Add the ipc shutdown event (#2652)

This event is triggered when the connection to the ipc is about to
shutdown because of a user action such as with a `restart` or `exit`
command. The `change` field indicates why the ipc is shutting down. It
can be either "restart" or "exit".

fixes #2318

src/commands.c

@@ -1562,7 +1562,7 @@ void cmd_exit(I3_CMD) {
 #ifdef I3_ASAN_ENABLED
     __lsan_do_leak_check();
 #endif
-    ipc_shutdown();
+    ipc_shutdown(SHUTDOWN_REASON_EXIT);
     unlink(config.ipc_socket_path);
     xcb_disconnect(conn);
     exit(0);
@@ -1595,7 +1595,7 @@ void cmd_reload(I3_CMD) {
  */
 void cmd_restart(I3_CMD) {
     LOG("restarting i3\n");
-    ipc_shutdown();
+    ipc_shutdown(SHUTDOWN_REASON_RESTART);
     unlink(config.ipc_socket_path);
     /* We need to call this manually since atexit handlers don’t get called
      * when exec()ing */

src/ipc.c

@@ -62,11 +62,39 @@ void ipc_send_event(const char *event, uint32_t message_type, const char *payloa
 }
 
 /*
- * Calls shutdown() on each socket and closes it. This function to be called
+ * For shutdown events, we send the reason for the shutdown.
+ */
+static void ipc_send_shutdown_event(shutdown_reason_t reason) {
+    yajl_gen gen = ygenalloc();
+    y(map_open);
+
+    ystr("change");
+
+    if (reason == SHUTDOWN_REASON_RESTART) {
+        ystr("restart");
+    } else if (reason == SHUTDOWN_REASON_EXIT) {
+        ystr("exit");
+    }
+
+    y(map_close);
+
+    const unsigned char *payload;
+    ylength length;
+
+    y(get_buf, &payload, &length);
+    ipc_send_event("shutdown", I3_IPC_EVENT_SHUTDOWN, (const char *)payload);
+
+    y(free);
+}
+
+/*
+ * Calls shutdown() on each socket and closes it. This function is to be called
  * when exiting or restarting only!
  *
  */
-void ipc_shutdown(void) {
+void ipc_shutdown(shutdown_reason_t reason) {
+    ipc_send_shutdown_event(reason);
+
     ipc_client *current;
     while (!TAILQ_EMPTY(&all_clients)) {
         current = TAILQ_FIRST(&all_clients);

src/util.c

@@ -259,7 +259,7 @@ void i3_restart(bool forget_layout) {
 
     restore_geometry();
 
-    ipc_shutdown();
+    ipc_shutdown(SHUTDOWN_REASON_RESTART);
 
     LOG("restarting \"%s\"...\n", start_argv[0]);
     /* make sure -a is in the argument list or add it */