feat(server): allow embed with listen (#25709)

connection from any channel or stdio will unblock
remote_ui_wait_for_attach.  Wait on stdio only if
only —embed specified, if both —embed and
—listen then wait on any channel.

runtime/doc/starting.txt

@@ -390,6 +390,10 @@ argument.
 <		Then startup will continue without waiting for `nvim_ui_attach`.
 		This is equivalent to: >
 		    nvim --headless --cmd "call stdioopen({'rpc': v:true})"
+<
+		Embedders that use the UI protocol on a socket connection must
+		pass |--listen| as well as |--embed|: >
+		    nvim --embed --listen addr
 
 <		See also: |ui-startup| |channel-stdio|
 

src/nvim/api/ui.c

@@ -126,18 +126,24 @@ void remote_ui_disconnect(uint64_t channel_id)
   xfree(ui);
 }
 
-/// Wait until ui has connected on stdio channel.
-void remote_ui_wait_for_attach(void)
+/// Wait until ui has connected on stdio channel if only_stdio
+/// is true, otherwise any channel.
+void remote_ui_wait_for_attach(bool only_stdio)
 {
-  Channel *channel = find_channel(CHAN_STDIO);
-  if (!channel) {
-    // this function should only be called in --embed mode, stdio channel
-    // can be assumed.
-    abort();
-  }
+  if (only_stdio) {
+    Channel *channel = find_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,
-                            map_has(uint64_t, &connected_uis, CHAN_STDIO));
+    LOOP_PROCESS_EVENTS_UNTIL(&main_loop, channel->events, -1,
+                              map_has(uint64_t, &connected_uis, CHAN_STDIO));
+  } else {
+    LOOP_PROCESS_EVENTS_UNTIL(&main_loop, main_loop.events, -1,
+                              ui_active());
+  }
 }
 
 /// Activates UI events on the channel.

src/nvim/main.c

@@ -392,9 +392,10 @@ int main(int argc, char **argv)
   // Wait for UIs to set up Nvim or show early messages
   // and prompts (--cmd, swapfile dialog, …).
   bool use_remote_ui = (embedded_mode && !headless_mode);
+  bool listen_and_embed = params.listen_addr != NULL;
   if (use_remote_ui) {
     TIME_MSG("waiting for UI");
-    remote_ui_wait_for_attach();
+    remote_ui_wait_for_attach(!listen_and_embed);
     TIME_MSG("done waiting for UI");
     firstwin->w_prev_height = firstwin->w_height;  // may have changed
   }

test/functional/ui/embed_spec.lua

@@ -2,10 +2,13 @@ local uv = require'luv'
 
 local helpers = require('test.functional.helpers')(after_each)
 local Screen = require('test.functional.ui.screen')
+local thelpers = require('test.functional.terminal.helpers')
 
 local feed = helpers.feed
 local eq = helpers.eq
 local clear = helpers.clear
+local ok = helpers.ok
+
 
 local function test_embed(ext_linegrid)
   local screen
@@ -133,3 +136,60 @@ describe('--embed UI', function()
     ]]}
   end)
 end)
+
+describe('--embed --listen UI', function()
+  it('waits for connection on listening address', function()
+    helpers.skip(helpers.is_os('win'))
+    clear()
+    local child_server = assert(helpers.new_pipename())
+
+    local screen = thelpers.screen_setup(0,
+      string.format(
+        [=[["%s", "--embed", "--listen", "%s", "-u", "NONE", "-i", "NONE", "--cmd", "%s"]]=],
+        helpers.nvim_prog, child_server, helpers.nvim_set))
+    screen:expect{grid=[[
+      {1: }                                                 |
+                                                        |
+                                                        |
+                                                        |
+                                                        |
+                                                        |
+      {3:-- TERMINAL --}                                    |
+    ]]}
+
+    local child_session = helpers.connect(child_server)
+
+    local info_ok, apiinfo = child_session:request('nvim_get_api_info')
+    assert(info_ok)
+    assert(#apiinfo == 2)
+
+    child_session:request('nvim_exec2', [[
+      let g:vim_entered=0
+      autocmd VimEnter * call execute("let g:vim_entered=1")
+    ]], {})
+
+    -- g:vim_entered shouldn't be set to 1 until after attach
+    local var_ok, var = child_session:request('nvim_get_var', 'vim_entered')
+    assert(var_ok)
+    ok(var == 0)
+
+    local child_screen = Screen.new(40, 6)
+    child_screen:attach(nil, child_session)
+    child_screen:expect{grid=[[
+      ^                                        |
+      {1:~                                       }|
+      {1:~                                       }|
+      {1:~                                       }|
+      {1:~                                       }|
+                                              |
+    ]], attr_ids={
+      [1] = {foreground = Screen.colors.Blue, bold = true};
+    }}
+
+    -- g:vim_entered should now be set to 1
+    var_ok, var = child_session:request('nvim_get_var', 'vim_entered')
+    assert(var_ok)
+    ok(var == 1)
+
+  end)
+end)