fix: callback handle error with non-strings

src/luv.c

@@ -706,7 +706,8 @@ LUALIB_API int luv_cfpcall(lua_State* L, int nargs, int nresult, int flags) {
     break;
   case LUA_ERRMEM:
     if ((flags & LUVF_CALLBACK_NOERRMSG) == 0)
-      fprintf(stderr, "System Error: %s\n", lua_tostring(L, -1));
+      fprintf(stderr, "System Error: %s\n",
+              luaL_tolstring(L, lua_absindex(L, -1), NULL));
     if ((flags & LUVF_CALLBACK_NOEXIT) == 0)
       exit(-1);
     lua_pop(L, 1);
@@ -716,7 +717,8 @@ LUALIB_API int luv_cfpcall(lua_State* L, int nargs, int nresult, int flags) {
   case LUA_ERRERR:
   default:
     if ((flags & LUVF_CALLBACK_NOERRMSG) == 0)
-      fprintf(stderr, "Uncaught Error: %s\n", lua_tostring(L, -1));
+      fprintf(stderr, "Uncaught Error: %s\n",
+              luaL_tolstring(L, lua_absindex(L, -1), NULL));
     if ((flags & LUVF_CALLBACK_NOEXIT) == 0)
       exit(-1);
     lua_pop(L, 1);

tests/manual-test-cb-non-string-error.lua

@@ -0,0 +1,24 @@
+return require('lib/tap')(function (test)
+  test("timer callback with errors", function(print, p, expect, uv)
+    local Error = {}
+    Error.__index = Error
+
+    function Error.new(msg)
+        local o = setmetatable({}, Error)
+        o.msg = assert(msg)
+        return o
+    end
+
+    function Error:__tostring()
+        return assert(tostring(self.msg))
+    end
+
+    local timer = uv.new_timer()
+    timer:start(10, 0, function()
+      timer:stop()
+      timer:close()
+      local e = Error.new('Error in timeout callback')
+      error(e)
+    end)
+  end)
+end)