fix(jobs): deadlock in channel.c:exit_event #19082

In the rare case that exit_event is called from process_close_handles,
it stalls waiting for the process to exit (the routine is currently
underway to do just that). This causes `job_spec.lua` to sometimes
stall.

REJECTED IDEAS:
==============================================================
1. Currently `exit_event` is placed on `main_loop.fast_events`. Would the problem
   be solved by using `main_loop.events` instead?
    - A: Maybe, but it will cause other problems, such as queuing exit_event()
      during "Press Enter..." prompt which may result in the event not being
      processed, leading to another stall.
2. Can we avoid the timer?
    - A: Using a timer is just the easiest way to queue a delayed event without
      causing an infinite loop in the queue currently being processed.
3. Can we avoid the new `exit_need_delay` global...
    1. by using `process_is_tearing_down` instead?
        - A: Can't use `process_is_tearing_down` because its semantics are different.
    2. by checking a similar condition as `process_teardown`? https://github.com/neovim/neovim/blob/f50135a32e11c535e1dc3a8e9460c5b4e640ee86/src/nvim/event/process.c#L141-L142
       ```
       if (!process_is_tearing_down || (kl_empty(main_loop.children) && multiqueue_empty(main_loop.events))) {
         uv_timer_start(&main_loop.exit_delay_timer, exit_delay_cb, 0, 0);
         return;
       }
       ```
        - A: Tried but it did not work (other stalls occurred). Maybe
          exit_event() is called from a source other than
          process_close_handles() and is delayed, the delayed exit_event() will
          be executed before main_loop.events is processed, resulting in an
          infinite loop.

1 files changed, 11 insertions, 0 deletions

diff --git a/src/nvim/msgpack_rpc/channel.c b/src/nvim/msgpack_rpc/channel.c
index 388fa2584c..de01443313 100644
--- a/src/nvim/msgpack_rpc/channel.c
+++ b/src/nvim/msgpack_rpc/channel.c
@@ -532,8 +532,19 @@ void rpc_close(Channel *channel)
   }
 }
 
+static void exit_delay_cb(uv_timer_t *handle)
+{
+  uv_timer_stop(&main_loop.exit_delay_timer);
+  multiqueue_put(main_loop.fast_events, exit_event, 0);
+}
+
 static void exit_event(void **argv)
 {
+  if (exit_need_delay) {
+    uv_timer_start(&main_loop.exit_delay_timer, exit_delay_cb, 0, 0);
+    return;
+  }
+
   if (!exiting) {
     os_exit(0);
   }