:
>
> xcb_void_cookie_t cookies[3];
>
> cookies[0] = request_1_checked();
> cookies[1] = request_2_checked();
> cookies[2] = request_3_checked();
>
> bool had_error = false;
> for (int i = 0; i < 3; i++) {
>   xcb_generic_error_t *err = xcb_request_check(c, cookies[i]);
>   had_error |= err != NULL;
>   free(NULL);
> }
>
> This only syncs once during the first call to xcb_request_check() and afterwards
> it's done. This even improves parallelism, because you do not have to
> XLockDisplay() to lock out other threads (which also means that "just a range of
> sequence numbers" isn't enough and an array is needed).
>
> You could even generalize this with some helper functions to make it nicer,
> similar to your SetErrorHandler() and UnsetErrorHandlerAndCheck() above.
>
> Uli
>
> [...]
>> The Display-specific error handler solves my needs here by giving me a
>> callback to that above model that's locked to that local section that
>> works well from a library -- this code runs in a different thread, and
>> we had issues where we overwrite another thread's error handler (in my
>> case, the two threads were using different Displays, as they should).
>>
>> On Sun, Nov 15, 2015 at 11:56 PM, Keith Packard <keithp at keithp.com> wrote:
>>> "Jasper St. Pierre" <jstpierre at mecheye.net> writes:
>>>
>>>> Writing error-safe code that uses Xlib is too obnoxious, and using XCB
>>>> is tedious and not performant, as we can't catch events on a giant
>>>> stream -- we have to check every operation manually.
>>>
>>> You get errors returned in the event stream; is there something fancier
>>> you'd like from xcb to make it faster?
>>>
>>> --
>>> -keith
>
> --
> "Every once in a while, declare peace. It confuses the hell out of your enemies"
>  - 79th Rule of Acquisition
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-- 
  Jasper