[PATCH v2 07/12] commit: avoid race when creating orphan commits

[Michael Haggerty <mhagger@alum.mit.edu>]

If HEAD doesn't point at anything during the initial check, then we
should make sure that it *still* doesn't point at anything when we are
ready to update the reference. Otherwise, another process might commit
while we are working (e.g., while we are waiting for the user to edit
the commit message) and we will silently overwrite it.

This fixes a failing test in t7516.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Stefan Beller <sbeller@google.com>
---
 builtin/commit.c        | 2 +-
 t/t7516-commit-races.sh | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/builtin/commit.c b/builtin/commit.c
index 8afb0ff..682f922 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1766,7 +1766,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	if (!transaction ||
 	    ref_transaction_update(transaction, "HEAD", sha1,
 				   current_head
-				   ? current_head->object.sha1 : NULL,
+				   ? current_head->object.sha1 : null_sha1,
 				   0, sb.buf, &err) ||
 	    ref_transaction_commit(transaction, &err)) {
 		rollback_index_files();
diff --git a/t/t7516-commit-races.sh b/t/t7516-commit-races.sh
index 08e6a6c..dd44ef2 100755
--- a/t/t7516-commit-races.sh
+++ b/t/t7516-commit-races.sh
@@ -12,7 +12,7 @@ test_expect_success 'set up editor' '
 	EOF
 '
 
-test_expect_failure 'race to create orphan commit' '
+test_expect_success 'race to create orphan commit' '
 	test_must_fail env EDITOR=./editor git commit --allow-empty &&
 	git show -s --pretty=format:%s >subject &&
 	grep -q hare subject &&
-- 
2.1.4