delete_ref(): use the usual convention for old_sha1

The ref_transaction_update() family of functions use the following
convention for their old_sha1 parameters:

* old_sha1 == NULL: Don't check the old value at all.
* is_null_sha1(old_sha1): Ensure that the reference didn't exist
  before the transaction.
* otherwise: Ensure that the reference had the specified value before
  the transaction.

delete_ref() had a different convention, namely treating
is_null_sha1(old_sha1) as "don't care". Change it to adhere to the
standard convention to reduce the scope for confusion.

Please note that it is now a bug to pass old_sha1=NULL_SHA1 to
delete_ref() (because it doesn't make sense to delete a reference that
you already know doesn't exist). This is consistent with the behavior
of ref_transaction_delete().

Most of the callers of delete_ref() never pass old_sha1=NULL_SHA1 to
delete_ref(), and are therefore unaffected by this change. The
two exceptions are:

* The call in cmd_update_ref(), which passed NULL_SHA1 if the old
  value passed in on the command line was 0{40} or the empty string.
  Change that caller to pass NULL in those cases.

  Arguably, it should be an error to call "update-ref -d" with the old
  value set to "does not exist", just as it is for the `--stdin`
  command "delete". But since this usage was accepted until now,
  continue to accept it.

* The call in delete_branches(), which could pass NULL_SHA1 if
  deleting a broken or symbolic ref. Change it to pass NULL in these
  cases.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/builtin/update-ref.c b/builtin/update-ref.c
index 160c7ac1d3..6763cf1837 100644
--- a/builtin/update-ref.c
+++ b/builtin/update-ref.c
@@ -422,7 +422,13 @@ int cmd_update_ref(int argc, const char **argv, const char *prefix)
 	if (no_deref)
 		flags = REF_NODEREF;
 	if (delete)
-		return delete_ref(refname, oldval ? oldsha1 : NULL, flags);
+		/*
+		 * For purposes of backwards compatibility, we treat
+		 * NULL_SHA1 as "don't care" here:
+		 */
+		return delete_ref(refname,
+				  (oldval && !is_null_sha1(oldsha1)) ? oldsha1 : NULL,
+				  flags);
 	else
 		return update_ref(msg, refname, sha1, oldval ? oldsha1 : NULL,
 				  flags, UPDATE_REFS_DIE_ON_ERR);