libxl: fork: Share SIGCHLD handler amongst ctxs

libxl: fork: Share SIGCHLD handler amongst ctxs

Previously, an application which had multiple libxl ctxs in multiple
threads, would have to itself plumb SIGCHLD through to each ctx.
Instead, permit multiple libxl ctxs to all share the SIGCHLD handler.

We keep a list of all the ctxs which are interested in SIGCHLD and
notify all of their self-pipes.

In more detail:

 * sigchld_owner, the ctx* of the SIGCHLD owner, is replaced by
   sigchld_users, a list of SIGCHLD users.

 * Each ctx keeps track of whether it is on the users list, so that
   libxl__sigchld_needed and libxl__sigchld_notneeded now instead of
   idempotently installing and removing the handler, idempotently add
   or remove the ctx from the list.

   We ensure that we always have the SIGCHLD handler installed
   iff the sigchld_users list is nonempty.  To make this a bit
   easier we make sigchld_installhandler_core and
   sigchld_removehandler_core idempotent.

   Specifically, the call sites for sigchld_installhandler_core and
   sigchld_removehandler_core are updated to manipulate sigchld_users
   and only call the install or remove functions as applicable.

 * In the signal handler we walk the list of SIGCHLD users and write
   to each of their self-pipes.  That means that we need to arrange to
   defer SIGCHLD when we are manipulating the list (to avoid the
   signal handler interrupting our list manipulation); this is quite
   tiresome to arrange.

   The code as written will, on the first installation of the SIGCHLD
   handler, firstly install the real handler, then immediately replace
   it with the deferral handler.  Doing it this way makes the code
   clearer as it makes the SIGCHLD deferral machinery much more
   self-contained (and hence easier to reason about).

 * The first part of libxl__sigchld_notneeded is broken out into a new
   function sigchld_user_remove (which is also needed during for
   postfork).  And of course that first part of the function is now
   rather different, as explained above.

 * sigchld_installhandler_core no longer takes the gc argument,
   because it now deals with SIGCHLD for all ctxs.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Jim Fehlig <jfehlig@suse.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
---
v3: Include bugfixes from "Fixup SIGCHLD sharing" patch:

    * Use a mutex for defer_sigchld, to guard against concurrency
      between the thread calling defer_sigchld and an instance of the
      primary signal handler on another thread.

    * libxl_sigchld_owner_libxl_always is incompatible with SIGCHLD
      sharing.  Document this correctly.

    Fix "have have" error in comment.

    Move removal of newly unused variables to previous patch.

v2.1: Provide feature test macro LIBXL_HAVE_SIGCHLD_SHARING