[PATCH v6 4/7] refs: add support for transactional symref updates

From: Karthik Nayak <karthik.188@gmail.com>
Date: May 3, 2024, 12:41 UTC
Message-ID: <20240503124115.252413-5-knayak@gitlab.com>
In-Reply-To: <20240503124115.252413-1-knayak@gitlab.com>

From: Karthik Nayak <karthik.188@gmail.com>

The reference backends currently support transactional reference updates. While this is exposed to users via 'git-update-ref' and its '--stdin' mode, it is also used internally within various commands.

However, we never supported transactional updates of symrefs. Let's add support for symrefs in both the 'files' and the 'reftable' backend.

Here, we add and use `ref_update_has_null_new_value()`, a helper function which is used to check if there is a new_value in a reference update. The new value could either be a symref target `new_target` or a OID `new_oid`.

With this, now transactional updates (verify, create, delete, update)
can be used for:
- regular refs
- symbolic refs
- conversion of regular to symbolic refs and vice versa

This also allows us to expose this to users via new commands in 'git-update-ref' in the future.

Note that a dangling symref update does not record a new reflog entry, which is unchanged before and after this commit.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
---
 refs.c                  |  15 ++++-
 refs/files-backend.c    | 137 ++++++++++++++++++++++++++++++++++------
 refs/refs-internal.h    |   7 ++
 refs/reftable-backend.c |  77 +++++++++++++++++-----
 4 files changed, 198 insertions(+), 38 deletions(-)

diff --git a/refs.c b/refs.c
index 203a101988..acb153b6f8 100644
--- a/refs.c
+++ b/refs.c
@@ -1217,6 +1217,8 @@ void ref_transaction_free(struct ref_transaction *transaction)
 
 	for (i = 0; i < transaction->nr; i++) {
 		free(transaction->updates[i]->msg);
+		free((char *)transaction->updates[i]->new_target);
+		free((char *)transaction->updates[i]->old_target);
 		free(transaction->updates[i]);
 	}
 	free(transaction->updates);
@@ -1247,10 +1249,13 @@ struct ref_update *ref_transaction_add_update(
 
 	update->flags = flags;
 
-	if (flags & REF_HAVE_NEW)
+	update->new_target = xstrdup_or_null(new_target);
+	update->old_target = xstrdup_or_null(old_target);
+	if ((flags & REF_HAVE_NEW) && new_oid)
 		oidcpy(&update->new_oid, new_oid);
-	if (flags & REF_HAVE_OLD)
+	if ((flags & REF_HAVE_OLD) && old_oid)
 		oidcpy(&update->old_oid, old_oid);
+
 	update->msg = normalize_reflog_message(msg);
 	return update;
 }
@@ -1286,6 +1291,7 @@ int ref_transaction_update(struct ref_transaction *transaction,
 	flags &= REF_TRANSACTION_UPDATE_ALLOWED_FLAGS;
 
 	flags |= (new_oid ? REF_HAVE_NEW : 0) | (old_oid ? REF_HAVE_OLD : 0);
+	flags |= (new_target ? REF_HAVE_NEW : 0) | (old_target ? REF_HAVE_OLD : 0);
 
 	ref_transaction_add_update(transaction, refname, flags,
 				   new_oid, old_oid, new_target,
@@ -2814,3 +2820,8 @@ int copy_existing_ref(const char *oldref, const char *newref, const char *logmsg
 {
 	return refs_copy_existing_ref(get_main_ref_store(the_repository), oldref, newref, logmsg);
 }
+
+int ref_update_has_null_new_value(struct ref_update *update)
+{
+	return !update->new_target && is_null_oid(&update->new_oid);
+}
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 74a713090c..e1f0ca74c0 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -2434,6 +2434,36 @@ static const char *original_update_refname(struct ref_update *update)
 	return update->refname;
 }
 
