read-tree: narrow scope of index expansion for --prefix

When `unpack_trees` is provided with a prefix, the tree traversal that occurs
in it requires that the base directory path given to it is a traversable
directory in the index. In order to adhere to this requirement, the index is
expanded if the given prefix is equal to or a subdirectory of a sparse
directory in the index.

Signed-off-by: Victoria Dye <[email protected]>

Author: vdye

builtin/read-tree.c

@@ -217,8 +217,7 @@ 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) {

t/t1092-sparse-checkout-compatibility.sh

@@ -1258,7 +1258,13 @@ 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
 '
 
 # NEEDSWORK: a sparse-checkout behaves differently from a full checkout

unpack-trees.c

@@ -1741,6 +1741,58 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 		ensure_full_index(o->dst_index);
 	}
 
+	/*
+	 * If the prefix is equal to or contained within a sparse directory, the
+	 * index needs to be expanded to traverse with the specified prefix. Note
+	 * that only the src_index is checked because the prefix is only specified
+	 * in cases where src_index == dst_index.
+	 */
+	if (o->prefix && o->src_index->sparse_index) {
+		int i, ce_len;
+		struct cache_entry *ce;
+		int prefix_len = strlen(o->prefix);
+
+		if (prefix_len > 0) {
+			for (i = 0; i < o->src_index->cache_nr; i++) {
+				ce = o->src_index->cache[i];
+				ce_len = ce_namelen(ce);
+
+				if (!S_ISSPARSEDIR(ce->ce_mode))
+					continue;
+
+				/*
+				 * Normalize comparison length for cache entry vs. prefix -
+				 * either may have a trailing slash, which we do not want to
+				 * compare (can assume both are directories).
+				 */
+				if (ce->name[ce_len - 1] == '/')
+					ce_len--;
+				if (o->prefix[prefix_len - 1] == '/')
+					prefix_len--;
+
+				/*
+				 * If prefix length is shorter, then it is either a parent to
+				 * this sparse directory, or a completely different path. In
+				 * either case, we don't need to expand the index
+				 */
+				if (prefix_len < ce_len)
+					continue;
+
+				/*
+				 * Avoid the case of expanding the index with a prefix
+				 * a/beta for a sparse directory a/b.
+				 */
+				if (ce_len < prefix_len && o->prefix[ce_len] != '/')
+					continue;
+
+				if (!strncmp(ce->name, o->prefix, ce_len)) {
+					ensure_full_index(o->src_index);
+					break;
+				}
+			}
+		}
+	}
+
 	if (!core_apply_sparse_checkout || !o->update)
 		o->skip_sparse_checkout = 1;
 	if (!o->skip_sparse_checkout && !o->pl) {