refs: support rejection in batch updates during F/D checks

The `refs_verify_refnames_available()` is used to batch check refnames
for F/D conflicts. While this is the more performant alternative than
its individual version, it does not provide rejection capabilities on a
single update level. For batched updates, this would mean a rejection of
the entire transaction whenever one reference has a F/D conflict.

Modify the function to call `ref_transaction_maybe_set_rejected()` to
check if a single update can be rejected. Since this function is only
internally used within 'refs/' and we want to pass in a `struct
ref_transaction *` as a variable. We also move and mark
`refs_verify_refnames_available()` to 'refs-internal.h' to be an
internal function.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 42 insertions, 12 deletions

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 9620dd86fb..8b20e40401 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -677,16 +677,18 @@ static void unlock_ref(struct ref_lock *lock)
  * - Generate informative error messages in the case of failure
  */
 static enum ref_transaction_error lock_raw_ref(struct files_ref_store *refs,
-					       const char *refname,
+					       struct ref_update *update,
+					       size_t update_idx,
 					       int mustexist,
 					       struct string_list *refnames_to_check,
 					       const struct string_list *extras,
 					       struct ref_lock **lock_p,
 					       struct strbuf *referent,
-					       unsigned int *type,
 					       struct strbuf *err)
 {
 	enum ref_transaction_error ret = REF_TRANSACTION_ERROR_GENERIC;
+	const char *refname = update->refname;
+	unsigned int *type = &update->type;
 	struct ref_lock *lock;
 	struct strbuf ref_file = STRBUF_INIT;
 	int attempts_remaining = 3;
@@ -785,6 +787,8 @@ retry:
 
 	if (files_read_raw_ref(&refs->base, refname, &lock->old_oid, referent,
 			       type, &failure_errno)) {
+		struct string_list_item *item;
+
 		if (failure_errno == ENOENT) {
 			if (mustexist) {
 				/* Garden variety missing reference. */
@@ -864,7 +868,9 @@ retry:
 		 * make sure there is no existing packed ref that conflicts
 		 * with refname. This check is deferred so that we can batch it.
 		 */
-		string_list_append(refnames_to_check, refname);
+		item = string_list_append(refnames_to_check, refname);
+		item->util = xmalloc(sizeof(update_idx));
+		memcpy(item->util, &update_idx, sizeof(update_idx));
 	}
 
 	ret = 0;
@@ -2547,6 +2553,7 @@ struct files_transaction_backend_data {
  */
 static enum ref_transaction_error lock_ref_for_update(struct files_ref_store *refs,
 						      struct ref_update *update,
+						      size_t update_idx,
 						      struct ref_transaction *transaction,
 						      const char *head_ref,
 						      struct string_list *refnames_to_check,
@@ -2575,9 +2582,9 @@ static enum ref_transaction_error lock_ref_for_update(struct files_ref_store *re
 	if (lock) {
 		lock->count++;
 	} else {
-		ret = lock_raw_ref(refs, update->refname, mustexist,
+		ret = lock_raw_ref(refs, update, update_idx, mustexist,
 				   refnames_to_check, &transaction->refnames,
-				   &lock, &referent, &update->type, err);
+				   &lock, &referent, err);
 		if (ret) {
 			char *reason;
 
@@ -2849,7 +2856,7 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 	for (i = 0; i < transaction->nr; i++) {
 		struct ref_update *update = transaction->updates[i];
 
-		ret = lock_ref_for_update(refs, update, transaction,
+		ret = lock_ref_for_update(refs, update, i, transaction,
 					  head_ref, &refnames_to_check,
 					  err);
 		if (ret) {
@@ -2905,7 +2912,8 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 	 * So instead, we accept the race for now.
 	 */
 	if (refs_verify_refnames_available(refs->packed_ref_store, &refnames_to_check,
-					   &transaction->refnames, NULL, 0, err)) {
+					   &transaction->refnames, NULL, transaction,
+					   0, err)) {
 		ret = REF_TRANSACTION_ERROR_NAME_CONFLICT;
 		goto cleanup;
 	}
@@ -2951,7 +2959,7 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 
 cleanup:
 	free(head_ref);
-	string_list_clear(&refnames_to_check, 0);
+	string_list_clear(&refnames_to_check, 1);
 
