symbolic-ref: refuse to set syntactically invalid target

You can feed absolute garbage to symbolic-ref as a target like:

  git symbolic-ref HEAD refs/heads/foo..bar

While this doesn't technically break the repo entirely (our "is it a git
directory" detector looks only for "refs/" at the start), we would never
resolve such a ref, as the ".." is invalid within a refname.

Let's flag these as invalid at creation time to help the caller realize
that what they're asking for is bogus.

A few notes:

  - We use REFNAME_ALLOW_ONELEVEL here, which lets:

     git update-ref refs/heads/foo FETCH_HEAD

    continue to work. It's unclear whether anybody wants to do something
    so odd, but it does work now, so this is erring on the conservative
    side. There's a test to make sure we didn't accidentally break this,
    but don't take that test as an endorsement that it's a good idea, or
    something we might not change in the future.

  - The test in t4202-log.sh checks how we handle such an invalid ref on
    the reading side, so it has to be updated to touch the HEAD file
    directly.

  - We need to keep our HEAD-specific check for "does it start with
    refs/". The ALLOW_ONELEVEL flag means we won't be enforcing that for
    other refs, but HEAD is special here because of the checks in
    validate_headref().

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/t/t1401-symbolic-ref.sh b/t/t1401-symbolic-ref.sh
index 9fb0b90f25..0c204089b8 100755
--- a/t/t1401-symbolic-ref.sh
+++ b/t/t1401-symbolic-ref.sh
@@ -165,4 +165,14 @@ test_expect_success 'symbolic-ref can resolve d/f name (ENOTDIR)' '
 	test_cmp expect actual
 '
 
+test_expect_success 'symbolic-ref refuses invalid target for non-HEAD' '
+	test_must_fail git symbolic-ref refs/heads/invalid foo..bar
+'
+
+test_expect_success 'symbolic-ref allows top-level target for non-HEAD' '
+	git symbolic-ref refs/heads/top-level FETCH_HEAD &&
+	git update-ref FETCH_HEAD HEAD &&
+	test_cmp_rev top-level HEAD
+'
+
 test_done