delete_ref(): use the usual convention for old_sha1mh/init-delete-refs-api

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, 0 insertions, 8 deletions

diff --git a/refs.c b/refs.c
index d5c3f8de1b..c5086ae70c 100644
--- a/refs.c
+++ b/refs.c
@@ -2833,14 +2833,6 @@ int delete_ref(const char *refname, const unsigned char *old_sha1,
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
 
-	/*
-	 * Treat NULL_SHA1 and NULL alike, to mean "we don't care what
-	 * the old value of the reference was (or even if it didn't
-	 * exist)":
-	 */
-	if (old_sha1 && is_null_sha1(old_sha1))
-		old_sha1 = NULL;
-
 	transaction = ref_transaction_begin(&err);
 	if (!transaction ||
 	    ref_transaction_delete(transaction, refname, old_sha1,