I'm excited to see someone else join my effort, thanks for continuing my 
effort! Also, less work for me :)

On 04.12.2019 21:39, Emily Shaffer wrote:

  static int file_callback(const struct option *opt, const char *arg, int unset)
  {
  	struct grep_opt *grep_opt = opt->value;
-	int from_stdin;
  	FILE *patterns;
  	int lno = 0;
  	struct strbuf sb = STRBUF_INIT;
  
  	BUG_ON_OPT_NEG(unset);
  
-	from_stdin = !strcmp(arg, "-");
-	patterns = from_stdin ? stdin : fopen(arg, "r");
+	patterns_from_stdin = !strcmp(arg, "-");
+
+	if (patterns_from_stdin && pathspec_from_stdin)

To my understanding, this check will not work as expected. 
`file_callback` will be called at the moment of parsing args. 
`pathspec_from_stdin` is only initialized later.

Maybe it would be better to convert `file_callback` into a regular 
function and call it after the options were parsed, similar to how 
`pathspec_from_file` is parsed later?

This will also allow to move global variables into local scope and 
resolve other small issues raised by other reviewers.

+test_expect_success 'grep with two stdin inputs fails' '
+	test_must_fail git grep --pathspec-from-file - --patterns-from-file - <pathspecs
+'
+

It is usually a good idea to test for specific error, like this:

   test_must_fail git grep --pathspec-from-file - --patterns-from-file - 
<pathspecs 2>err &&
   test_i18ngrep "cannot specify both patterns and pathspec via stdin" 
err &&