[PATCH v3] pull, fetch: fix segfault in --set-upstream option

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Fix a segfault in the --set-upstream option added in
24bc1a12926 (pull, fetch: add --set-upstream option, 2019-08-19) added
in v2.24.0.

The code added there did not do the same checking we do for "git
branch" itself since 8efb8899cfe (branch: segfault fixes and
validation, 2013-02-23), which in turn fixed the same sort of segfault
I'm fixing now in "git branch --set-upstream-to", see
6183d826ba6 (branch: introduce --set-upstream-to, 2012-08-20).

The warning message I'm adding here is an amalgamation of the error
added for "git branch" in 8efb8899cfe, and the error output
install_branch_config() itself emits, i.e. it trims "refs/heads/" from
the name and says "branch X on remote", not "branch refs/heads/X on
remote".

I think it would make more sense to simply die() here, but in the
other checks for --set-upstream added in 24bc1a12926 we issue a
warning() instead. Let's do the same here for consistency for now.

There was an earlier submitted alternate way of fixing this in [1],
due to that patch breaking threading with the original report at [2] I
didn't notice it before authoring this version. I think the more
detailed warning message here is better, and we should also have tests
for this behavior.

1. https://lore.kernel.org/git/20210706162238.575988-1-clemens@endorphin.org/
2. https://lore.kernel.org/git/CAG6gW_uHhfNiHGQDgGmb1byMqBA7xa8kuH1mP-wAPEe5Tmi2Ew@mail.gmail.com/

Reported-by: Clemens Fruhwirth <clemens@endorphin.org>
Reported-by: Jan Pokorný <poki@fnusa.cz>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---

A v3 that functionally behaves the same way, but uses a more idiomatic
way of calling skip_prefix() to strip the "refs/heads/*" prefix, if
present.

Range-diff against v2:
1:  9e846b76959 ! 1:  68899471206 pull, fetch: fix segfault in --set-upstream option
    @@ builtin/fetch.c: 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);

 builtin/fetch.c         | 10 ++++++++++
 t/t5553-set-upstream.sh | 22 ++++++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/builtin/fetch.c b/builtin/fetch.c
index e064687dbdc..28fa168133a 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1625,6 +1625,16 @@ static int do_fetch(struct transport *transport,
 			}
 		}
 		if (source_ref) {
+			if (!branch) {
+				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);
+				goto skip;
+			}
+
 			if (!strcmp(source_ref->name, "HEAD") ||
 			    starts_with(source_ref->name, "refs/heads/"))
 				install_branch_config(0,
diff --git a/t/t5553-set-upstream.sh b/t/t5553-set-upstream.sh
index b1d614ce18c..7d12ceff702 100755
--- a/t/t5553-set-upstream.sh
+++ b/t/t5553-set-upstream.sh
@@ -91,6 +91,17 @@ test_expect_success 'fetch --set-upstream with valid URL sets upstream to URL' '
 	check_config_missing other2
 '
 
+test_expect_success 'fetch --set-upstream with a detached HEAD' '
+	git checkout HEAD^0 &&
+	test_when_finished "git checkout -" &&
+	cat >expect <<-\EOF &&
+	warning: could not set upstream of HEAD to '"'"'main'"'"' from '"'"'upstream'"'"' when it does not point to any branch.
+	EOF
+	git fetch --set-upstream upstream main 2>actual.raw &&
+	grep ^warning: actual.raw >actual &&
+	test_cmp expect actual
+'
+
 # tests for pull --set-upstream
 
 test_expect_success 'setup bare parent pull' '
@@ -178,4 +189,15 @@ test_expect_success 'pull --set-upstream with valid URL and branch sets branch'
 	check_config_missing other2
 '
 
+test_expect_success 'pull --set-upstream with a detached HEAD' '
+	git checkout HEAD^0 &&
+	test_when_finished "git checkout -" &&
+	cat >expect <<-\EOF &&
+	warning: could not set upstream of HEAD to '"'"'main'"'"' from '"'"'upstream'"'"' when it does not point to any branch.
+	EOF
+	git pull --set-upstream upstream main 2>actual.raw &&
+	grep ^warning: actual.raw >actual &&
+	test_cmp expect actual
+'
+
 test_done
-- 
2.33.0.741.g4db85f1eb27