Re: [PATCH v3 16/19] branch.c: use ref transaction for all ref updates

Change create_branch to use a ref transaction when creating the new branch.
ref_transaction_create will check that the ref does not already exist and fail
otherwise meaning that we no longer need to keep a lock on the ref during the
setup_tracking. This simplifies the code since we can now do the transaction
in one single step.

If the forcing flag is false then use ref_transaction_create since this will
fail if the ref already exist. Otherwise use ref_transaction_update.

This also fixes a race condition in the old code where two concurrent
create_branch could race since the lock_any_ref_for_update/write_ref_sha1
did not protect against the ref already existsing. I.e. one thread could end up
overwriting a branch even if the forcing flag is false.

Signed-off-by: Ronnie Sahlberg <redacted>
---
 branch.c | 39 +++++++++++++++++++++++++--------------
 1 file changed, 25 insertions(+), 14 deletions(-)

diff --git a/branch.c b/branch.c
index 660097b..23cde1e 100644
--- a/branch.c
+++ b/branch.c

@@ -226,7 +226,6 @@ void create_branch(const char *head,
 		   int force, int reflog, int clobber_head,
 		   int quiet, enum branch_track track)
 {
-	struct ref_lock *lock = NULL;
 	struct commit *commit;
 	unsigned char sha1[20];
 	char *real_ref, msg[PATH_MAX + 20];

@@ -285,15 +284,6 @@ void create_branch(const char *head,
 		die(_("Not a valid branch point: '%s'."), start_name);
 	hashcpy(sha1, commit->object.sha1);
 
-	if (!dont_change_ref) {
-		lock = lock_any_ref_for_update(ref.buf, NULL, 0, NULL);
-		if (!lock)
-			die_errno(_("Failed to lock ref for update"));
-	}
-
-	if (reflog)
-		log_all_ref_updates = 1;
-
 	if (forcing)
 		snprintf(msg, sizeof msg, "branch: Reset to %s",
 			 start_name);

@@ -301,13 +291,34 @@ void create_branch(const char *head,
 		snprintf(msg, sizeof msg, "branch: Created from %s",
 			 start_name);
 
+	if (reflog)
+		log_all_ref_updates = 1;
+
+	if (!dont_change_ref) {
+		struct ref_transaction *transaction;
+		char *err = NULL;
+
+		transaction = ref_transaction_begin();
+		if (forcing) {
+			if (!transaction ||
+			    ref_transaction_update(transaction, ref.buf, sha1,
+						   NULL, 0, 0) ||
+			    ref_transaction_commit(transaction, msg, &err))
+			  die_errno(_("%s: failed to write ref: %s"),
+				    ref.buf, err);
+		} else {
+			if (!transaction ||
+			    ref_transaction_create(transaction, ref.buf, sha1,
+						   0) ||
+			    ref_transaction_commit(transaction, msg, &err))
+			  die_errno(_("%s: failed to write ref: %s"),
+				    ref.buf, err);
+		}