refs: allow callers to supply old OIDs for batch deletion

refs_delete_refs() performs unconditional deletions, so callers cannot
preserve old values that they have already resolved. Consequently,
reference-transaction hooks see a null old OID.

Let callers provide an optional array of expected old OIDs in parallel with
the refname list. When the array is provided, delete the ref at position N
only if it still points at the OID at position N. A null OID requests an
unconditional deletion for refs whose old value cannot be resolved, such as
broken refs.

Use REF_TRANSACTION_ALLOW_FAILURE when old OIDs are supplied. This retains
the helper's best-effort behavior: an old-OID mismatch rejects that deletion
while independent deletions in the batch can still proceed.

Signed-off-by: Maciej Ciemborowicz <maciej.ciemborowicz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bisect.c

@@ -1206,7 +1206,7 @@ int bisect_clean_state(void)
 	string_list_append(&refs_for_removal, "BISECT_EXPECTED_REV");
 	result = refs_delete_refs(get_main_ref_store(the_repository),
 				  "bisect: remove", &refs_for_removal,
-				  REF_NO_DEREF);
+				  NULL, REF_NO_DEREF);
 	string_list_clear(&refs_for_removal, 0);
 	unlink_or_warn(git_path_bisect_ancestors_ok());
 	unlink_or_warn(git_path_bisect_log());

builtin/branch.c

@@ -351,7 +351,8 @@ static int delete_branches(int argc, const char **argv, int kinds,
 	}
 
 	if (!(flags & DELETE_BRANCH_DRY_RUN) &&
-	    refs_delete_refs(get_main_ref_store(the_repository), NULL, &refs_to_delete, REF_NO_DEREF))
+	    refs_delete_refs(get_main_ref_store(the_repository), NULL,
+			     &refs_to_delete, NULL, REF_NO_DEREF))
 		ret = 1;
 
 	for_each_string_list_item(item, &refs_to_delete) {

builtin/fetch.c

@@ -1486,7 +1486,7 @@ static int prune_refs(struct display_state *display_state,
 		} else {
 			result = refs_delete_refs(get_main_ref_store(the_repository),
 						  "fetch: prune", &refnames,
-						  0);
+						  NULL, 0);
 		}
 	}
 

builtin/remote.c

@@ -1073,7 +1073,7 @@ static int rm(int argc, const char **argv, const char *prefix,
 	if (!result)
 		result = refs_delete_refs(get_main_ref_store(the_repository),
 					  "remote: remove", &branches,
-					  REF_NO_DEREF);
+					  NULL, REF_NO_DEREF);
 	string_list_clear(&branches, 0);
 
 	if (skipped.nr) {
@@ -1645,7 +1645,8 @@ static int prune_remote(const char *remote, int dry_run)
 
 	if (!dry_run)
 		result |= refs_delete_refs(get_main_ref_store(the_repository),
-					   "remote: prune", &refs_to_prune, 0);
+					   "remote: prune", &refs_to_prune,
+					   NULL, 0);
 
 	for_each_string_list_item(item, &states.stale) {
 		const char *refname = item->util;

builtin/tag.c

@@ -122,7 +122,8 @@ static int delete_tags(const char **argv)
 	struct string_list_item *item;
 
 	result = for_each_tag_name(argv, collect_tags, (void *)&refs_to_delete);
-	if (refs_delete_refs(get_main_ref_store(the_repository), NULL, &refs_to_delete, REF_NO_DEREF))
+	if (refs_delete_refs(get_main_ref_store(the_repository), NULL,
+			     &refs_to_delete, NULL, REF_NO_DEREF))
 		result = 1;
 
 	for_each_string_list_item(item, &refs_to_delete) {

refs.c

@@ -16,6 +16,7 @@
 #include "refs/refs-internal.h"
 #include "hook.h"
 #include "object-name.h"
+#include "oid-array.h"
 #include "odb.h"
 #include "object.h"
 #include "path.h"
@@ -3069,34 +3070,60 @@ void ref_transaction_for_each_rejected_update(struct ref_transaction *transactio
 	}
 }
 
