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Core Wayland window system code and protocol
74df22befe
I've noticed a blocking problem in Wayland's event-loop code when updating timer event sources. The problem occurs if you update the timer at a point after is has expired, but before it has been dispatched, i.e. from an event callback that happens during the same epoll wakeup. When the timer is subsequently dispatched, wl_event_source_timer_dispatch blocks for the duration of the new timeout in its call to read() from the timer fd (which is the expected behaviour according to the man page for timerfd_settime). This isn't too uncommon a scenario - for example, a socket with an associated timeout timer. You'd typically want to update the timer when reading from the socket. This is how I noticed the issue, since I was setting a timeout of 1 minute, and saw my server blocking for this duration! The following patch adds a (currently failing) test case to Wayland's event-loop-test.c. It demonstrates the problem using two timers, which are set to expire at the same time. The first timer to receive its expiry callback updates the other timer with a much larger timeout, which then causes the test to block for this timeout before calling the second timer's callback. As for a fix, I'm not so sure (which is why I thought I'd post the failing test case first to show what I mean). I notice that it doesn't actually do anything with the value read from the timerfd socket, which gives the number of times the timer expired since the last read, or when the timer was last updated (which blocks if the timer hasn't yet expired). I believe this value should always read as 1 anyway, since we don't use periodic timers. A simple fix would be to use the TFD_NONBLOCK option when creating the timerfd, ensuring that the read call won't block. We'd then have to ignore the case when the read returns EAGAIN. |
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What is Wayland
Wayland is a project to define a protocol for a compositor to talk to
its clients as well as a library implementation of the protocol. The
compositor can be a standalone display server running on Linux kernel
modesetting and evdev input devices, an X application, or a wayland
client itself. The clients can be traditional applications, X servers
(rootless or fullscreen) or other display servers.
The wayland protocol is essentially only about input handling and
buffer management. The compositor receives input events and forwards
them to the relevant client. The clients creates buffers and renders
into them and notifies the compositor when it needs to redraw. The
protocol also handles drag and drop, selections, window management and
other interactions that must go through the compositor. However, the
protocol does not handle rendering, which is one of the features that
makes wayland so simple. All clients are expected to handle rendering
themselves, typically through cairo or OpenGL.
The weston compositor is a reference implementation of a wayland
compositor and the weston repository also includes a few example
clients.
Building the wayland libraries is fairly simple, aside from libffi,
they don't have many dependencies:
$ git clone git://anongit.freedesktop.org/wayland/wayland
$ cd wayland
$ ./autogen.sh --prefix=PREFIX
$ make
$ make install
where PREFIX is where you want to install the libraries. See
http://wayland.freedesktop.org for more complete build instructions
for wayland, weston, xwayland and various toolkits.