introduce the sync IPC command

Sending the sync command via IPC ensures pending IPC messages are handled by i3
before the sync response is read. This is rarely useful for direct IPC
connections to i3, but becomes useful when synchronizing with i3bar, which might
have pending IPC messages in response to button clicks.

src/ipc.c

@@ -1173,9 +1173,68 @@ IPC_HANDLER(send_tick) {
     DLOG("Sent tick event\n");
 }
 
+struct sync_state {
+    char *last_key;
+    uint32_t rnd;
+    xcb_window_t window;
+};
+
+static int _sync_json_key(void *extra, const unsigned char *val, size_t len) {
+    struct sync_state *state = extra;
+    FREE(state->last_key);
+    state->last_key = scalloc(len + 1, 1);
+    memcpy(state->last_key, val, len);
+    return 1;
+}
+
+static int _sync_json_int(void *extra, long long val) {
+    struct sync_state *state = extra;
+    if (strcasecmp(state->last_key, "rnd") == 0) {
+        state->rnd = val;
+    } else if (strcasecmp(state->last_key, "window") == 0) {
+        state->window = (xcb_window_t)val;
+    }
+    return 1;
+}
+
+IPC_HANDLER(sync) {
+    yajl_handle p;
+    yajl_status stat;
+
+    /* Setup the JSON parser */
+    static yajl_callbacks callbacks = {
+        .yajl_map_key = _sync_json_key,
+        .yajl_integer = _sync_json_int,
+    };
+
+    struct sync_state state;
+    memset(&state, '\0', sizeof(struct sync_state));
+    p = yalloc(&callbacks, (void *)&state);
+    stat = yajl_parse(p, (const unsigned char *)message, message_size);
+    FREE(state.last_key);
+    if (stat != yajl_status_ok) {
+        unsigned char *err;
+        err = yajl_get_error(p, true, (const unsigned char *)message,
+                             message_size);
+        ELOG("YAJL parse error: %s\n", err);
+        yajl_free_error(p, err);
+
+        const char *reply = "{\"success\":false}";
+        ipc_send_message(fd, strlen(reply), I3_IPC_REPLY_TYPE_SYNC, (const uint8_t *)reply);
+        yajl_free(p);
+        return;
+    }
+    yajl_free(p);
+
+    DLOG("received IPC sync request (rnd = %d, window = 0x%08x)\n", state.rnd, state.window);
+    sync_respond(state.window, state.rnd);
+    const char *reply = "{\"success\":true}";
+    ipc_send_message(fd, strlen(reply), I3_IPC_REPLY_TYPE_SYNC, (const uint8_t *)reply);
+}
+
 /* The index of each callback function corresponds to the numeric
  * value of the message type (see include/i3/ipc.h) */
-handler_t handlers[11] = {
+handler_t handlers[12] = {
     handle_run_command,
     handle_get_workspaces,
     handle_subscribe,
@@ -1187,6 +1246,7 @@ handler_t handlers[11] = {
     handle_get_binding_modes,
     handle_get_config,
     handle_send_tick,
+    handle_sync,
 };
 
 /*