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

[Derrick Stolee via GitGitGadget]

From: Derrick Stolee <dsto...@microsoft.com>

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 <dsto...@microsoft.com>
---
 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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