libxl: Introduce libxl__ev_immediate

This new ev allows to arrange a non-reentrant callback to be called.
This happen immediately after the current event is processed and after
other ev_immediates that would have already been registered.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

tools/libxl/libxl_event.c

@@ -914,6 +914,15 @@ int libxl__ev_devstate_wait(libxl__ao *ao, libxl__ev_devstate *ds,
     return rc;
 }
 
+/*
+ * immediate non-reentrant callback
+ */
+
+void libxl__ev_immediate_register(libxl__egc *egc, libxl__ev_immediate *ei)
+{
+    LIBXL_STAILQ_INSERT_TAIL(&egc->ev_immediates, ei, entry);
+}
+
 /*
  * domain death/destruction
  */
@@ -1395,6 +1404,17 @@ static void egc_run_callbacks(libxl__egc *egc)
     EGC_GC;
     libxl_event *ev, *ev_tmp;
     libxl__aop_occurred *aop, *aop_tmp;
+    libxl__ev_immediate *ei;
+
+    while (!LIBXL_STAILQ_EMPTY(&egc->ev_immediates)) {
+        ei = LIBXL_STAILQ_FIRST(&egc->ev_immediates);
+        LIBXL_STAILQ_REMOVE_HEAD(&egc->ev_immediates, entry);
+        CTX_LOCK;
+        /* This callback is internal to libxl and expects CTX to be
+         * locked. */
+        ei->callback(egc, ei);
+        CTX_UNLOCK;
+    }
 
     LIBXL_TAILQ_FOREACH_SAFE(ev, &egc->occurred_for_callback, link, ev_tmp) {
         LIBXL_TAILQ_REMOVE(&egc->occurred_for_callback, ev, link);

tools/libxl/libxl_internal.h

@@ -200,6 +200,7 @@ typedef struct libxl__ev_slowlock libxl__ev_slowlock;
 typedef struct libxl__dm_resume_state libxl__dm_resume_state;
 typedef struct libxl__ao_device libxl__ao_device;
 typedef struct libxl__multidev libxl__multidev;
+typedef struct libxl__ev_immediate libxl__ev_immediate;
 
 typedef struct libxl__domain_create_state libxl__domain_create_state;
 typedef void libxl__domain_create_cb(struct libxl__egc *egc,
@@ -363,6 +364,20 @@ struct libxl__ev_child {
     LIBXL_LIST_ENTRY(struct libxl__ev_child) entry;
 };
 
+/* libxl__ev_immediate
+ *
+ * Allow to call a non-reentrant callback.
+ *
+ * `callback' will be called immediately as a new event.
+ */
+struct libxl__ev_immediate {
+    /* filled by user */
+    void (*callback)(libxl__egc *, libxl__ev_immediate *);
+    /* private to libxl__ev_immediate */
+    LIBXL_STAILQ_ENTRY(libxl__ev_immediate) entry;
+};
+void libxl__ev_immediate_register(libxl__egc *, libxl__ev_immediate *);
+
 /*
  * Lock for device hotplug, qmp_lock.
  *
@@ -733,6 +748,7 @@ struct libxl__egc {
     struct libxl__event_list occurred_for_callback;
     LIBXL_TAILQ_HEAD(, libxl__ao) aos_for_callback;
     LIBXL_TAILQ_HEAD(, libxl__aop_occurred) aops_for_callback;
+    LIBXL_STAILQ_HEAD(, libxl__ev_immediate) ev_immediates;
 };
 
 struct libxl__aop_occurred {
@@ -2322,6 +2338,7 @@ _hidden libxl_device_model_version libxl__default_device_model(libxl__gc *gc);
         LIBXL_TAILQ_INIT(&(egc).occurred_for_callback); \
         LIBXL_TAILQ_INIT(&(egc).aos_for_callback);      \
         LIBXL_TAILQ_INIT(&(egc).aops_for_callback);     \
+        LIBXL_STAILQ_INIT(&(egc).ev_immediates);        \
     } while(0)
 
 _hidden void libxl__egc_cleanup(libxl__egc *egc);