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>

builtin/branch.c

@@ -253,7 +253,8 @@ static int delete_branches(int argc, const char **argv, int force, int kinds,
 			continue;
 		}
 
-		if (delete_ref(name, sha1, REF_NODEREF)) {
+		if (delete_ref(name, is_null_sha1(sha1) ? NULL : sha1,
+			       REF_NODEREF)) {
 			error(remote_branch
 			      ? _("Error deleting remote-tracking branch '%s'")
 			      : _("Error deleting branch '%s'"),

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);

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,

refs.h

@@ -240,11 +240,11 @@ extern int read_ref_at(const char *refname, unsigned int flags,
 extern int reflog_exists(const char *refname);
 
 /*
- * Delete the specified reference. If old_sha1 is non-NULL and not
- * NULL_SHA1, then verify that the current value of the reference is
- * old_sha1 before deleting it. If old_sha1 is NULL or NULL_SHA1,
- * delete the reference if it exists, regardless of its old value.
- * flags is passed through to ref_transaction_delete().
+ * Delete the specified reference. If old_sha1 is non-NULL, then
+ * verify that the current value of the reference is old_sha1 before
+ * deleting it. If old_sha1 is NULL, delete the reference if it
+ * exists, regardless of its old value. It is an error for old_sha1 to
+ * be NULL_SHA1. flags is passed through to ref_transaction_delete().
  */
 extern int delete_ref(const char *refname, const unsigned char *old_sha1,
 		      unsigned int flags);