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[PATCH v5 6/8] refs: implement batch reference update support

From
Karthik Nayak <karthik.188@gmail.com>
Date
Mar 27, 2025, 11:13 UTC
Message-ID
<20250327-245-partially-atomic-ref-updates-v5-6-4db2a3e34404@gmail.com>
In-Reply-To
<20250327-245-partially-atomic-ref-updates-v5-0-4db2a3e34404@gmail.com>

Git supports making reference updates with or without transactions. Updates with transactions are generally better optimized. But transactions are all or nothing. This means, if a user wants to batch updates to take advantage of the optimizations without the hard requirement that all updates must succeed, there is no way currently to do so. Particularly with the reftable backend where batching multiple reference updates is more efficient than performing them sequentially.

Introduce batched update support with a new flag, 'REF_TRANSACTION_ALLOW_FAILURE'. Batched updates while different from transactions, use the transaction infrastructure under the hood. When enabled, this flag allows individual reference updates that would typically cause the entire transaction to fail due to non-system-related errors to be marked as rejected while permitting other updates to proceed. System errors referred by 'REF_TRANSACTION_ERROR_GENERIC' continue to result in the entire transaction failing. This approach enhances flexibility while preserving transactional integrity where necessary.

The implementation introduces several key components:
  - Add 'rejection_err' field to struct `ref_update` to track failed
    updates with failure reason.
  - Add a new struct `ref_transaction_rejections` and a field within
    `ref_transaction` to this struct to allow quick iteration over
    rejected updates.
  - Modify reference backends (files, packed, reftable) to handle
    partial transactions by using `ref_transaction_set_rejected()`
    instead of failing the entire transaction when
    `REF_TRANSACTION_ALLOW_FAILURE` is set.
  - Add `ref_transaction_for_each_rejected_update()` to let callers
    examine which updates were rejected and why.

This foundational change enables batched update support throughout the reference subsystem. A following commit will expose this capability to users by adding a `--batch-updates` flag to 'git-update-ref(1)', providing both a user-facing feature and a testable implementation.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
---
 refs.c                  | 61 +++++++++++++++++++++++++++++++++++++++++++++++++
 refs.h                  | 22 ++++++++++++++++++
 refs/files-backend.c    | 12 +++++++++-
 refs/packed-backend.c   | 27 ++++++++++++++++++++--
 refs/refs-internal.h    | 26 +++++++++++++++++++++
 refs/reftable-backend.c | 12 +++++++++-
 6 files changed, 156 insertions(+), 4 deletions(-)
diff --git a/refs.c b/refs.c
index 3d0b53d56e..b34ec198f5 100644
--- a/refs.c
+++ b/refs.c
@@ -1176,6 +1176,10 @@ struct ref_transaction *ref_store_transaction_begin(struct ref_store *refs,
 	tr->ref_store = refs;
 	tr->flags = flags;
 	string_list_init_dup(&tr->refnames);
+
+	if (flags & REF_TRANSACTION_ALLOW_FAILURE)
+		CALLOC_ARRAY(tr->rejections, 1);
+
 	return tr;
 }
 
@@ -1206,11 +1210,45 @@ void ref_transaction_free(struct ref_transaction *transaction)
 		free((char *)transaction->updates[i]->old_target);
 		free(transaction->updates[i]);
 	}
+
+	if (transaction->rejections)
+		free(transaction->rejections->update_indices);
+	free(transaction->rejections);
+
 	string_list_clear(&transaction->refnames, 0);
 	free(transaction->updates);
 	free(transaction);
 }
 
+int ref_transaction_maybe_set_rejected(struct ref_transaction *transaction,
+				       size_t update_idx,
+				       enum ref_transaction_error err)
+{
+	if (update_idx >= transaction->nr)
+		BUG("trying to set rejection on invalid update index");
+
+	if (!(transaction->flags & REF_TRANSACTION_ALLOW_FAILURE))
+		return 0;
+
+	if (!transaction->rejections)
+		BUG("transaction not inititalized with failure support");
+
+	/*
+	 * Don't accept generic errors, since these errors are not user
+	 * input related.
+	 */
+	if (err == REF_TRANSACTION_ERROR_GENERIC)
+		return 0;
+
+	transaction->updates[update_idx]->rejection_err = err;
+	ALLOC_GROW(transaction->rejections->update_indices,
+		   transaction->rejections->nr + 1,
+		   transaction->rejections->alloc);
+	transaction->rejections->update_indices[transaction->rejections->nr++] = update_idx;
+
+	return 1;
+}
+
 struct ref_update *ref_transaction_add_update(
 		struct ref_transaction *transaction,
 		const char *refname, unsigned int flags,
@@ -1236,6 +1274,7 @@ struct ref_update *ref_transaction_add_update(
 	transaction->updates[transaction->nr++] = update;
 
 	update->flags = flags;
+	update->rejection_err = 0;
 
 	update->new_target = xstrdup_or_null(new_target);
 	update->old_target = xstrdup_or_null(old_target);
@@ -2728,6 +2767,28 @@ void ref_transaction_for_each_queued_update(struct ref_transaction *transaction,
 	}
 }
 