-int refs_delete_refs(struct ref_store *refs, const char *logmsg,
-		     struct string_list *refnames, unsigned int flags)
+struct delete_refs_rejection_data {
+	int failures;
+};
+
+static void delete_refs_rejection_handler(const char *refname,
+					  const struct object_id *old_oid UNUSED,
+					  const struct object_id *new_oid UNUSED,
+					  const char *old_target UNUSED,
+					  const char *new_target UNUSED,
+					  enum ref_transaction_error err,
+					  const char *details,
+					  void *cb_data)
 {
+	struct delete_refs_rejection_data *data = cb_data;
+
+	warning(_("could not delete reference %s: %s"), refname,
+		details ? details : ref_transaction_error_msg(err));
+	data->failures = 1;
+}
+
+int refs_delete_refs(struct ref_store *refs, const char *logmsg,
+		     struct string_list *refnames,
+		     const struct oid_array *old_oids,
+		     unsigned int flags)
+{
+	struct delete_refs_rejection_data rejection_data = { 0 };
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
-	struct string_list_item *item;
+	size_t i;
 	int ret = 0, failures = 0;
 	char *msg;
 
 	if (!refnames->nr)
 		return 0;
+	if (old_oids && old_oids->nr != refnames->nr)
+		BUG("refname and old OID counts do not match");
 
 	msg = normalize_reflog_message(logmsg);
 
-	/*
-	 * Since we don't check the references' old_oids, the
-	 * individual updates can't fail, so we can pack all of the
-	 * updates into a single transaction.
-	 */
-	transaction = ref_store_transaction_begin(refs, 0, &err);
+	transaction = ref_store_transaction_begin(refs,
+			old_oids ? REF_TRANSACTION_ALLOW_FAILURE : 0, &err);
 	if (!transaction) {
 		ret = error("%s", err.buf);
 		goto out;
 	}
 
-	for_each_string_list_item(item, refnames) {
+	for (i = 0; i < refnames->nr; i++) {
+		struct string_list_item *item = &refnames->items[i];
+		const struct object_id *old_oid = old_oids ? &old_oids->oid[i] : NULL;
+
+		if (old_oid && is_null_oid(old_oid))
+			old_oid = NULL;
 		ret = ref_transaction_delete(transaction, item->string,
-					     NULL, NULL, flags, msg, &err);
+					     old_oid, NULL, flags, msg, &err);
 		if (ret) {
 			warning(_("could not delete reference %s: %s"),
 				item->string, err.buf);
@@ -3112,9 +3139,14 @@ int refs_delete_refs(struct ref_store *refs, const char *logmsg,
 			      refnames->items[0].string, err.buf);
 		else
 			error(_("could not delete references: %s"), err.buf);
-	}
+	} else if (old_oids)
+		ref_transaction_for_each_rejected_update(transaction,
+						 delete_refs_rejection_handler,
+						 &rejection_data);
 
 out:
+	if (rejection_data.failures)
+		failures = 1;
 	if (!ret && failures)
 		ret = -1;
 	ref_transaction_free(transaction);

refs.h

@@ -9,6 +9,7 @@
 struct fsck_options;
 struct object_id;
 struct ref_store;
+struct oid_array;
 struct strbuf;
 struct string_list;
 struct string_list_item;
@@ -623,13 +624,21 @@ int refs_delete_ref(struct ref_store *refs, const char *msg,
 		    unsigned int flags);
 
 /*
- * Delete the specified references. If there are any problems, emit
+ * Delete the specified references. If old_oids is non-NULL, it must contain
+ * an entry for each refname, in the same order. Each non-null OID is used to
+ * verify the current value of the corresponding reference before deleting
+ * it. A null OID requests an unconditional deletion, which allows callers to
+ * include broken refs whose old value cannot be resolved.
+ *
+ * If there are any problems, emit
  * errors but attempt to keep going (i.e., the deletes are not done in
  * an all-or-nothing transaction). msg and flags are passed through to
  * ref_transaction_delete().
  */
 int refs_delete_refs(struct ref_store *refs, const char *msg,
-		     struct string_list *refnames, unsigned int flags);
+		     struct string_list *refnames,
+		     const struct oid_array *old_oids,
+		     unsigned int flags);
 
 /** Delete a reflog */
 int refs_delete_reflog(struct ref_store *refs, const char *refname);

t/helper/test-ref-store.c

@@ -132,7 +132,7 @@ static int cmd_delete_refs(struct ref_store *refs, const char **argv)
 	while (*argv)
 		string_list_append(&refnames, *argv++);
 
-	result = refs_delete_refs(refs, msg, &refnames, flags);
+	result = refs_delete_refs(refs, msg, &refnames, NULL, flags);
 	string_list_clear(&refnames, 0);
 	return result;
 }