[PATCH wayland v3] client: Introduce proxy wrappers

[Pekka Paalanen]

On Thu, 28 Apr 2016 15:31:31 +0800
Jonas Ådahl <jadahl at gmail.com> wrote:

> Using the libwayland-client client API with multiple threads with
> thread local queues are prone to race conditions.
> 
> The problem is that one thread can read and queue events after another
> thread creates a proxy but before it sets the queue.
> 
> This may result in the event to the proxy being silently dropped, or
> potentially dispatched on the wrong thread had the creating thread set
> the implementation before setting the queue.
> 
> This patch introduces API to solve this case by introducing "proxy
> wrappers". In short, a proxy wrapper is a wl_proxy struct that will
> never itself proxy any events, but may be used by the client to set a
> queue, and use it instead of the original proxy when sending requests
> that creates new proxies. When sending requests, the wrapper will
> work in the same way as the normal proxy object, but the proxy created
> by sending a request (for example wl_display.sync) will inherit to the
> same proxy queue as the wrapper.
> 
> https://bugs.freedesktop.org/show_bug.cgi?id=91273
> 
> Signed-off-by: Jonas Ådahl <jadahl at gmail.com>
> ---
> 
> Changes since v2:
> 
>  - Reworded the commit message and added bugzilla link
>  - Changed some wl_log("warning: ..."); return; to wl_abort(...);
>  - Made wl_proxy_create_wrapper() set errno on error (including EINVAL)
>  - Made it allowed to create a wrapper from a wrapper - this was an arbitrary
>    restriction, and I believe it makes sense to allow it
>  - Changed flags == ... to flags & ...
>  - Documentation fixes

Hi Jonas,

hmm, but if you allow creating a wrapper 'B' from a wrapper 'A', you
also allow creating a wrapper for a dead underlying real proxy 'P'.
Like this:

1. create 'A' from 'P'
2. destroy 'P'
3. create 'B' from 'A'

That happens because a wrapper can never get the "dead" flags.

Will that be a problem?

I'm not sure, so I'd like to err on the safe side: forbid creating a
wrapper from a wrapper for now. If someone needs it, we can allow it
later. We cannot deny it later that easily. How's that?

Or, do not error out on creating a wrapper from a dead object. Creating
a wrapper from a dead object and sending request on that is no
different to sending a request on the original proxy, right?
Maybe that would make it even a bit easier to use, as the only case of
wrapper creation failing would be OOM.

Otherwise all the changes look good to me, and the above question is
the only issue barring my R-b.


Thanks,
pq

>  src/wayland-client-core.h |   6 +++
>  src/wayland-client.c      | 127 +++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 132 insertions(+), 1 deletion(-)
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