+void ref_transaction_for_each_rejected_update(struct ref_transaction *transaction,
+					      ref_transaction_for_each_rejected_update_fn cb,
+					      void *cb_data)
+{
+	if (!transaction->rejections)
+		return;
+
+	for (size_t i = 0; i < transaction->rejections->nr; i++) {
+		size_t update_index = transaction->rejections->update_indices[i];
+		struct ref_update *update = transaction->updates[update_index];
+
+		if (!update->rejection_err)
+			continue;
+
+		cb(update->refname,
+		   (update->flags & REF_HAVE_OLD) ? &update->old_oid : NULL,
+		   (update->flags & REF_HAVE_NEW) ? &update->new_oid : NULL,
+		   update->old_target, update->new_target,
+		   update->rejection_err, cb_data);
+	}
+}
+
 int refs_delete_refs(struct ref_store *refs, const char *logmsg,
 		     struct string_list *refnames, unsigned int flags)
 {
diff --git a/refs.h b/refs.h
index f009cdae7d..c48c800478 100644
--- a/refs.h
+++ b/refs.h
@@ -667,6 +667,13 @@ enum ref_transaction_flag {
 	 * either be absent or null_oid.
 	 */
 	REF_TRANSACTION_FLAG_INITIAL = (1 << 0),
+
+	/*
+	 * The transaction mechanism by default fails all updates if any conflict
+	 * is detected. This flag allows transactions to partially apply updates
+	 * while rejecting updates which do not match the expected state.
+	 */
+	REF_TRANSACTION_ALLOW_FAILURE = (1 << 1),
 };
 
 /*
@@ -897,6 +904,21 @@ void ref_transaction_for_each_queued_update(struct ref_transaction *transaction,
 					    ref_transaction_for_each_queued_update_fn cb,
 					    void *cb_data);
 
+/*
+ * Execute the given callback function for each of the reference updates which
+ * have been rejected in the given transaction.
+ */
+typedef void ref_transaction_for_each_rejected_update_fn(const char *refname,
+							 const struct object_id *old_oid,
+							 const struct object_id *new_oid,
+							 const char *old_target,
+							 const char *new_target,
+							 enum ref_transaction_error err,
+							 void *cb_data);
+void ref_transaction_for_each_rejected_update(struct ref_transaction *transaction,
+					      ref_transaction_for_each_rejected_update_fn cb,
+					      void *cb_data);
+
 /*
  * Free `*transaction` and all associated data.
  */
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 4f27f7652c..256c69b942 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -2852,8 +2852,15 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 		ret = lock_ref_for_update(refs, update, transaction,
 					  head_ref, &refnames_to_check,
 					  err);
-		if (ret)
+		if (ret) {
+			if (ref_transaction_maybe_set_rejected(transaction, i, ret)) {
+				strbuf_reset(err);
+				ret = 0;
+
+				continue;
+			}
 			goto cleanup;
+		}
 
 		if (update->flags & REF_DELETING &&
 		    !(update->flags & REF_LOG_ONLY) &&
@@ -3151,6 +3158,9 @@ static int files_transaction_finish(struct ref_store *ref_store,
 		struct ref_update *update = transaction->updates[i];
 		struct ref_lock *lock = update->backend_data;
 
