Subject: [RFC PATCH 1/5] t1011: add testcase demonstrating accidental loss of user modifications

From: Elijah Newren <[email-masked]>
Date: Sat, 8 Jan 2022 20:57:28 -0800

If a user has a file with local modifications that is not marked as
SKIP_WORKTREE, but the sparsity patterns are such that it should be
marked that way, and the user then invokes a command like

   * git checkout -q HEAD^

or

   * git read-tree -mu HEAD^

Then the file will be deleted along with all the users' modifications.
Add a testcase demonstrating this problem.

Note: This bug only triggers if something other than 'HEAD' is given;
if the commands above had specified 'HEAD', then the users' file would
be left alone.

Signed-off-by: Elijah Newren <[email-masked]>
---
 t/t1011-read-tree-sparse-checkout.sh | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/t/t1011-read-tree-sparse-checkout.sh b/t/t1011-read-tree-sparse-checkout.sh
index 24092c09a9..1b2395b8a8 100755
--- a/t/t1011-read-tree-sparse-checkout.sh
+++ b/t/t1011-read-tree-sparse-checkout.sh
@@ -187,6 +187,27 @@ test_expect_success 'read-tree updates worktree, absent case' '
 	test ! -f init.t
 '
 
+test_expect_success 'read-tree will not throw away dirty changes, non-sparse' '
+	echo "/*" >.git/info/sparse-checkout &&
+	read_tree_u_must_succeed -m -u HEAD &&
+
+	echo dirty >init.t &&
+	read_tree_u_must_fail -m -u HEAD^ &&
+	test_path_is_file init.t &&
+	grep -q dirty init.t
+'
+
+test_expect_failure 'read-tree will not throw away dirty changes, sparse' '
+	echo "/*" >.git/info/sparse-checkout &&
+	read_tree_u_must_succeed -m -u HEAD &&
+
+	echo dirty >init.t &&
+	echo sub/added >.git/info/sparse-checkout &&
+	read_tree_u_must_fail -m -u HEAD^ &&
+	test_path_is_file init.t &&
+	grep -q dirty init.t
+'
+
 test_expect_success 'read-tree updates worktree, dirty case' '
 	echo sub/added >.git/info/sparse-checkout &&
 	git checkout -f top &&
-- 
2.34.1.442.ge63c19bdd2.dirty