fix(lua): don't crash when lua_tolstring returns NULL for error value

src/nvim/lua/executor.c

@@ -123,6 +123,34 @@ lua_State *get_global_lstate(void)
   return global_lstate;
 }
 
+/// get error on top of stack as a string
+///
+/// Might alter the top value on stack in place (but doesn't change stack height)
+///
+/// "error" points to memory on the lua stack, use
+/// or duplicate the string before using "lstate" again
+///
+/// @param[out] len length of error (can be NULL)
+static const char *nlua_get_error(lua_State *lstate, size_t *len)
+{
+  if (luaL_getmetafield(lstate, -1, "__tostring")) {
+    if (lua_isfunction(lstate, -1) && luaL_callmeta(lstate, -2, "__tostring")) {
+      // call __tostring, convert the result replace error with it
+      lua_replace(lstate, -3);
+    }
+    // pop __tostring.
+    lua_pop(lstate, 1);
+  }
+
+  const char *str = lua_tolstring(lstate, -1, len);
+  if (str == NULL) {
+    str = "[?????]";
+    *len = 7;
+  }
+
+  return str;
+}
+
 /// Convert lua error into a Vim error message
 ///
 /// @param  lstate  Lua interpreter state.
@@ -131,22 +159,7 @@ void nlua_error(lua_State *const lstate, const char *const msg)
   FUNC_ATTR_NONNULL_ALL
 {
   size_t len;
-  const char *str = NULL;
+  const char *str = nlua_get_error(lstate, &len);
-
-  if (luaL_getmetafield(lstate, -1, "__tostring")) {
-    if (lua_isfunction(lstate, -1) && luaL_callmeta(lstate, -2, "__tostring")) {
-      // call __tostring, convert the result and pop result.
-      str = lua_tolstring(lstate, -1, &len);
-      lua_pop(lstate, 1);
-    }
-    // pop __tostring.
-    lua_pop(lstate, 1);
-  }
-
-  if (!str) {
-    // defer to lua default conversion, this will render tables as [NULL].
-    str = lua_tolstring(lstate, -1, &len);
-  }
 
   if (in_script) {
     fprintf(stderr, msg, (int)len, str);
@@ -216,7 +229,9 @@ static int nlua_luv_cfpcall(lua_State *lstate, int nargs, int nresult, int flags
       // consider out of memory errors unrecoverable, just like xmalloc()
       preserve_exit(e_outofmem);
     }
-    const char *error = lua_tostring(lstate, -1);
+
+    size_t len;
+    const char *error = nlua_get_error(lstate, &len);
 
     multiqueue_put(main_loop.events, nlua_luv_error_event,
                    xstrdup(error), (void *)(intptr_t)kCallback);

test/functional/lua/loop_spec.lua

@@ -136,4 +136,29 @@ describe('vim.uv', function()
   it("is equal to require('luv')", function()
     eq(true, exec_lua("return vim.uv == require('luv')"))
   end)
+
+  it("doesn't crash on async callbacks throwing nil error", function()
+    local screen = Screen.new(50, 4)
+    screen:attach()
+    screen:set_default_attr_ids {
+      [1] = { bold = true, reverse = true },
+      [2] = { background = Screen.colors.Red, foreground = Screen.colors.Grey100 },
+      [3] = { foreground = Screen.colors.SeaGreen4, bold = true },
+    }
+
+    exec_lua [[
+      local idle = vim.uv.new_idle()
+      idle:start(function()
+        idle:stop()
+        error()
+      end)
+    ]]
+
+    screen:expect([[
+      {1:                                                  }|
+      {2:Error executing luv callback:}                     |
+      {2:[?????]}                                           |
+      {3:Press ENTER or type command to continue}^           |
+    ]])
+  end)
 end)