+		if (update->rejection_err)
+			continue;
+
 		if (update->flags & REF_NEEDS_COMMIT ||
 		    update->flags & REF_LOG_ONLY) {
 			if (parse_and_write_reflog(refs, update, lock, err)) {
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index d90bd815a3..debca86a2b 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1327,10 +1327,11 @@ static int packed_ref_store_remove_on_disk(struct ref_store *ref_store,
  * remain locked when it is done.
  */
 static enum ref_transaction_error write_with_updates(struct packed_ref_store *refs,
-						     struct string_list *updates,
+						     struct ref_transaction *transaction,
 						     struct strbuf *err)
 {
 	enum ref_transaction_error ret = REF_TRANSACTION_ERROR_GENERIC;
+	struct string_list *updates = &transaction->refnames;
 	struct ref_iterator *iter = NULL;
 	size_t i;
 	int ok;
@@ -1411,6 +1412,13 @@ static enum ref_transaction_error write_with_updates(struct packed_ref_store *re
 						    "reference already exists",
 						    update->refname);
 					ret = REF_TRANSACTION_ERROR_CREATE_EXISTS;
+
+					if (ref_transaction_maybe_set_rejected(transaction, i, ret)) {
+						strbuf_reset(err);
+						ret = 0;
+						continue;
+					}
+
 					goto error;
 				} else if (!oideq(&update->old_oid, iter->oid)) {
 					strbuf_addf(err, "cannot update ref '%s': "
@@ -1419,6 +1427,13 @@ static enum ref_transaction_error write_with_updates(struct packed_ref_store *re
 						    oid_to_hex(iter->oid),
 						    oid_to_hex(&update->old_oid));
 					ret = REF_TRANSACTION_ERROR_INCORRECT_OLD_VALUE;
+
+					if (ref_transaction_maybe_set_rejected(transaction, i, ret)) {
+						strbuf_reset(err);
+						ret = 0;
+						continue;
+					}
+
 					goto error;
 				}
 			}
@@ -1456,6 +1471,13 @@ static enum ref_transaction_error write_with_updates(struct packed_ref_store *re
 					    update->refname,
 					    oid_to_hex(&update->old_oid));
 				ret = REF_TRANSACTION_ERROR_NONEXISTENT_REF;
+
+				if (ref_transaction_maybe_set_rejected(transaction, i, ret)) {
+					strbuf_reset(err);
+					ret = 0;
+					continue;
+				}
+
 				goto error;
 			}
 		}
@@ -1521,6 +1543,7 @@ static enum ref_transaction_error write_with_updates(struct packed_ref_store *re
 write_error:
 	strbuf_addf(err, "error writing to %s: %s",
 		    get_tempfile_path(refs->tempfile), strerror(errno));
+	ret = REF_TRANSACTION_ERROR_GENERIC;
 
 error:
 	ref_iterator_free(iter);
@@ -1679,7 +1702,7 @@ static int packed_transaction_prepare(struct ref_store *ref_store,
 		data->own_lock = 1;
 	}
 
-	ret = write_with_updates(refs, &transaction->refnames, err);
+	ret = write_with_updates(refs, transaction, err);
 	if (ret)
 		goto failure;
 
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 3f1d19abd9..73a5379b73 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -123,6 +123,12 @@ struct ref_update {
 	 */
 	uint64_t index;
 
+	/*
+	 * Used in batched reference updates to mark if a given update
+	 * was rejected.
+	 */
+	enum ref_transaction_error rejection_err;
+
 	/*
 	 * If this ref_update was split off of a symref update via
 	 * split_symref_update(), then this member points at that
@@ -142,6 +148,13 @@ int refs_read_raw_ref(struct ref_store *ref_store, const char *refname,
 		      struct object_id *oid, struct strbuf *referent,
 		      unsigned int *type, int *failure_errno);
 
+/*
+ * Mark a given update as rejected with a given reason.
+ */
+int ref_transaction_maybe_set_rejected(struct ref_transaction *transaction,
+				       size_t update_idx,
+				       enum ref_transaction_error err);
+
 /*
  * Add a ref_update with the specified properties to transaction, and
  * return a pointer to the new object. This function does not verify
@@ -183,6 +196,18 @@ enum ref_transaction_state {
 	REF_TRANSACTION_CLOSED   = 2
 };
 
+/*
+ * Data structure to hold indices of updates which were rejected, for batched
+ * reference updates. While the updates themselves hold the rejection error,
+ * this structure allows a transaction to iterate only over the rejected
+ * updates.
+ */
+struct ref_transaction_rejections {
+	size_t *update_indices;
+	size_t alloc;
+	size_t nr;
+};
+
 /*
  * Data structure for holding a reference transaction, which can
  * consist of checks and updates to multiple references, carried out
@@ -195,6 +220,7 @@ struct ref_transaction {
 	size_t alloc;
 	size_t nr;
 	enum ref_transaction_state state;
+	struct ref_transaction_rejections *rejections;
 	void *backend_data;
 	unsigned int flags;
 	uint64_t max_index;
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index bd6b042103..5db4a108b9 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -1371,8 +1371,15 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 					    transaction->updates[i],
 					    &refnames_to_check, head_type,
 					    &head_referent, &referent, err);
-		if (ret)
+		if (ret) {
+			if (ref_transaction_maybe_set_rejected(transaction, i, ret)) {
+				strbuf_reset(err);
+				ret = 0;
+
+				continue;
+			}
 			goto done;
+		}
 	}
 
 	ret = refs_verify_refnames_available(ref_store, &refnames_to_check,
@@ -1454,6 +1461,9 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 		struct reftable_transaction_update *tx_update = &arg->updates[i];
 		struct ref_update *u = tx_update->update;
 
