[Mesa-dev] [PATCH] egl: Add GetSyncValuesCHROMIUM extension.

[Jamey Sharp]

On Fri, Apr 18, 2014 at 4:49 PM, Ian Romanick <idr at freedesktop.org> wrote:
> On 04/18/2014 03:37 PM, Sarah Sharp wrote:
>> One open question:
>>
>> I've used the normal (error checked) version of xcb_dri2_get_msc.  The
>> GLX implementation of glXGetSyncValuesOML uses the unchecked version,
>> but I'm not convinced it's necessary.
>>
>> I talked to Jamey Sharp, and he doesn't understand why most of Mesa
>> calls the unchecked versions of XCB functions.  He thought most
>> developers would want to use the normal (checked) versions, since the
>> unchecked versions may segfault on errors.  Mesa can always call the
>> checked version, but ignore the error that's set, so it doesn't have to
>> use the unchecked version.
>
> When I added some XCB code to Mesa a couple years ago, it wasn't
> completely clear to me what the difference was between the checked and
> unchecked versions.  See 6ccda72 and 588042a.  My recollection was that
> the _checked versions returned a cookie that you had to do something
> with.  For a bunch of the cases, I think we don't want to do the extra
> bookkeeping (or we don't want the extra opportunity for a memory) leak.
>
> Is there more difference than I know?

The key difference, in the case of requests that expect a reply, is
what happens to an X error if the server sends you one of those
instead of the reply you were expecting. In the checked case, XCB
returns that error to you when you ask for the reply, through an
optional out-parameter in the reply function. If you use the unchecked
variant of the request, then the error is placed in the event queue;
if you're using Xlib, then by default it will be abort()ing your
client as soon as it pulls the error out of the event queue.

There's no opportunity for a memory leak from using the default (that
is, checked) version of requests that expect a reply. If you don't
want to look at the details of the X error, just pass NULL for the
third argument of the reply function, in which case XCB will free the
error struct for you.

For requests where you do not expect to get a reply, XCB defaults to
unchecked behavior, and you have to use the _checked variant and an
xcb_request_check call if you want to keep the error out of the event
queue. Since xcb_request_check may have to do a round-trip to confirm
that no error occurred, you probably don't usually want checked
behavior for these requests. But that's why the default depends on
whether the request was going to have a reply anyway.

So I *think* you'll find life is much better in error cases if you
just s/_unchecked//g throughout Mesa. You'll get better error
reporting where it's free, without adding extra round-trips or
opportunities for memory leaks where it's more complicated. But there
could be something about Mesa's use case I still don't understand.

Jamey