libflux: add flux_watcher_set_priority()

Problem: there is no way to influence the priority of a reactor
watcher.

Add flux_watcher_set_priority() which calls libev's ev_set_priority()
if the watcher supports it (no-op if not).

Implement support for set_priority() in the check watcher.

src/common/libflux/reactor.c

@@ -199,6 +199,14 @@ struct flux_watcher_ops * flux_watcher_get_ops (flux_watcher_t *w)
     return NULL;
 }
 
+void flux_watcher_set_priority (flux_watcher_t *w, int priority)
+{
+    if (w) {
+        if (w->ops->set_priority)
+            w->ops->set_priority (w, priority);
+    }
+}
+
 void flux_watcher_start (flux_watcher_t *w)
 {
     if (w) {
@@ -558,6 +566,11 @@ flux_watcher_t *flux_prepare_watcher_create (flux_reactor_t *r,
 /* Check
  */
 
+static void check_set_priority (flux_watcher_t *w, int priority)
+{
+    ev_set_priority ((ev_check *)w->data, priority);
+}
+
 static void check_start (flux_watcher_t *w)
 {
     ev_check_start (w->r->loop, (ev_check *)w->data);
@@ -576,6 +589,7 @@ static void check_cb (struct ev_loop *loop, ev_check *cw, int revents)
 }
 
 static struct flux_watcher_ops check_watcher = {
+    .set_priority = check_set_priority,
     .start = check_start,
     .stop = check_stop,
     .destroy = NULL,

src/common/libflux/reactor.h

@@ -72,6 +72,14 @@ typedef struct flux_watcher flux_watcher_t;
 typedef void (*flux_watcher_f)(flux_reactor_t *r, flux_watcher_t *w,
                                int revents, void *arg);
 
+/* Set the watcher priority.  The range is [-2:2] (default 0).
+ * Higher priority watchers run first.
+ * This is a no-op if the underlying watcher doesn't support it.
+ * If the priority is out of range, the max or min value is set.
+ * Currently only the check watcher supports it.
+ */
+void flux_watcher_set_priority (flux_watcher_t *w, int priority);
+
 void flux_watcher_start (flux_watcher_t *w);
 void flux_watcher_stop (flux_watcher_t *w);
 void flux_watcher_destroy (flux_watcher_t *w);
@@ -160,6 +168,7 @@ void flux_stat_watcher_get_rstat (flux_watcher_t *w,
  */
 
 struct flux_watcher_ops {
+    void (*set_priority) (flux_watcher_t *w, int priority);
     void (*start) (flux_watcher_t *w);
     void (*stop) (flux_watcher_t *w);
     void (*destroy) (flux_watcher_t *w);