+		if (u->rejection_err)
+			continue;
+
 		/*
 		 * Write a reflog entry when updating a ref to point to
 		 * something new in either of the following cases:

-- 
2.48.1
Previous: Karthik NayakNext: Karthik Nayak
Message 30 of 49 in “refs: introduce support for partial reference transactions”
  1. 0/6 refs: introduce support for partial reference transactionsKarthik Nayak, Feb 7, 2025
  2. 1/6 refs/files: remove duplicate check in `split_symref_update()`Karthik Nayak, Feb 7, 2025
  3. Patrick SteinhardtFeb 7, 2025
  4. Karthik NayakFeb 11, 2025
  5. 2/6 refs: move duplicate refname update check to generic layerKarthik Nayak, Feb 7, 2025
  6. Patrick SteinhardtFeb 7, 2025
  7. Karthik NayakFeb 11, 2025
  8. 3/6 refs/files: remove duplicate duplicates checkKarthik Nayak, Feb 7, 2025
  9. Patrick SteinhardtFeb 7, 2025
  10. 4/6 refs/reftable: extract code from the transaction preparationKarthik Nayak, Feb 7, 2025
  11. 5/6 refs: implement partial reference transaction supportKarthik Nayak, Feb 7, 2025
  12. Patrick SteinhardtFeb 7, 2025
  13. Karthik NayakFeb 21, 2025
  14. 6/6 update-ref: add --allow-partial flag for stdin modeKarthik Nayak, Feb 7, 2025
  15. Patrick SteinhardtFeb 7, 2025
  16. Karthik NayakFeb 21, 2025
  17. Phillip WoodFeb 11, 2025
  18. Phillip WoodFeb 11, 2025
  19. Karthik NayakFeb 12, 2025
  20. Karthik NayakFeb 12, 2025
  21. Phillip WoodFeb 19, 2025
  22. Patrick SteinhardtFeb 19, 2025
  23. Karthik NayakFeb 21, 2025
  24. 0/8 refs: introduce support for batched reference updatesKarthik Nayak, Mar 27, 2025
  25. 1/8 refs/files: remove redundant check in split_symref_update()Karthik Nayak, Mar 27, 2025
  26. 2/8 refs: move duplicate refname update check to generic layerKarthik Nayak, Mar 27, 2025
  27. 3/8 refs/files: remove duplicate duplicates checkKarthik Nayak, Mar 27, 2025
  28. 4/8 refs/reftable: extract code from the transaction preparationKarthik Nayak, Mar 27, 2025
  29. 5/8 refs: introduce enum-based transaction error typesKarthik Nayak, Mar 27, 2025
  30. 6/8 refs: implement batch reference update supportKarthik Nayak, Mar 27, 2025
  31. 7/8 refs: support rejection in batch updates during F/D checksKarthik Nayak, Mar 27, 2025
  32. 8/8 update-ref: add --batch-updates flag for stdin modeKarthik Nayak, Mar 27, 2025
  33. Jean-Noël AVILAMar 28, 2025
  34. Junio C HamanoMar 29, 2025
  35. Karthik NayakMar 29, 2025
  36. Patrick SteinhardtMar 28, 2025
  37. 0/8 refs: introduce support for batched reference updatesKarthik Nayak, Apr 8, 2025
  38. 1/8 refs/files: remove redundant check in split_symref_update()Karthik Nayak, Apr 8, 2025
  39. 2/8 refs: move duplicate refname update check to generic layerKarthik Nayak, Apr 8, 2025
  40. 3/8 refs/files: remove duplicate duplicates checkKarthik Nayak, Apr 8, 2025
  41. 4/8 refs/reftable: extract code from the transaction preparationKarthik Nayak, Apr 8, 2025
  42. 5/8 refs: introduce enum-based transaction error typesKarthik Nayak, Apr 8, 2025
  43. 6/8 refs: implement batch reference update supportKarthik Nayak, Apr 8, 2025
  44. 7/8 refs: support rejection in batch updates during F/D checksKarthik Nayak, Apr 8, 2025
  45. 8/8 update-ref: add --batch-updates flag for stdin modeKarthik Nayak, Apr 8, 2025
  46. Junio C HamanoApr 8, 2025
  47. Karthik NayakApr 8, 2025
  48. Junio C HamanoApr 8, 2025
  49. Karthik NayakApr 10, 2025

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