Subject: Re: [PATCH v2] pull, fetch: fix segfault in --set-upstream option

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Tue, 24 Aug 2021 10:49:26 +0200

On Tue, Aug 24 2021, Clemens Fruhwirth wrote:

> On Mon, 23 Aug 2021 at 14:56, Ævar Arnfjörð Bjarmason <[email-masked]> wrote:
>>
>> A proposed replacement of cf/fetch-set-upstream-while-detached which
>> as noted in What's Cooking has been languishing for a while. I changed
>> the die() in my version to a warning() as suggested by Junio & updated
>> the test and commit message accordingly.
>
> Thank you Ævar, for taking care of that and adding tests and my
> apologizes for not responding in a timely manner.

No worries.

>>  builtin/fetch.c         | 11 +++++++++++
>>  t/t5553-set-upstream.sh | 22 ++++++++++++++++++++++
>>  2 files changed, 33 insertions(+)
>>
>> diff --git a/builtin/fetch.c b/builtin/fetch.c
>> index 25740c13df1..ca487edd805 100644
>> --- a/builtin/fetch.c
>> +++ b/builtin/fetch.c
>> @@ -1623,6 +1623,17 @@ static int do_fetch(struct transport *transport,
>>                         }
>>                 }
>>                 if (source_ref) {
>> +                       if (!branch) {
>> +                               const char *shortname = NULL;
>> +                               if (!skip_prefix(source_ref->name,
>> +                                                "refs/heads/", &shortname))
>> +                                       shortname = source_ref->name;
>
> Is skip_prefix ever able to fail?
>
> - If yes, then shortname will be left pointing to NULL when we print
> the warning below. Warning probably won't die because of NULL, but it
> still will print something weird.

If it fails we'll use source_ref->name, so it won't be NULL...

> - If no, then an assert(!skip_prefix(source_ref->name, "refs/heads/",
> &shortname)) might be more idiomatic.

Which means that this would be a bug, since it's not handling the case
where the source doesn't start with refs/heads/*.

I.e. it's the same as doing it this way (on top of this v2), which
perhaps is a clearer way to express the same idea. What do you think?

diff --git a/builtin/fetch.c b/builtin/fetch.c
index ca487edd805..2bc9159690d 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1624,10 +1624,9 @@ static int do_fetch(struct transport *transport,
 		}
 		if (source_ref) {
 			if (!branch) {
-				const char *shortname = NULL;
-				if (!skip_prefix(source_ref->name,
-						 "refs/heads/", &shortname))
-					shortname = source_ref->name;
+				const char *shortname = source_ref->name;
+				skip_prefix(shortname, "refs/heads/", &shortname);
+
 				warning(_("could not set upstream of HEAD to '%s' from '%s' when "
 					  "it does not point to any branch."),
 					shortname, transport->remote->name);