libxl: event: Fix hang when mixing blocking and eventy calls

libxl: event: Fix hang when mixing blocking and eventy calls

If the application calls libxl with ao_how==0 and also makes calls
like _occurred, libxl will sometimes get stuck.

The bug happens as follows (for example):

  Thread A
       libxl_do_thing(,ao_how==0)
       libxl_do_thing starts, sets up some callbacks
       libxl_do_thing exit path calls AO_INPROGRESS
       libxl__ao_inprogress goes into event loop
       eventloop_iteration sleeps on:
          - do_thing's current fd set
          - sigchld pipe if applicable
          - its poller

  Thread B
       libxl_something_occurred
       the something is to do with do_thing, above
       do_thing_next_callback does some more work
       do_thing_next_callback becomes interested in fd N
       thread B returns to application

Note that nothing wakes up thread A.  A is not listening on fd N.  So
do_thing_* will not spot when fd N signals.  do_thing will not make
further timely progress.  If there is no timeout thread A will never
wake up.

The problem here occurs because thread A is waiting on an out of date
osevent set.

There is also the possibility that a thread might block waiting for
libxl osevents but outside libxl, eg if the application used
libxl_osevent_beforepoll.  We will deal with that in a moment.

See the big comment in libxl_event.c for a fairly formal correctness
argument.

This depends on libxl__egc_ao_cleanup_1_baton being called everywhere
an egc or ao is disposed of.  Firstly egcs: in this patch we rename
libxl__egc_cleanup, which means we catch all the disposal sites.
Secondly aos: these are disposed of by (i) AO_CREATE_FAIL
(ii) ao__inprogress and (iii) an event which completes the ao later.
(i) and (ii) we handle by adding the call to _baton.  In the case of
(iii) any such function must be an event-generating function so it has
an egc too, so it will pass on the baton when the egc is disposed.

Reported-by: George Dunlap <george.dunlap@citrix.com>
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>
---
v2: Call libxl__egc_ao_cleanup_1_baton (renamed from __egc_cleanup) on
    all exits from ao_inprogress, even requests for async processing.
    Fixes a remaining instance of this bug (!)
    This involves disposing of ao->poller somewhat earlier.

v2: New correctness arguments in libxl_event.c comment and
    in commit message.