fix(dirmonitor): deadlock if error handler jumps somewhere else (#1647)

* fix: deadlock if error handler jumps somewhere else

* docs(dirmonitor): fix wrong data type in error callback

* docs(dirmonitor): clarify coroutines and deadlocks

* docs(dirmonitor): wording

Co-authored-by: Guldoman <giulio.lettieri@gmail.com>

---------

Co-authored-by: Guldoman <giulio.lettieri@gmail.com>

src/api/dirmonitor.c

@@ -1,4 +1,5 @@
 #include "api.h"
+#include "lua.h"
 #include <SDL.h>
 #include <stdlib.h>
 #include <string.h>
@@ -25,13 +26,16 @@ int get_mode_dirmonitor();
 
 
 static int f_check_dir_callback(int watch_id, const char* path, void* L) {
-  lua_pushvalue(L, -1);
+  // using absolute indices from f_dirmonitor_check (2: callback, 3: error_callback)
+  lua_pushvalue(L, 2);
   if (path)
     lua_pushlstring(L, path, watch_id);
   else
     lua_pushnumber(L, watch_id);
-  lua_call(L, 1, 1);
-  int result = lua_toboolean(L, -1);
+
+  int result = 0;
+  if (lua_pcall(L, 1, 1, 3) == LUA_OK)
+    result = lua_toboolean(L, -1);
   lua_pop(L, 1);
   return !result;
 }
@@ -95,8 +99,20 @@ static int f_dirmonitor_unwatch(lua_State *L) {
 }
 
 
+static int f_noop(lua_State *L) { return 0; }
+
+
 static int f_dirmonitor_check(lua_State* L) {
   struct dirmonitor* monitor = luaL_checkudata(L, 1, API_TYPE_DIRMONITOR);
+  luaL_checktype(L, 2, LUA_TFUNCTION);
+  if (!lua_isnoneornil(L, 3)) {
+    luaL_checktype(L, 3, LUA_TFUNCTION);
+  } else {
+    lua_settop(L, 2);
+    lua_pushcfunction(L, f_noop);
+  }
+  lua_settop(L, 3);
+
   SDL_LockMutex(monitor->mutex);
   if (monitor->length < 0)
     lua_pushnil(L);