This fixes a deadlock which was seen in-the-wild with wine.
It could happen that two threads tried to read from the socket at the same time
and one of the thread got stuck inside of poll()/select().
The fix works by making sure that the writing thread doesn't steal the reading
thread's reply.
Debugged-by: Erich Hoover <ehoover@mines.edu>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54671
Signed-off-by: Uli Schlachter <psychon@znc.in>
+ /* The code allows two threads to call select()/poll() at the same time.
+ * First thread just wants to read, a second thread wants to write, too.
+ * We have to make sure that we don't steal the reading thread's reply
+ * and let it get stuck in select()/poll().
+ * So a thread may read if either:
+ * - There is no other thread that wants to read (the above situation
+ * did not occur).
+ * - It is the reading thread (above situation occurred).
+ */
+ int may_read = c->in.reading == 1 || !count;
- if((fd.revents & POLLIN) == POLLIN)
+ if(may_read && (fd.revents & POLLIN) == POLLIN)
- if(FD_ISSET(c->fd, &rfds))
+ if(may_read && FD_ISSET(c->fd, &rfds))
#endif
ret = ret && _xcb_in_read(c);
#endif
ret = ret && _xcb_in_read(c);