fix(rpc): trigger UILeave earlier on channel close (#38846)

Problem:
On exit, rpc_free() is called when processing main_loop.events after
libuv calls close callbacks of the channel's stream. However, when there
are no child processes, these libuv callbacks are called in loop_close()
instead of proc_teardown(), and main_loop.events isn't processed after
loop_close(). As a result, calling remote_ui_disconnect() in rpc_free()
causes UILeave to depend on the presence of child processes.

Solution:
Always call remote_ui_disconnect() in rpc_close_event(), and remove the
call in rpc_free().

(cherry picked from commit 5d66ef188f)

src/nvim/autocmd.c

@@ -2684,11 +2684,6 @@ void do_autocmd_uienter(uint64_t chanid, bool attached)
 {
   static bool recursive = false;
 
-#ifdef EXITFREE
-  if (entered_free_all_mem) {
-    return;
-  }
-#endif
   if (starting == NO_SCREEN) {
     return;  // user config hasn't been sourced yet
   }

src/nvim/msgpack_rpc/channel.c

@@ -494,32 +494,31 @@ static void rpc_close_event(void **argv)
 
   channel_decref(channel);
 
+  // No more I/O can happen on this channel. Remove UI if there is one attached.
+  // Do this here instead of in rpc_free() which isn't always called on exit, so that
+  // UILeave events behave consistently.
+  remote_ui_disconnect(channel->id, NULL, false);
+
   bool is_ui_client = ui_client_channel_id && channel->id == ui_client_channel_id;
-  if (is_ui_client || channel->streamtype == kChannelStreamStdio) {
-    if (!is_ui_client) {
-      // Avoid hanging when there are no other UIs and a prompt is triggered on exit.
-      remote_ui_disconnect(channel->id, NULL, false);
-    } else {
-      ui_client_attach_to_restarted_server();
-      if (ui_client_channel_id != channel->id) {
-        // Attached to new server. Don't exit.
-        return;
-      }
+  if (is_ui_client) {
+    ui_client_attach_to_restarted_server();
+    if (ui_client_channel_id != channel->id) {
+      // Attached to new server. Don't exit.
+      return;
     }
-    if (!channel->detach) {
-      if (channel->streamtype == kChannelStreamProc && ui_client_error_exit < 0) {
-        // Wait for the embedded server to exit instead of exiting immediately,
-        // as it's necessary to get the server's exit code in on_proc_exit().
-      } else {
-        exit_on_closed_chan(0);
-      }
+    if (channel->streamtype == kChannelStreamProc && ui_client_error_exit < 0) {
+      // Wait for the embedded server to exit instead of exiting immediately,
+      // as it's necessary to get the server's exit code in on_proc_exit().
+      return;
     }
+    exit_on_closed_chan(0);
+  } else if (channel->streamtype == kChannelStreamStdio && !channel->detach) {
+    exit_on_closed_chan(0);
   }
 }
 
 void rpc_free(Channel *channel)
 {
-  remote_ui_disconnect(channel->id, NULL, false);
   unpacker_teardown(channel->rpc.unpacker);
   xfree(channel->rpc.unpacker);
 

test/functional/api/ui_spec.lua

@@ -162,16 +162,65 @@ it('autocmds UIEnter/UILeave', function()
     autocmd UIEnter * call add(g:evs, "UIEnter") | let g:uienter_ev = deepcopy(v:event)
     autocmd UILeave * call add(g:evs, "UILeave") | let g:uileave_ev = deepcopy(v:event)
     autocmd VimEnter * call add(g:evs, "VimEnter")
+    autocmd VimLeave * call add(g:evs, "VimLeave")
   ]])
+
   local screen = Screen.new()
   eq({ chan = 1 }, eval('g:uienter_ev'))
+  eq({ 'VimEnter', 'UIEnter' }, eval('g:evs'))
+
   screen:detach()
   eq({ chan = 1 }, eval('g:uileave_ev'))
-  eq({
-    'VimEnter',
-    'UIEnter',
-    'UILeave',
-  }, eval('g:evs'))
+  eq({ 'VimEnter', 'UIEnter', 'UILeave' }, eval('g:evs'))
+
+  local servername = api.nvim_get_vvar('servername')
+
+  local session2 = n.connect(servername)
+  local status2, chan2 = session2:request('nvim_get_chan_info', 0)
+  t.ok(status2)
+
+  local session3 = n.connect(servername)
+  local status3, chan3 = session3:request('nvim_get_chan_info', 0)
+  t.ok(status3)
+
+  local screen2 = Screen.new(nil, nil, nil, session2)
+  eq({ chan = chan2.id }, eval('g:uienter_ev'))
+  eq({ 'VimEnter', 'UIEnter', 'UILeave', 'UIEnter' }, eval('g:evs'))
+
+  screen2:detach()
+  eq({ chan = chan2.id }, eval('g:uileave_ev'))
+  eq({ 'VimEnter', 'UIEnter', 'UILeave', 'UIEnter', 'UILeave' }, eval('g:evs'))
+
+  command('let g:evs = ["…"]')
+
+  screen2:attach(session2)
+  eq({ chan = chan2.id }, eval('g:uienter_ev'))
+  eq({ '…', 'UIEnter' }, eval('g:evs'))
+
+  Screen.new(nil, nil, nil, session3)
+  eq({ chan = chan3.id }, eval('g:uienter_ev'))
+  eq({ '…', 'UIEnter', 'UIEnter' }, eval('g:evs'))
+
+  screen:attach(n.get_session())
+  eq({ chan = 1 }, eval('g:uienter_ev'))
+  eq({ '…', 'UIEnter', 'UIEnter', 'UIEnter' }, eval('g:evs'))
+
+  session3:close()
+  t.retry(nil, 1000, function()
+    eq({}, api.nvim_get_chan_info(chan3.id))
+  end)
+  eq({ chan = chan3.id }, eval('g:uileave_ev'))
+  eq({ '…', 'UIEnter', 'UIEnter', 'UIEnter', 'UILeave' }, eval('g:evs'))
+
+  command('let g:evs = ["…"]')
+  command('autocmd UILeave * call writefile(g:evs, "Xevents.log")')
+  finally(function()
+    os.remove('Xevents.log')
+  end)
+  n.expect_exit(command, 'qall!')
+  n.check_close() -- Wait for process exit.
+  -- UILeave should have been triggered for both remaining UIs.
+  eq('…\nVimLeave\nUILeave\nUILeave\n', t.read_file('Xevents.log'))
 end)
 
 it('autocmds VimSuspend/VimResume #22041', function()