refs.c: add a flag to allow reflog updates to truncate the log

Add a flag that allows us to truncate the reflog before we write the
update.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 24 insertions, 3 deletions

diff --git a/refs.c b/refs.c
index 4fcef5291d..01d5ed5a97 100644
--- a/refs.c
+++ b/refs.c
@@ -3748,7 +3748,12 @@ int transaction_commit(struct ref_transaction *transaction,
 		}
 	}
 
-	/* Update all reflog files */
+	/*
+	 * Update all reflog files
+	 * We have already done all ref updates and deletes.
+	 * There is not much we can do here if there are any reflog
+	 * update errors other than complain.
+	 */
 	for (i = 0; i < n; i++) {
 		struct ref_update *update = updates[i];
 
@@ -3756,7 +3761,15 @@ int transaction_commit(struct ref_transaction *transaction,
 			continue;
 		if (update->reflog_fd == -1)
 			continue;
-
+		if (update->flags & REFLOG_TRUNCATE)
+			if (lseek(update->reflog_fd, 0, SEEK_SET) < 0 ||
+				ftruncate(update->reflog_fd, 0)) {
+				error("Could not truncate reflog: %s. %s",
+				      update->refname, strerror(errno));
+				rollback_lock_file(&update->reflog_lock);
+				update->reflog_fd = -1;
+				continue;
+			}
 		if (log_ref_write_fd(update->reflog_fd, update->old_sha1,
 				     update->new_sha1,
 				     update->committer, update->msg)) {
diff --git a/refs.h b/refs.h
index 479615f84e..2234750cf8 100644
--- a/refs.h
+++ b/refs.h
@@ -320,7 +320,15 @@ int transaction_delete_sha1(struct ref_transaction *transaction,
 			    struct strbuf *err);
 
 /*
- * Append a reflog entry for refname.
+ * Flags controlling transaction_update_reflog().
+ * REFLOG_TRUNCATE: Truncate the reflog.
+ *
+ * Flags >= 0x100 are reserved for internal use.
+ */
+#define REFLOG_TRUNCATE 0x01
+/*
+ * Append a reflog entry for refname. If the REFLOG_TRUNCATE flag is set
+ * this update will first truncate the reflog before writing the entry.
  */
 int transaction_update_reflog(struct ref_transaction *transaction,
 			      const char *refname,