Update joystick example with new failure mode

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

API.md

@@ -238,8 +238,20 @@ struct js_event {
     uint8_t  number;    /* axis/button number */
 } e;
 
+/*
+ * Called on any joystick event
+ *
+ * Note: We handle errors here by exiting, for a background daemon with
+ *       multiple watchers you may want another approach.
+ */
 static void joystick_cb(uev_t *w, void *arg, int events)
 {
+    if (UEV_ERROR == events) {
+        /* Possibly joystick was unplugged */
+        warn("Unrecoverable error, exiting");
+        uev_exit(w->ctx);
+    }
+
     read(w->fd, &e, sizeof(e));
 
     switch (e.type) {

examples/joystick.c

@@ -37,8 +37,15 @@ struct js_event {
 	uint8_t  number;	/* axis/button number */
 } e;
 
+/*
+ * Always check for UEV_ERROR in watcher callback!
+ * Possibly joystick was unplugged
+ */
 static void joystick_cb(uev_t *w, void *arg, int events)
 {
+	if (UEV_ERROR == events)
+		errx(EBADF, "Unrecoverable error, exiting");
+
 	if (read(w->fd, &e, sizeof(e)) < 0)
 		errx(errno, "Failed reading joystick event");