fix(worker): do not clean up workers in loop __gc

When setting up luv with luaopen_luv(), instead of cleaning up workers
in the __gc of the loop (which luv may not own), instead create an empty
userdata with a worker __gc and attach it to the lua_State.

Fixes #754

src/luv.c

@@ -852,11 +852,6 @@ static int loop_gc(lua_State *L) {
   while (uv_loop_close(loop)) {
     uv_run(loop, UV_RUN_DEFAULT);
   }
-  /* do cleanup in main thread */
-  lua_getglobal(L, "_THREAD");
-  if (lua_isnil(L, -1))
-    luv_work_cleanup();
-  lua_pop(L, 1);
   return 0;
 }
 
@@ -925,7 +920,11 @@ LUALIB_API int luaopen_luv (lua_State* L) {
   luv_dir_init(L);
 #endif
   luv_thread_init(L);
-  luv_work_init(L);
+
+  lua_getglobal(L, "_THREAD");
+  int is_main = lua_isnil(L, -1);
+  lua_pop(L, 1);
+  luv_work_init(L, is_main);
 
   luv_constants(L);
   lua_setfield(L, -2, "constants");

src/work.c

@@ -287,12 +287,12 @@ static void luv_key_init_once(void)
   idx_vms = 0;
 }
 
-static void luv_work_cleanup(void)
+static int luv_work_gc(lua_State *L)
 {
   unsigned int i;
 
   if (nvms == 0)
-    return;
+    return 0;
 
   for (i = 0; i < nvms && vms[i]; i++)
     release_vm_cb(vms[i]);
@@ -302,9 +302,10 @@ static void luv_work_cleanup(void)
 
   uv_mutex_destroy(&vm_mutex);
   nvms = 0;
+  return 0;
 }
 
-static void luv_work_init(lua_State* L) {
+static void luv_work_init(lua_State* L, int is_main) {
   luaL_newmetatable(L, "luv_work_ctx");
   lua_pushcfunction(L, luv_work_ctx_tostring);
   lua_setfield(L, -2, "__tostring");
@@ -314,5 +315,15 @@ static void luv_work_init(lua_State* L) {
   lua_setfield(L, -2, "__index");
   lua_pop(L, 1);
 
+  // do cleanup in main thread
+  if (is_main) {
+    lua_newuserdata(L, 0);
+    luaL_newmetatable(L, "luv_work.meta");
+    lua_pushcfunction(L, luv_work_gc);
+    lua_setfield(L, -2, "__gc");
+    lua_setmetatable(L, -2);
+    lua_setfield(L, LUA_REGISTRYINDEX, "luv_work");
+  }
+
   uv_once(&once_vmkey, luv_key_init_once);
 }