[PATCH v3 08/13] 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 ed04d1c..f2ce14e 100755
--- a/t/t7516-commit-races.sh
+++ b/t/t7516-commit-races.sh
@@ -3,7 +3,7 @@
 test_description='git commit races'
 . ./test-lib.sh
 
-test_expect_failure 'race to create orphan commit' '
+test_expect_success 'race to create orphan commit' '
 	write_script hare-editor <<-\EOF &&
 	git commit --allow-empty -m hare
 	EOF
-- 
2.1.4