startup: wait for embedder before executing startup commands and files

Give embeders a chance to set up nvim, by processing a request before
startup. This allows an external UI to show messages and prompts from
--cmd and buffer loading (e.g. swap files)

src/nvim/main.c

@@ -344,6 +344,24 @@ int main(int argc, char **argv)
     p_lpl = false;
   }
 
+  // give embedders a chance to set up nvim, by processing a request before
+  // startup. This allows an external UI to show messages and prompts from
+  // --cmd and buffer loading (e.g. swap files)
+  bool early_ui = false;
+  if (embedded_mode) {
+    TIME_MSG("waiting for embedder to make request");
+    rpc_wait_for_request();
+    TIME_MSG("done waiting for embedder");
+
+    if (ui_active()) {
+      // prepare screen now, so external UIs can display messages
+      starting = NO_BUFFERS;
+      screenclear();
+      early_ui = true;
+      TIME_MSG("initialized screen early for embedder");
+    }
+  }
+
   // Execute --cmd arguments.
   exe_pre_commands(&params);
 
@@ -456,8 +474,10 @@ int main(int argc, char **argv)
 
   setmouse();  // may start using the mouse
 
-  if (exmode_active) {
-    must_redraw = CLEAR;  // Don't clear the screen when starting in Ex mode.
+  if (exmode_active || early_ui) {
+    // Don't clear the screen when starting in Ex mode, or when an
+    // embedding UI might have displayed messages
+    must_redraw = CLEAR;
   } else {
     screenclear();  // clear screen
     TIME_MSG("clearing screen");

src/nvim/msgpack_rpc/channel.c

@@ -40,6 +40,8 @@
 static PMap(cstr_t) *event_strings = NULL;
 static msgpack_sbuffer out_buffer;
 
+static bool got_stdio_request = false;
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "msgpack_rpc/channel.c.generated.h"
 #endif
@@ -329,6 +331,9 @@ static void handle_request(Channel *channel, msgpack_object *request)
     send_error(channel, request_id, error.msg);
     api_clear_error(&error);
     api_free_array(args);
+    if (channel->id == CHAN_STDIO) {
+      got_stdio_request = true;
+    }
     return;
   }
 
@@ -344,6 +349,9 @@ static void handle_request(Channel *channel, msgpack_object *request)
     if (is_get_mode && !input_blocking()) {
       // Defer the event to a special queue used by os/input.c. #6247
       multiqueue_put(ch_before_blocking_events, on_request_event, 1, evdata);
+      if (channel->id == CHAN_STDIO) {
+        got_stdio_request = true;
+      }
     } else {
       // Invoke immediately.
       on_request_event((void **)&evdata);
@@ -378,6 +386,11 @@ static void on_request_event(void **argv)
   channel_decref(channel);
   xfree(e);
   api_clear_error(&error);
+  bool is_api_info = handler.fn == handle_nvim_get_api_info;
+  // api info is used to initiate client library, ignore it
+  if (channel->id == CHAN_STDIO && !is_api_info) {
+    got_stdio_request = true;
+  }
 }
 
 static bool channel_write(Channel *channel, WBuffer *buffer)
@@ -744,3 +757,16 @@ static void log_msg_close(FILE *f, msgpack_object msg)
 }
 #endif
 
+/// Wait until embedder has done a request
+void rpc_wait_for_request(void)
+{
+  Channel *channel = find_rpc_channel(CHAN_STDIO);
+  if (!channel) {
+    // this function should only be called in --embed mode, stdio channel
+    // can be assumed.
+    abort();
+  }
+
+  LOOP_PROCESS_EVENTS_UNTIL(&main_loop, channel->events, -1, got_stdio_request);
+}
+

src/nvim/syntax.c

@@ -6332,7 +6332,6 @@ int load_colors(char_u *name)
   apply_autocmds(EVENT_COLORSCHEME, name, curbuf->b_fname, FALSE, curbuf);
 
   recursive = false;
-  ui_refresh();
 
   return retval;
 }
@@ -6885,7 +6884,7 @@ void do_highlight(const char *line, const bool forceit, const bool init)
       // "fg", which have been changed now.
       highlight_attr_set_all();
 
-      if (!ui_is_external(kUINewgrid)) {
+      if (!ui_is_external(kUINewgrid) && starting != NO_SCREEN) {
         // Older UIs assume that we clear the screen after normal group is
         // changed
         ui_refresh();