fix(jobwait): always drain process event queues #15970

Problem:
jobwait() returns early if the job was stopped, but the job might have
pending callbacks on its event queue which are required to complete its
teardown. State such as term->closed might not be updated yet (by the
pending callbacks), so codepaths such as :bdelete think the job is still
running.

Solution:
Always flush the job's event queue before returning from jobwait().

ref #15349

Co-authored-by: Gregory Anders <greg@gpanders.com>

runtime/doc/nvim_terminal_emulator.txt

@@ -13,7 +13,7 @@ from the connected program.
 Terminal buffers behave like normal buffers, except:
 - With 'modifiable', lines can be edited but not deleted.
 - 'scrollback' controls how many lines are kept.
-- Output is followed if the cursor is on the last line.
+- Output is followed ("tailed") if cursor is on the last line.
 - 'modified' is the default. You can set 'nomodified' to avoid a warning when
   closing the terminal buffer.
 - 'bufhidden' defaults to "hide".

src/nvim/eval/funcs.c

@@ -5328,14 +5328,19 @@ static void f_jobwait(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   TV_LIST_ITER_CONST(args, arg, {
     Channel *chan = NULL;
     if (TV_LIST_ITEM_TV(arg)->v_type != VAR_NUMBER
-        || !(chan = find_job(TV_LIST_ITEM_TV(arg)->vval.v_number, false))) {
+        || !(chan = find_channel(TV_LIST_ITEM_TV(arg)->vval.v_number))
+        || chan->streamtype != kChannelStreamProc) {
+      jobs[i] = NULL;  // Invalid job.
+    } else if (process_is_stopped(&chan->stream.proc)) {
+      // Job is stopped but not fully destroyed.
+      // Ensure all callbacks on its event queue are executed. #15402
+      process_wait(&chan->stream.proc, -1, NULL);
       jobs[i] = NULL;  // Invalid job.
     } else {
       jobs[i] = chan;
       channel_incref(chan);
       if (chan->stream.proc.status < 0) {
-        // Process any pending events on the job's queue before temporarily
-        // replacing it.
+        // Flush any events in the job's queue before temporarily replacing it.
         multiqueue_process_events(chan->events);
         multiqueue_replace_parent(chan->events, waiting_jobs);
       }

test/functional/terminal/buffer_spec.lua

@@ -7,6 +7,7 @@ local eq, neq = helpers.eq, helpers.neq
 local write_file = helpers.write_file
 local command= helpers.command
 local exc_exec = helpers.exc_exec
+local matches = helpers.matches
 
 describe(':terminal buffer', function()
   local screen
@@ -259,6 +260,14 @@ describe(':terminal buffer', function()
     eq('Vim(wqall):E948: Job still running', exc_exec('wqall'))
   end)
 
+  it('stops running jobs with :quit', function()
+    -- Open in a new window to avoid terminating the nvim instance
+    command('split')
+    command('terminal')
+    command('set nohidden')
+    command('quit')
+  end)
+
   it('does not segfault when pasting empty buffer #13955', function()
     feed_command('terminal')
     feed('<c-\\><c-n>')