libxl: abort bootloader invocation when domain dies

Cancel the bootloader (specifically, by sending it a signal) if the
domain is seen to disappear from xenstore.

We use a new utility event source libxl__domaindeathcheck which
provides a convenient wrapper for the xenstore watch.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

Changes since v8:
 * Fixed the commit message summary line.

Changes since v7:
 * Add a comment explaining why we use a watch on the domain's
   xenstore path rather than @releaseDomain.
 * Fix typo in error message.
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>

1 files changed, 47 insertions, 0 deletions

diff --git a/tools/libxl/libxl_event.c b/tools/libxl/libxl_event.c
index 63719b2b5d..03d0498020 100644
--- a/tools/libxl/libxl_event.c
+++ b/tools/libxl/libxl_event.c
@@ -589,6 +589,53 @@ int libxl__ev_devstate_wait(libxl__gc *gc, libxl__ev_devstate *ds,
 }
 
 /*
+ * domain death/destruction
+ */
+
+/*
+ * We use a xenstore watch on the domain's path, rather than using an
+ * @releaseDomain watch and asking the hypervisor.  This is simpler
+ * because turning @releaseDomain into domain-specific information is
+ * complicated.
+ *
+ * It is also sufficient for our callers, which are generally trying
+ * to do cleanup of their own execution state on domain death, for the
+ * following reason: if the domain is destroyed then either (a) the
+ * entries in xenstore have already been deleted, in which case the
+ * test here works or (b) they have not in which case something has
+ * gone very badly wrong and we are going to leak those xenstore
+ * entries, in which case trying to avoid leaking other stuff is
+ * futile.
+ */
+
+static void domaindeathcheck_callback(libxl__egc *egc, libxl__ev_xswatch *w,
+                            const char *watch_path, const char *event_path)
+{
+    libxl__domaindeathcheck *dc = CONTAINER_OF(w, *dc, watch);
+    EGC_GC;
+    const char *p = libxl__xs_read(gc, XBT_NULL, watch_path);
+    if (p) return;
+
+    if (errno!=ENOENT) {
+        LIBXL__EVENT_DISASTER(egc,"failed to read xenstore"
+                              " for domain detach check", errno, 0);
+        return;
+    }
+
+    LOG(ERROR,"%s: domain %"PRIu32" removed (%s no longer in xenstore)",
+        dc->what, dc->domid, watch_path);
+    dc->callback(egc, dc);
+}
+
+int libxl__domaindeathcheck_start(libxl__gc *gc,
+                                  libxl__domaindeathcheck *dc)
+{
+    const char *path = GCSPRINTF("/local/domain/%"PRIu32, dc->domid);
+    return libxl__ev_xswatch_register(gc, &dc->watch,
+                                      domaindeathcheck_callback, path);
+}
+
+/*
  * osevent poll
  */