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, 2 insertions, 0 deletions

diff --git a/builtin/symbolic-ref.c b/builtin/symbolic-ref.c
index e547a08d6c..1b0f10225f 100644
--- a/builtin/symbolic-ref.c
+++ b/builtin/symbolic-ref.c
@@ -71,6 +71,8 @@ int cmd_symbolic_ref(int argc, const char **argv, const char *prefix)
 		if (!strcmp(argv[0], "HEAD") &&
 		    !starts_with(argv[1], "refs/"))
 			die("Refusing to point HEAD outside of refs/");
+		if (check_refname_format(argv[1], REFNAME_ALLOW_ONELEVEL) < 0)
+			die("Refusing to set '%s' to invalid ref '%s'", argv[0], argv[1]);
 		ret = !!create_symref(argv[0], argv[1], msg);
 		break;
 	default: