[PATCH 07/18] libxl: fork: Provide ..._always_selective_reap

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

Applications exist which want to use libxl in an event-driven mode but
which do not integrate child termination into their event system, but
instead reap all their own children synchronously.

In such an application libxl must own SIGCHLD but avoid reaping any
children that don't belong to libxl.

Provide libxl_sigchld_owner_libxl_always_selective_reap which has this
behaviour.

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>

---
v2: Document the new mode in the big "Subprocess handling" comment.
---
 tools/libxl/libxl_event.h |   11 +++++++++++
 tools/libxl/libxl_fork.c  |    7 +++++++
 2 files changed, 18 insertions(+)

diff --git a/tools/libxl/libxl_event.h b/tools/libxl/libxl_event.h
index 3c93955..824ac88 100644
--- a/tools/libxl/libxl_event.h
+++ b/tools/libxl/libxl_event.h
@@ -474,6 +474,12 @@ void libxl_osevent_occurred_timeout(libxl_ctx *ctx, void *for_libxl)
  *       and provides a callback to be notified of their exit
  *       statues.  The application must have only one libxl_ctx
  *       configured this way.
+ *
+ *     libxl_sigchld_owner_libxl_always_selective_reap:
+ *
+ *       The application expects to reap all of its own children
+ *       synchronously, and does not use SIGCHLD.  libxl is
+ *       to install a SIGCHLD handler.
  */
 
 
@@ -491,6 +497,11 @@ typedef enum {
     /* libxl owns SIGCHLD all the time, and the application is
      * relying on libxl's event loop for reaping its children too. */
     libxl_sigchld_owner_libxl_always,
+
+    /* libxl owns SIGCHLD all the time, but it must only reap its own
+     * children.  The application will reap its own children
+     * synchronously with waitpid, without the assistance of SIGCHLD. */
+    libxl_sigchld_owner_libxl_always_selective_reap,
 } libxl_sigchld_owner;
 
 typedef struct {
diff --git a/tools/libxl/libxl_fork.c b/tools/libxl/libxl_fork.c
index b2325e0..16e17f6 100644
--- a/tools/libxl/libxl_fork.c
+++ b/tools/libxl/libxl_fork.c
@@ -268,6 +268,7 @@ static bool chldmode_ours(libxl_ctx *ctx, bool creating)
     case libxl_sigchld_owner_mainloop:
         return 0;
     case libxl_sigchld_owner_libxl_always:
+    case libxl_sigchld_owner_libxl_always_selective_reap:
         return 1;
     }
     abort();
@@ -398,6 +399,12 @@ static void sigchld_selfpipe_handler(libxl__egc *egc, libxl__ev_fd *ev,
     int e = libxl__self_pipe_eatall(selfpipe);
     if (e) LIBXL__EVENT_DISASTER(egc, "read sigchld pipe", e, 0);
 
+    if (CTX->childproc_hooks->chldowner
+        == libxl_sigchld_owner_libxl_always_selective_reap) {
+        childproc_checkall(egc);
+        return;
+    }
+
     while (chldmode_ours(CTX, 0) /* in case the app changes the mode */) {
         int status;
         pid_t pid = checked_waitpid(egc, -1, &status);
-- 
1.7.10.4