refactor(channel): eliminate special case in on_proc_exit() #32485

Problem:
on_proc_exit() has a special-case that assumes that the UI client will
never spawn more than 1 child process.

Solution:
If the Nvim server exits, the stream EOF will trigger `rpc_close()` in
the UI client, so we don't need the special case in `on_proc_exit`.
Pass `Channel.exit_status` from `rpc_close()` so that the correct exit
code is reflected.

src/nvim/channel.h

@@ -40,7 +40,7 @@ struct Channel {
   CallbackReader on_data;
   CallbackReader on_stderr;
   Callback on_exit;
-  int exit_status;
+  int exit_status;  ///< Process exit-code (if the channel wraps a process).
 
   bool callback_busy;
   bool callback_scheduled;

src/nvim/event/proc.c

@@ -437,11 +437,6 @@ static void on_proc_exit(Proc *proc)
   Loop *loop = proc->loop;
   ILOG("child exited: pid=%d status=%d" PRIu64, proc->pid, proc->status);
 
-  // XXX: This assumes the TUI never spawns any other processes...?
-  if (ui_client_channel_id) {
-    exit_on_closed_chan(proc->status);
-  }
-
   // Process has terminated, but there could still be data to be read from the
   // OS. We are still in the libuv loop, so we cannot call code that polls for
   // more data directly. Instead delay the reading after the libuv loop by

src/nvim/msgpack_rpc/channel.c

@@ -486,15 +486,15 @@ void rpc_close(Channel *channel)
   channel->rpc.closed = true;
   channel_decref(channel);
 
-  if (ui_client_channel_id && channel->id == ui_client_channel_id) {
-    assert(!channel->detach);  // `Channel.detach` is not currently used by the UI client.
-    exit_on_closed_chan(0);
-  } else if (channel->streamtype == kChannelStreamStdio) {
-    // Avoid hanging when there are no other UIs and a prompt is triggered on exit.
-    remote_ui_disconnect(channel->id);
+  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);
+    }
 
     if (!channel->detach) {
-      exit_on_closed_chan(0);
+      exit_on_closed_chan(channel->exit_status == -1 ? 0 : channel->exit_status);
     }
   }
 }