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>

data/core/dirwatch.lua

@@ -91,6 +91,7 @@ end
 -- designed to be run inside a coroutine.
 function dirwatch:check(change_callback, scan_time, wait_time)
   local had_change = false
+  local last_error
   self.monitor:check(function(id)
     had_change = true
     if self.monitor:mode() == "single" then
@@ -102,7 +103,10 @@ function dirwatch:check(change_callback, scan_time, wait_time)
     elseif self.reverse_watched[id] then
       change_callback(self.reverse_watched[id])
     end
+  end, function(err)
+    last_error = err
   end)
+  if last_error ~= nil then error(last_error) end
   local start_time = system.get_time()
   for directory, old_modified in pairs(self.scanned) do
     if old_modified then

docs/api/dirmonitor.lua

@@ -45,10 +45,14 @@ function dirmonitor:unwatch(fd_or_path) end
 ---edited, removed or added. A file descriptor will be passed to the
 ---callback in "multiple" mode or a path in "single" mode.
 ---
+---If an error occurred during the callback execution, the error callback will be called with the error object.
+---This callback should not manipulate coroutines to avoid deadlocks.
+---
 ---@param callback dirmonitor.callback
+---@param error_callback fun(error: any): nil
 ---
 ---@return boolean? changes True when changes were detected.
-function dirmonitor:check(callback) end
+function dirmonitor:check(callback, error_callback) end
 
 ---
 ---Get the working mode for the current file system monitoring backend.

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);