pygmainloop: avoid lockups if multiple glib.MainLoop exist

If multiple glib.MainLoop() sources exist, then we will add multiple
watches, and when python writes a byte to the wakeup pipe, all of the
sources will try to read it; only one will succeed and the others
will block. Set both ends of the pipe nonblocking to avoid this.

https://bugzilla.gnome.org/show_bug.cgi?id=663068

gi/_glib/pygmainloop.c

@@ -217,7 +217,10 @@ pyg_signal_watch_new(void)
 	    g_error("Cannot create main loop pipe: %s\n",
 	            g_strerror(errno));
 
-        /* Make the write end of the fd non blocking */
+        /* Make both ends of the fd non blocking */
+        flag = fcntl(pipe_fds[0], F_GETFL, 0);
+        flag |= O_NONBLOCK;
+        fcntl(pipe_fds[0], F_SETFL, flag);
         flag = fcntl(pipe_fds[1], F_GETFL, 0);
         flag |= O_NONBLOCK;
         fcntl(pipe_fds[1], F_SETFL, flag);