libxl: event: poller pipe optimisation

Track in userland whether the poller pipe is nonempty.  This saves us
writing many many bytes to the pipe if nothing ever reads them.

This is going to be relevant in a moment, where we are going to create
a situation where this will happen quite a lot.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Tested-by: George Dunlap <george.dunlap@citrix.com>

tools/libxl/libxl_event.c

@@ -1417,6 +1417,7 @@ static void afterpoll_internal(libxl__egc *egc, libxl__poller *poller,
     }
 
     if (afterpoll_check_fd(poller,fds,nfds, poller->wakeup_pipe[0],POLLIN)) {
+        poller->pipe_nonempty = 0;
         int e = libxl__self_pipe_eatall(poller->wakeup_pipe[0]);
         if (e) LIBXL__EVENT_DISASTER(gc, "read wakeup", e, 0);
     }
@@ -1809,6 +1810,8 @@ void libxl__poller_put(libxl_ctx *ctx, libxl__poller *p)
 
 void libxl__poller_wakeup(libxl__gc *gc, libxl__poller *p)
 {
+    if (p->pipe_nonempty) return;
+    p->pipe_nonempty = 1;
     int e = libxl__self_pipe_wakeup(p->wakeup_pipe[1]);
     if (e) LIBXL__EVENT_DISASTER(gc, "cannot poke watch pipe", e, 0);
 }

tools/libxl/libxl_internal.h

@@ -625,6 +625,7 @@ struct libxl__poller {
     int (*fd_rindices)[3]; /* see libxl_event.c:beforepoll_internal */
 
     int wakeup_pipe[2]; /* 0 means no fd allocated */
+    bool pipe_nonempty;
 
     /*
      * We also use the poller to record whether any fds have been