sparse-checkout: add config to disable deleting dirs

The clean_tracked_sparse_directories() method deletes the tracked
directories that go out of scope when the sparse-checkout cone changes,
at least in cone mode. This is new behavior, but is recommended based on
our understanding of how users are interacting with the feature in most
cases.

It is possible that some users will object to the new behavior, so
create a new configuration option 'index.deleteSparseDirectories' that
can be set to 'false' to make clean_tracked_sparse_directories() do
nothing. This will keep all untracked files in the working tree and
cause performance problems with the sparse index, but those trade-offs
are for the user to decide.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

Documentation/config/index.txt

@@ -1,3 +1,9 @@
+index.deleteSparseDirectories::
+	When enabled, the cone mode sparse-checkout feature will delete
+	directories that are outside of the sparse-checkout cone, unless
+	such a directory contains an untracked, non-ignored file. Defaults
+	to true.
+
 index.recordEndOfIndexEntries::
 	Specifies whether the index file should include an "End Of Index
 	Entry" section. This reduces index load time on multiprocessor

builtin/sparse-checkout.c

@@ -102,7 +102,7 @@ static int sparse_checkout_list(int argc, const char **argv)
 
 static void clean_tracked_sparse_directories(struct repository *r)
 {
-	int i, was_full = 0;
+	int i, value, was_full = 0;
 	struct strbuf path = STRBUF_INIT;
 	size_t pathlen;
 	struct string_list_item *item;
@@ -119,6 +119,13 @@ static void clean_tracked_sparse_directories(struct repository *r)
 	    !r->index->sparse_checkout_patterns->use_cone_patterns)
 		return;
 
+	/*
+	 * Users can disable this behavior.
+	 */
+	if (!repo_config_get_bool(r, "index.deletesparsedirectories", &value) &&
+	    !value)
+		return;
+
 	/*
 	 * Use the sparse index as a data structure to assist finding
 	 * directories that are safe to delete. This conversion to a

t/t1091-sparse-checkout-builtin.sh

@@ -666,6 +666,10 @@ test_expect_success 'cone mode clears ignored subdirectories' '
 	git -C repo status --porcelain=v2 >out &&
 	test_must_be_empty out &&
 
+	git -C repo -c index.deleteSparseDirectories=false sparse-checkout reapply &&
+	test_path_is_dir repo/folder1 &&
+	test_path_is_dir repo/deep/deeper2 &&
+
 	git -C repo sparse-checkout reapply &&
 	test_path_is_missing repo/folder1 &&
 	test_path_is_missing repo/deep/deeper2 &&