[RFC PATCH v2 00/29] libxl: Cancelling asynchronous operations

[Ian Jackson <ian.jackson@eu.citrix.com>]

This is v2 of my work-in-progress series to support cancellation of
long-running libxl operations.

There are many improvements since v1, but the basic structure remains
the same and the external API remains unchanged.

I have rebased this onto current staging.  I have compiled it but
NOT EXECUTED IT AT ALL.  Euan, I thought it would be useful to give
you something you could start to work on building against.

I wouldn't recommend testing it yet until I've at least smoke tested
it to see that things still work if you don't cancel them.

Here's a list of the patches:

  01/29  libxl: Further fix exit paths from libxl_device_events_handler
  02/29  libxl: Comment cleanups
  03/29  libxl: suspend: switch_logdirty_done takes rc
  04/29  libxl: suspend: common suspend callbacks take rc
  05/29  libxl: suspend: Return correct error from callbacks
  06/29  libxl: Use libxl__xswait* in libxl__ao_device
  07/29  libxl: xswait/devstate: Move xswait to before devstate
  08/29  libxl: devstate: Use libxl__xswait*
  09/29  libxl: New error codes CANCELLED etc.
  10/29  libxl: events: Make timeout and async exec setup take an ao, not a gc
  11/29  libxl: events: Make libxl__async_exec_* pass caller an rc
  12/29  libxl: events: Permit timeouts to signal cancellation
  13/29  libxl: domain create: Do not destroy on cancellation
  14/29  libxl: ao: Record ultimate parent of a nested ao
  15/29  libxl: ao: Count the nested progeny of an ao
  16/29  libxl: ao: Provide manip_refcnt
  17/29  libxl: cancellation: Provide public ao cancellation API
  18/29  libxl: cancellation: Provide explicit internal cancel check API
  19/29  libxl: cancellation: Make timeouts cancellable
  20/29  libxl: cancellation: Note that driver domain task cannot be cancelled
  21/29  libxl: cancellation: Make spawns cancellable
  22/29  libxl: Introduce DOMAIN_DESTROYED error code
  23/29  libxl: cancellation: Support cancellation where we spot domain death
  24/29  libxl: Introduce FILLZERO
  25/29  libxl: cancellation: Preparations for save/restore cancellation
  26/29  libxl: cancellation: Handle SIGTERM in save/restore helper
  27/29  libxl: cancellation: Cancel libxc save/restore
  28/29  libxl: ao: datacopier callback gets an rc
  29/29  libxl: cancellation: Make datacopiers cancellable