+/*
+ * Check whether the old_target values stored in update are consistent
+ * with current_target, which is the symbolic reference's current value.
+ * If everything is OK, return 0; otherwise, write an error message to
+ * err and return -1.
+ */
+static int check_old_target(struct ref_update *update,
+			    const char *current_target,
+			    struct strbuf *err)
+{
+	if (!update->old_target)
+		BUG("called without old_target set");
+
+	if (!strcmp(update->old_target, current_target))
+		return 0;
+
+	if (!strcmp(current_target, ""))
+		strbuf_addf(err, "cannot lock ref '%s': "
+			    "reference is missing but expected %s",
+			    original_update_refname(update),
+			    update->old_target);
+	else
+		strbuf_addf(err, "cannot lock ref '%s': "
+			    "is at %s but expected %s",
+			    original_update_refname(update),
+			    current_target, update->old_target);
+
+	return -1;
+}
+
 /*
  * Check whether the REF_HAVE_OLD and old_oid values stored in update
  * are consistent with oid, which is the reference's current value. If
@@ -2494,7 +2524,7 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 
 	files_assert_main_repository(refs, "lock_ref_for_update");
 
-	if ((update->flags & REF_HAVE_NEW) && is_null_oid(&update->new_oid))
+	if ((update->flags & REF_HAVE_NEW) && ref_update_has_null_new_value(update))
 		update->flags |= REF_DELETING;
 
 	if (head_ref) {
@@ -2537,7 +2567,14 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 					ret = TRANSACTION_GENERIC_ERROR;
 					goto out;
 				}
-			} else if (check_old_oid(update, &lock->old_oid, err)) {
+			}
+
+			if (update->old_target) {
+				if (check_old_target(update, referent.buf, err)) {
+					ret = TRANSACTION_GENERIC_ERROR;
+					goto out;
+				}
+			} else if  (check_old_oid(update, &lock->old_oid, err)) {
 				ret = TRANSACTION_GENERIC_ERROR;
 				goto out;
 			}
@@ -2558,7 +2595,17 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 	} else {
 		struct ref_update *parent_update;
 
-		if (check_old_oid(update, &lock->old_oid, err)) {
+		/*
+		 * Even if the ref is a regular ref, if `old_target` is set, we
+		 * check the referent value. Ideally `old_target` should only
+		 * be set for symrefs, but we're strict about its usage.
+		 */
+		if (update->old_target) {
+			if (check_old_target(update, referent.buf, err)) {
+				ret = TRANSACTION_GENERIC_ERROR;
+				goto out;
+			}
+		} else if  (check_old_oid(update, &lock->old_oid, err)) {
 			ret = TRANSACTION_GENERIC_ERROR;
 			goto out;
 		}
@@ -2576,9 +2623,27 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 		}
 	}
 
-	if ((update->flags & REF_HAVE_NEW) &&
-	    !(update->flags & REF_DELETING) &&
-	    !(update->flags & REF_LOG_ONLY)) {
+	if (update->new_target && !(update->flags & REF_LOG_ONLY)) {
+		if (create_symref_lock(refs, lock, update->refname, update->new_target, err)) {
+			ret = TRANSACTION_GENERIC_ERROR;
+			goto out;
+		}
+
+		if (close_ref_gently(lock)) {
+			strbuf_addf(err, "couldn't close '%s.lock'",
+				    update->refname);
+			ret = TRANSACTION_GENERIC_ERROR;
+			goto out;
+		}
+
+		/*
+		 * Once we have created the symref lock, the commit
+		 * phase of the transaction only needs to commit the lock.
+		 */
+		update->flags |= REF_NEEDS_COMMIT;
+	} else if ((update->flags & REF_HAVE_NEW) &&
+		   !(update->flags & REF_DELETING) &&
+		   !(update->flags & REF_LOG_ONLY)) {
 		if (!(update->type & REF_ISSYMREF) &&
 		    oideq(&lock->old_oid, &update->new_oid)) {
 			/*
@@ -2841,6 +2906,43 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 	return ret;
 }
 
