[PATCH 5/7] read-tree: narrow scope of index expansion for '--prefix'

["Victoria Dye via GitGitGadget" <gitgitgadget@gmail.com>]

From: Victoria Dye <vdye@github.com>

When 'git read-tree' is provided with a prefix, expand the index only if the
prefix is equivalent to a sparse directory or contained within one. If the
index is not expanded in these cases, 'ce_in_traverse_path' will indicate
that the relevant sparse directory is not in the prefix/traverse path,
skipping past it and not unpacking the appropriate tree(s).

If the prefix is in-cone, its sparse subdirectories (if any) will be
traversed correctly without index expansion.

Signed-off-by: Victoria Dye <vdye@github.com>
---
 builtin/read-tree.c                      |  3 +--
 t/t1092-sparse-checkout-compatibility.sh |  8 ++++++-
 unpack-trees.c                           | 30 ++++++++++++++++++++++++
 3 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/builtin/read-tree.c b/builtin/read-tree.c
index c2fdbc2657f..a7b7f822281 100644
--- a/builtin/read-tree.c
+++ b/builtin/read-tree.c
@@ -213,8 +213,7 @@ int cmd_read_tree(int argc, const char **argv, const char *cmd_prefix)
 	if (opts.merge && !opts.index_only)
 		setup_work_tree();
 
-	/* TODO: audit sparse index behavior in unpack_trees */
-	if (opts.skip_sparse_checkout || opts.prefix)
+	if (opts.skip_sparse_checkout)
 		ensure_full_index(&the_index);
 
 	if (opts.merge) {
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index ae44451a0a9..a404be0a10f 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -1415,7 +1415,13 @@ test_expect_success 'sparse index is not expanded: read-tree' '
 	do
 		ensure_not_expanded read-tree -mu $MERGE_TREES &&
 		ensure_not_expanded reset --hard HEAD || return 1
-	done
+	done &&
+
+	rm -rf sparse-index/deep/deeper2 &&
+	ensure_not_expanded add . &&
+	ensure_not_expanded commit -m "test" &&
+
+	ensure_not_expanded read-tree --prefix=deep/deeper2 -u deepest
 '
 
 test_expect_success 'ls-files' '
diff --git a/unpack-trees.c b/unpack-trees.c
index 360844bda3a..dba122a02bb 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1739,6 +1739,36 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 		setup_standard_excludes(o->dir);
 	}
 
+	/*
+	 * If the prefix is equal to or contained within a sparse directory, the
+	 * index needs to be expanded to traverse with the specified prefix.
+	 */
+	if (o->prefix && o->src_index->sparse_index) {
+		int prefix_len = strlen(o->prefix);
+
+		while (prefix_len > 0 && o->prefix[prefix_len - 1] == '/')
+			prefix_len--;
+
+		if (prefix_len > 0) {
+			struct strbuf ce_prefix = STRBUF_INIT;
+			strbuf_grow(&ce_prefix, prefix_len + 1);
+			strbuf_add(&ce_prefix, o->prefix, prefix_len);
+			strbuf_addch(&ce_prefix, '/');
+
+			/*
+			 * If the prefix is not inside the sparse cone, then the
+			 * index is explicitly expanded if it is found as a sparse
+			 * directory, or implicitly expanded (by 'index_name_pos')
+			 * if the path is inside a sparse directory.
+			 */
+			if (!path_in_cone_mode_sparse_checkout(ce_prefix.buf, o->src_index) &&
+			    index_name_pos(o->src_index, ce_prefix.buf, ce_prefix.len) >= 0)
+				ensure_full_index(o->src_index);
+
+			strbuf_release(&ce_prefix);
+		}
+	}
+
 	if (!core_apply_sparse_checkout || !o->update)
 		o->skip_sparse_checkout = 1;
 	if (!o->skip_sparse_checkout && !o->pl) {
-- 
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