[PATCH v2] grep: improve errors for unmatched ( and )

["Ahelenia Ziemiańska" <nabijaczleweli@nabijaczleweli.xyz>]

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Imagine you want to grep for (. Easy:

  $ git grep '('
  fatal: unmatched parenthesis

uhoh. This is plainly wrong. Unless you know specifically that
(a) git grep has expression groups and that
(b) the only way to work around them is by doing -- '(' or -e '('

Similarly,

  $ git grep ')'
  fatal: incomplete pattern expression: )

is somehow worse. ")" is a complete regular expression pattern.
Of course, the error wants to say "group" here.
In this case it's also not "incomplete", it's unmatched.
But whatever.

Make them return

  $ ./git grep '('
  fatal: unmatched ( for expression group
  $ ./git grep ')'
  fatal: incomplete pattern expression group: )

which hopefully are clearer in indicating that it's not the expression
that's wrong (since no pattern had been parsed at all), but rather that
it's been misconstrued as a grouping operator.

Link: https://bugs.debian.org/1051205
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
---
On Fri, Mar 22, 2024 at 09:41:40AM -0700, Junio C Hamano wrote:
> Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz> writes:
> > uhoh. This is plainly wrong. Unless you know specifically that
> > (a) git grep has expression groups and that
> > (b) the only way to work around them is by doing -- '(' or -e '('
> I do not think "--" (end of options and beginning of pathspec)
> marker would work for that purpose, UNLESS you are talking about a
> file whose name is an open parenthesis.
False. -- turns all subsequent parameters into arguments, and if
there is no -e, the first argument is the pattern, and all the
subsequent ones are paths. This is normal [git] grep behaviour.

> Just keep "-e '('" in the
> description and drop the double-dash there.
Disagree. This is one of the two methodologies I've devised to work
around this in the past, and it's one of the two methodologies that
work.

All else applied.

 grep.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/grep.c b/grep.c
index 5f23d1a..ac34bfe 100644
--- a/grep.c
+++ b/grep.c
@@ -621,7 +621,7 @@ static struct grep_expr *compile_pattern_atom(struct grep_pat **list)
 		*list = p->next;
 		x = compile_pattern_or(list);
 		if (!*list || (*list)->token != GREP_CLOSE_PAREN)
-			die("unmatched parenthesis");
+			die("unmatched ( for expression group");
 		*list = (*list)->next;
 		return x;
 	default:
@@ -792,7 +792,7 @@ void compile_grep_patterns(struct grep_opt *opt)
 	if (p)
 		opt->pattern_expression = compile_pattern_expr(&p);
 	if (p)
-		die("incomplete pattern expression: %s", p->pattern);
+		die("incomplete pattern expression group: %s", p->pattern);
 
 	if (opt->no_body_match && opt->pattern_expression)
 		opt->pattern_expression = grep_not_expr(opt->pattern_expression);
-- 
2.39.2

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