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Re: [PATCH v2] ref-filter.c: pass empty-string as NULL to atom parsers

Taylor Blau <me@xxxxxxxxxxxx> writes:

> On Tue, Oct 03, 2017 at 08:55:01AM +0900, Junio C Hamano wrote:
>> Jonathan Nieder <jrnieder@xxxxxxxxx> writes:
>> > The above does a nice job of explaining
>> >
>> >  - what this change is going to do
>> >  - how it's good for the internal code structure / maintainability
>> >
>> > What it doesn't tell me about is why the user-facing effect won't
>> > cause problems.  Is there no atom where %(atom:) was previously
>> > accepted and did something meaningful that this may break?

This loose end needs to be tied.  

I replaced "let's assume that doing this change is OK" from the
original log message with something a bit stronger, as with this
change, we are saying that it is forbidden to treat %(atom) and
%(atom:) differently.  I also recorded the result of due-diligence
survey of the current code to suggest that the change would be OK
for current users.

-- >8 --
From: Taylor Blau <me@xxxxxxxxxxxx>
Date: Mon, 2 Oct 2017 09:10:34 -0700
Subject: [PATCH] ref-filter.c: pass empty-string as NULL to atom parsers

Peff points out that different atom parsers handle the empty
"sub-argument" list differently. An example of this is the format

Since callers often use `string_list_split` (which splits the empty
string with any delimiter as a 1-ary string_list containing the empty
string), this makes handling empty sub-argument strings non-ergonomic.

Let's fix this by declaring that atom parser implementations must
not care about distinguishing between the empty string "%(refname:)"
and no sub-arguments "%(refname)".  Current code aborts, either with
"unrecognised arg" (e.g. "refname:") or "does not take args"
(e.g. "body:") as an error message.

Signed-off-by: Taylor Blau <me@xxxxxxxxxxxx>
Reviewed-by: Jeff King <peff@xxxxxxxx>
Reviewed-by: Jonathan Nieder <jrnieder@xxxxxxxxx>
Signed-off-by: Junio C Hamano <gitster@xxxxxxxxx>
 ref-filter.c            | 10 +++++++++-
 t/t6300-for-each-ref.sh |  1 +
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/ref-filter.c b/ref-filter.c
index bc591f4f3d..f3e53d4448 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -415,8 +415,16 @@ static int parse_ref_filter_atom(const struct ref_format *format,
 	REALLOC_ARRAY(used_atom, used_atom_cnt);
 	used_atom[at].name = xmemdupz(atom, ep - atom);
 	used_atom[at].type = valid_atom[i].cmp_type;
-	if (arg)
+	if (arg) {
 		arg = used_atom[at].name + (arg - atom) + 1;
+		if (!*arg) {
+			/*
+			 * Treat empty sub-arguments list as NULL (i.e.,
+			 * "%(atom:)" is equivalent to "%(atom)").
+			 */
+			arg = NULL;
+		}
+	}
 	memset(&used_atom[at].u, 0, sizeof(used_atom[at].u));
 	if (valid_atom[i].parser)
 		valid_atom[i].parser(format, &used_atom[at], arg);
diff --git a/t/t6300-for-each-ref.sh b/t/t6300-for-each-ref.sh
index 2274a4b733..edc1bd8eab 100755
--- a/t/t6300-for-each-ref.sh
+++ b/t/t6300-for-each-ref.sh
@@ -51,6 +51,7 @@ test_atom() {
 test_atom head refname refs/heads/master
+test_atom head refname: refs/heads/master
 test_atom head refname:short master
 test_atom head refname:lstrip=1 heads/master
 test_atom head refname:lstrip=2 master