Ensure control socket is closed first, and wait for it to close.

src/server/control.c

@@ -78,6 +78,14 @@ void control_destroy(struct control *control)
     free(control);
 }
 
+void control_wait_for_close(struct control *control)
+{
+    NULLCHECK(control);
+    while (!fd_is_closed(control->control_fd)) {
+       usleep(10000);
+    }
+}
+
 struct control_client *control_client_create(struct flexnbd *flexnbd,
 					     int client_fd,
 					     struct mbox *state_mbox)

src/server/control.h

@@ -47,6 +47,7 @@ struct control_client {
 struct control *control_create(struct flexnbd *,
 			       const char *control_socket_name);
 void control_signal_close(struct control *);
+void control_wait_for_close(struct control *control);
 void control_destroy(struct control *);
 
 void *control_runner(void *);

src/server/serve.c

@@ -820,14 +820,21 @@ void serve_cleanup(struct server *params,
     void *status;
 
     info("cleaning up");
-
+    
-    if (params->server_fd) {
+    /* Close the control socket, and wait for it to close before proceeding.
-	close(params->server_fd);
+     * If we do not wait, we risk a race condition with the tail supervisor
-    }
+     * sending a status command, and deadlocking the mirroring.  */
-
-    /* close the control socket too */
     if (params->flexnbd && params->flexnbd->control) {
+	debug("closing control socket");
         control_signal_close(params->flexnbd->control);
+
+	debug("waiting for control socket to close");
+	control_wait_for_close(params->flexnbd->control);
+    }
+    
+    if (params->server_fd) {
+	debug("closing server_fd");
+	close(params->server_fd);
     }
 
     /* need to stop background build if we're killed very early on */