+static int parse_and_write_reflog(struct files_ref_store *refs,
+				  struct ref_update *update,
+				  struct ref_lock *lock,
+				  struct strbuf *err)
+{
+	if (update->new_target) {
+		/*
+		 * We want to get the resolved OID for the target, to ensure
+		 * that the correct value is added to the reflog.
+		 */
+		if (!refs_resolve_ref_unsafe(&refs->base, update->new_target,
+					     RESOLVE_REF_READING,
+					     &update->new_oid, NULL)) {
+			/*
+			 * TODO: currently we skip creating reflogs for dangling
+			 * symref updates. It would be nice to capture this as
+			 * zero oid updates however.
+			 */
+			return 0;
+		}
+	}
+
+	if (files_log_ref_write(refs, lock->ref_name, &lock->old_oid,
+				&update->new_oid, update->msg, update->flags, err)) {
+		char *old_msg = strbuf_detach(err, NULL);
+
+		strbuf_addf(err, "cannot update the ref '%s': %s",
+			    lock->ref_name, old_msg);
+		free(old_msg);
+		unlock_ref(lock);
+		update->backend_data = NULL;
+		return -1;
+	}
+
+	return 0;
+}
+
 static int files_transaction_finish(struct ref_store *ref_store,
 				    struct ref_transaction *transaction,
 				    struct strbuf *err)
@@ -2871,23 +2973,20 @@ static int files_transaction_finish(struct ref_store *ref_store,
 
 		if (update->flags & REF_NEEDS_COMMIT ||
 		    update->flags & REF_LOG_ONLY) {
-			if (files_log_ref_write(refs,
-						lock->ref_name,
-						&lock->old_oid,
-						&update->new_oid,
-						update->msg, update->flags,
-						err)) {
-				char *old_msg = strbuf_detach(err, NULL);
-
-				strbuf_addf(err, "cannot update the ref '%s': %s",
-					    lock->ref_name, old_msg);
-				free(old_msg);
-				unlock_ref(lock);
-				update->backend_data = NULL;
+			if (parse_and_write_reflog(refs, update, lock, err)) {
 				ret = TRANSACTION_GENERIC_ERROR;
 				goto cleanup;
 			}
 		}
+
+		/*
+		 * We try creating a symlink, if that succeeds we continue to the
+		 * next update. If not, we try and create a regular symref.
+		 */
+		if (update->new_target && prefer_symlink_refs)
+			if (!create_ref_symlink(lock, update->new_target))
+				continue;
+
 		if (update->flags & REF_NEEDS_COMMIT) {
 			clear_loose_ref_cache(refs);
 			if (commit_ref(lock)) {
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 108f4ec419..03eda70ea4 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -749,4 +749,11 @@ void base_ref_store_init(struct ref_store *refs, struct repository *repo,
  */
 struct ref_store *maybe_debug_wrap_ref_store(const char *gitdir, struct ref_store *store);
 
+/*
+ * Helper function to check if the new value is null, this
+ * takes into consideration that the update could be a regular
+ * ref or a symbolic ref.
+ */
+int ref_update_has_null_new_value(struct ref_update *update);
+
 #endif /* REFS_REFS_INTERNAL_H */
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index 6104471199..54c4d8b771 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -856,7 +856,7 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 			 * There is no need to write the reference deletion
 			 * when the reference in question doesn't exist.
 			 */
-			 if (u->flags & REF_HAVE_NEW && !is_null_oid(&u->new_oid)) {
+			 if ((u->flags & REF_HAVE_NEW) && !ref_update_has_null_new_value(u)) {
 				 ret = queue_transaction_update(refs, tx_data, u,
 								&current_oid, err);
 				 if (ret)
@@ -907,8 +907,10 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 				 * intertwined with the locking in files-backend.c.
 				 */
 				new_update = ref_transaction_add_update(
-						transaction, referent.buf, new_flags,
-						&u->new_oid, &u->old_oid, NULL, NULL, u->msg);
+					transaction, referent.buf, new_flags,
+					&u->new_oid, &u->old_oid, u->new_target,
+					u->old_target, u->msg);
+
 				new_update->parent_update = u;
 
