Subject: [PATCH v4 16/17] sparse-checkout: write using lockfile

From: "Derrick Stolee via GitGitGadget" <[email-masked]>
Date: Tue, 15 Oct 2019 13:56:03 +0000

From: Derrick Stolee <[email-masked]>

If two 'git sparse-checkout set' subcommands are launched at the
same time, the behavior can be unexpected as they compete to write
the sparse-checkout file and update the working directory.

Take a lockfile around the writes to the sparse-checkout file. In
addition, acquire this lock around the working directory update
to avoid two commands updating the working directory in different
ways.

Signed-off-by: Derrick Stolee <[email-masked]>
---
 builtin/sparse-checkout.c          | 15 ++++++++++++---
 t/t1091-sparse-checkout-builtin.sh |  7 +++++++
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/builtin/sparse-checkout.c b/builtin/sparse-checkout.c
index 595955d4fc..b319966a79 100644
--- a/builtin/sparse-checkout.c
+++ b/builtin/sparse-checkout.c
@@ -305,6 +305,8 @@ static int write_patterns_and_update(struct pattern_list *pl)
 {
 	char *sparse_filename;
 	FILE *fp;
+	int fd;
+	struct lock_file lk = LOCK_INIT;
 	int result;
 
 	if (!core_apply_sparse_checkout) {
@@ -314,21 +316,28 @@ static int write_patterns_and_update(struct pattern_list *pl)
 
 	result = update_working_directory(pl);
 
+	sparse_filename = get_sparse_checkout_filename();
+	fd = hold_lock_file_for_update(&lk, sparse_filename,
+				      LOCK_DIE_ON_ERROR);
+
+	result = update_working_directory(pl);
 	if (result) {
+		rollback_lock_file(&lk);
+		free(sparse_filename);
 		clear_pattern_list(pl);
 		update_working_directory(NULL);
 		return result;
 	}
 
-	sparse_filename = get_sparse_checkout_filename();
-	fp = fopen(sparse_filename, "w");
+	fp = fdopen(fd, "w");
 
 	if (core_sparse_checkout_cone)
 		write_cone_to_file(fp, pl);
 	else
 		write_patterns_to_file(fp, pl);
 
-	fclose(fp);
+	fflush(fp);
+	commit_lock_file(&lk);
 
 	free(sparse_filename);
 	clear_pattern_list(pl);
diff --git a/t/t1091-sparse-checkout-builtin.sh b/t/t1091-sparse-checkout-builtin.sh
index ecb5a63671..48493e1012 100755
--- a/t/t1091-sparse-checkout-builtin.sh
+++ b/t/t1091-sparse-checkout-builtin.sh
@@ -262,4 +262,11 @@ test_expect_success 'revert to old sparse-checkout on bad update' '
 	test_cmp dir expect
 '
 
+test_expect_success 'fail when lock is taken' '
+	test_when_finished rm -rf repo/.git/info/sparse-checkout.lock &&
+	touch repo/.git/info/sparse-checkout.lock &&
+	test_must_fail git -C repo sparse-checkout set deep 2>err &&
+	test_i18ngrep "File exists" err
+'
+
 test_done
-- 
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