Jeff King <p...@peff.net> writes:

> On Thu, Sep 01, 2016 at 02:08:08PM -0700, Junio C Hamano wrote:
>
>> I think we should.
>> 
>> t1401 expects to be able to, but if you really do it:
>> 
>>      $ cd /tmp
>>      $ git init throwaway
>>         $ cd throwaway
>>         $ git symbolic-ref -d HEAD
>> 
>> the setup machinery considers that you are no longer in a working
>> tree that is controlled by a repository at .git/ because .git/ is
>> no longer a valid repository, so you cannot even do
>> 
>>      $ git symbolic-ref HEAD refs/heads/master
>> 
>> to recover.
>
> Yes, I think we should, too. The same reasoning from afe5d3d (symbolic
> ref: refuse non-ref targets in HEAD, 2009-01-29) applies.

-- >8 --
Subject: symbolic-ref -d: do not allow removal of HEAD

If you delete the symbolic-ref HEAD from a repository, Git no longer
considers it valid, and even "git symbolic-ref HEAD refs/heads/master"
would not be able to recover from that state.

In the spirit similar to afe5d3d5 ("symbolic ref: refuse non-ref
targets in HEAD", 2009-01-29), forbid removal of HEAD.

Signed-off-by: Junio C Hamano <gits...@pobox.com>
---

 I decided against it for now for no good reason, other than I am a
 bit superstitious, but it may be a good idea to move these safety
 checks to delete_ref() and create_symref() in the longer term.

 builtin/symbolic-ref.c  |  2 ++
 t/t1401-symbolic-ref.sh | 19 ++++++++++++-------
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/builtin/symbolic-ref.c b/builtin/symbolic-ref.c
index 9c29a64..96eed94 100644
--- a/builtin/symbolic-ref.c
+++ b/builtin/symbolic-ref.c
@@ -56,6 +56,8 @@ int cmd_symbolic_ref(int argc, const char **argv, const char 
*prefix)
                ret = check_symref(argv[0], 1, 0, 0);
                if (ret)
                        die("Cannot delete %s, not a symbolic ref", argv[0]);
+               if (!strcmp(argv[0], "HEAD"))
+                       die("deleting '%s' is not allowed", argv[0]);
                return delete_ref(argv[0], NULL, REF_NODEREF);
        }
 
diff --git a/t/t1401-symbolic-ref.sh b/t/t1401-symbolic-ref.sh
index ca3fa40..5c30f94 100755
--- a/t/t1401-symbolic-ref.sh
+++ b/t/t1401-symbolic-ref.sh
@@ -33,18 +33,23 @@ test_expect_success 'symbolic-ref refuses bare sha1' '
 '
 reset_to_sane
 
-test_expect_success 'symbolic-ref deletes HEAD' '
-       git symbolic-ref -d HEAD &&
+test_expect_success 'HEAD cannot be removed' '
+       test_must_fail git symbolic-ref -d HEAD
+'
+
+test_expect_success 'symbolic-ref can be deleted' '
+       git symbolic-ref NOTHEAD refs/heads/foo &&
+       git symbolic-ref -d NOTHEAD &&
        test_path_is_file .git/refs/heads/foo &&
-       test_path_is_missing .git/HEAD
+       test_path_is_missing .git/NOTHEAD
 '
 reset_to_sane
 
-test_expect_success 'symbolic-ref deletes dangling HEAD' '
-       git symbolic-ref HEAD refs/heads/missing &&
-       git symbolic-ref -d HEAD &&
+test_expect_success 'symbolic-ref can delete dangling symref' '
+       git symbolic-ref NOTHEAD refs/heads/missing &&
+       git symbolic-ref -d NOTHEAD &&
        test_path_is_missing .git/refs/heads/missing &&
-       test_path_is_missing .git/HEAD
+       test_path_is_missing .git/NOTHEAD
 '
 reset_to_sane
 

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