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, 2 insertions, 0 deletions

diff --git a/src/nvim/event/process.c b/src/nvim/event/process.c
index 1ec11f1eb6..e029f778f6 100644
--- a/src/nvim/event/process.c
+++ b/src/nvim/event/process.c
@@ -386,11 +386,13 @@ static void process_close_handles(void **argv)
 {
   Process *proc = argv[0];
 
+  exit_need_delay++;
   flush_stream(proc, &proc->out);
   flush_stream(proc, &proc->err);
 
   process_close_streams(proc);
   process_close(proc);
+  exit_need_delay--;
 }
 
 static void on_process_exit(Process *proc)