 				/*
@@ -938,7 +940,22 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 		 * individual refs. But the error messages match what the files
 		 * backend returns, which keeps our tests happy.
 		 */
-		if (u->flags & REF_HAVE_OLD && !oideq(&current_oid, &u->old_oid)) {
+		if (u->old_target) {
+			if (strcmp(referent.buf, u->old_target)) {
+				if (!strcmp(referent.buf, ""))
+					strbuf_addf(err, "verifying symref target: '%s': "
+						    "reference is missing but expected %s",
+						    original_update_refname(u),
+						    u->old_target);
+				else
+					strbuf_addf(err, "verifying symref target: '%s': "
+						    "is at %s but expected %s",
+						    original_update_refname(u),
+						    referent.buf, u->old_target);
+				ret = -1;
+				goto done;
+			}
+		} else if ((u->flags & REF_HAVE_OLD) && !oideq(&current_oid, &u->old_oid)) {
 			if (is_null_oid(&u->old_oid))
 				strbuf_addf(err, _("cannot lock ref '%s': "
 					    "reference already exists"),
@@ -1043,7 +1060,7 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 		 * - `core.logAllRefUpdates` tells us to create the reflog for
 		 *   the given ref.
 		 */
-		if (u->flags & REF_HAVE_NEW && !(u->type & REF_ISSYMREF) && is_null_oid(&u->new_oid)) {
+		if ((u->flags & REF_HAVE_NEW) && !(u->type & REF_ISSYMREF) && ref_update_has_null_new_value(u)) {
 			struct reftable_log_record log = {0};
 			struct reftable_iterator it = {0};
 
@@ -1084,24 +1101,50 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 			   (u->flags & REF_FORCE_CREATE_REFLOG ||
 			    should_write_log(&arg->refs->base, u->refname))) {
 			struct reftable_log_record *log;
+			int create_reflog = 1;
+
+			if (u->new_target) {
+				if (!refs_resolve_ref_unsafe(&arg->refs->base, u->new_target,
+							     RESOLVE_REF_READING, &u->new_oid, NULL)) {
+					/*
+					 * TODO: currently we skip creating reflogs for dangling
+					 * symref updates. It would be nice to capture this as
+					 * zero oid updates however.
+					 */
+					create_reflog = 0;
+				}
+			}
 
-			ALLOC_GROW(logs, logs_nr + 1, logs_alloc);
-			log = &logs[logs_nr++];
-			memset(log, 0, sizeof(*log));
-
-			fill_reftable_log_record(log);
-			log->update_index = ts;
-			log->refname = xstrdup(u->refname);
-			memcpy(log->value.update.new_hash, u->new_oid.hash, GIT_MAX_RAWSZ);
-			memcpy(log->value.update.old_hash, tx_update->current_oid.hash, GIT_MAX_RAWSZ);
-			log->value.update.message =
-				xstrndup(u->msg, arg->refs->write_options.block_size / 2);
+			if (create_reflog) {
+				ALLOC_GROW(logs, logs_nr + 1, logs_alloc);
+				log = &logs[logs_nr++];
+				memset(log, 0, sizeof(*log));
+
+				fill_reftable_log_record(log);
+				log->update_index = ts;
+				log->refname = xstrdup(u->refname);
+				memcpy(log->value.update.new_hash, u->new_oid.hash, GIT_MAX_RAWSZ);
+				memcpy(log->value.update.old_hash, tx_update->current_oid.hash, GIT_MAX_RAWSZ);
+				log->value.update.message =
+					xstrndup(u->msg, arg->refs->write_options.block_size / 2);
+			}
 		}
 
 		if (u->flags & REF_LOG_ONLY)
 			continue;
 
-		if (u->flags & REF_HAVE_NEW && is_null_oid(&u->new_oid)) {
+		if (u->new_target) {
+			struct reftable_ref_record ref = {
+				.refname = (char *)u->refname,
+				.value_type = REFTABLE_REF_SYMREF,
+				.value.symref = (char *)u->new_target,
+				.update_index = ts,
+			};
+
+			ret = reftable_writer_add_ref(writer, &ref);
+			if (ret < 0)
+				goto done;
+		} else if ((u->flags & REF_HAVE_NEW) && ref_update_has_null_new_value(u)) {
 			struct reftable_ref_record ref = {
 				.refname = (char *)u->refname,
 				.update_index = ts,

-- 
2.43.GIT