fix(lua): close vim.defer_fn() timer if vim.schedule() failed (#37647)

Problem:
Using vim.defer_fn() just before Nvim exit leaks luv handles.

Solution:
Make vim.schedule() return an error message if scheduling failed.
Make vim.defer_fn() close timer if vim.schedule() failed.

1 files changed, 5 insertions, 2 deletions

diff --git a/src/nvim/lua/executor.c b/src/nvim/lua/executor.c
index 78d4515057..b76244cc2d 100644
--- a/src/nvim/lua/executor.c
+++ b/src/nvim/lua/executor.c
@@ -407,17 +407,20 @@ static int nlua_schedule(lua_State *const lstate)
     return lua_error(lstate);
   }
 
+  lua_pushnil(lstate);
   // If main_loop is closing don't schedule tasks to run in the future,
   // otherwise any refs allocated here will not be cleaned up.
   if (main_loop.closing) {
-    return 0;
+    lua_pushliteral(lstate, "main loop is closing");
+    return 2;
   }
 
   LuaRef cb = nlua_ref_global(lstate, 1);
   // Pass along UI event handler to disable on error.
   multiqueue_put(main_loop.events, nlua_schedule_event, (void *)(ptrdiff_t)cb,
                  (void *)(ptrdiff_t)ui_event_ns_id);
-  return 0;
+  lua_pushnil(lstate);
+  return 2;
 }
 
 // Dummy timer callback. Used by f_wait().