 	if (ret)
 		files_transaction_cleanup(refs, transaction);
@@ -3097,7 +3105,8 @@ static int files_transaction_finish_initial(struct files_ref_store *refs,
 	}
 
 	if (refs_verify_refnames_available(&refs->base, &refnames_to_check,
-					   &affected_refnames, NULL, 1, err)) {
+					   &affected_refnames, NULL, transaction,
+					   1, err)) {
 		packed_refs_unlock(refs->packed_ref_store);
 		ret = REF_TRANSACTION_ERROR_NAME_CONFLICT;
 		goto cleanup;
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 73a5379b73..f868870851 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -806,4 +806,20 @@ enum ref_transaction_error ref_update_check_old_target(const char *referent,
  */
 int ref_update_expects_existing_old_ref(struct ref_update *update);
 
+/*
+ * Same as `refs_verify_refname_available()`, but checking for a list of
+ * refnames instead of only a single item. This is more efficient in the case
+ * where one needs to check multiple refnames.
+ *
+ * If using batched updates, then individual updates are marked rejected,
+ * reference backends are then in charge of not committing those updates.
+ */
+enum ref_transaction_error refs_verify_refnames_available(struct ref_store *refs,
+					  const struct string_list *refnames,
+					  const struct string_list *extras,
+					  const struct string_list *skip,
+					  struct ref_transaction *transaction,
+					  unsigned int initial_transaction,
+					  struct strbuf *err);
+
 #endif /* REFS_REFS_INTERNAL_H */
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index 8fb7d6cc71..a461d1b8e0 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -1074,6 +1074,7 @@ static enum ref_transaction_error prepare_single_update(struct reftable_ref_stor
 							struct ref_transaction *transaction,
 							struct reftable_backend *be,
 							struct ref_update *u,
+							size_t update_idx,
 							struct string_list *refnames_to_check,
 							unsigned int head_type,
 							struct strbuf *head_referent,
@@ -1149,6 +1150,7 @@ static enum ref_transaction_error prepare_single_update(struct reftable_ref_stor
 	if (ret < 0)
 		return REF_TRANSACTION_ERROR_GENERIC;
 	if (ret > 0 && !ref_update_expects_existing_old_ref(u)) {
+		struct string_list_item *item;
 		/*
 		 * The reference does not exist, and we either have no
 		 * old object ID or expect the reference to not exist.
@@ -1158,7 +1160,9 @@ static enum ref_transaction_error prepare_single_update(struct reftable_ref_stor
 		 * can output a proper error message instead of failing
 		 * at a later point.
 		 */
-		string_list_append(refnames_to_check, u->refname);
+		item = string_list_append(refnames_to_check, u->refname);
+		item->util = xmalloc(sizeof(update_idx));
+		memcpy(item->util, &update_idx, sizeof(update_idx));
 
 		/*
 		 * There is no need to write the reference deletion
@@ -1368,7 +1372,7 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 
 	for (i = 0; i < transaction->nr; i++) {
 		ret = prepare_single_update(refs, tx_data, transaction, be,
-					    transaction->updates[i],
+					    transaction->updates[i], i,
 					    &refnames_to_check, head_type,
 					    &head_referent, &referent, err);
 		if (ret) {
@@ -1384,6 +1388,7 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 
 	ret = refs_verify_refnames_available(ref_store, &refnames_to_check,
 					     &transaction->refnames, NULL,
+					     transaction,
 					     transaction->flags & REF_TRANSACTION_FLAG_INITIAL,
 					     err);
 	if (ret < 0)
@@ -1402,7 +1407,7 @@ done:
 	}
 	strbuf_release(&referent);
 	strbuf_release(&head_referent);
-	string_list_clear(&refnames_to_check, 0);
+	string_list_clear(&refnames_to_check, 1);
 
 	return ret;
 }