index: make sure to write cached subtrees if parent is invalidated

If e.g. the root tree is invalidated, we still want to write out
its children, since those may still have valid cache entries.

src/tree-cache.c

@@ -280,10 +280,8 @@ static void write_tree(git_buf *out, git_tree_cache *tree)
 
 	git_buf_printf(out, "%s%c%zd %"PRIuZ"\n", tree->name, 0, tree->entry_count, tree->children_count);
 
-	if (tree->entry_count == -1)
-		return;
-
-	git_buf_put(out, (const char *) &tree->oid, GIT_OID_RAWSZ);
+	if (tree->entry_count != -1)
+		git_buf_put(out, (const char *) &tree->oid, GIT_OID_RAWSZ);
 
 	for (i = 0; i < tree->children_count; i++)
 		write_tree(out, tree->children[i]);

tests/index/cache.c

@@ -119,6 +119,59 @@ void test_index_cache__read_tree_no_children(void)
 	git_index_free(index);
 }
 
+void test_index_cache__two_levels(void)
+{
+	git_tree *tree;
+	git_oid tree_id;
+	git_index *index;
+	git_index_entry entry;
+	const git_tree_cache *tree_cache;
+
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_index_clear(index));
+
+	memset(&entry, 0x0, sizeof(entry));
+	entry.mode = GIT_FILEMODE_BLOB;
+	cl_git_pass(git_oid_fromstr(&entry.id, "a8233120f6ad708f843d861ce2b7228ec4e3dec6"));
+	entry.path = "top-level.txt";
+	cl_git_pass(git_index_add(index, &entry));
+
+	entry.path = "subdir/file.txt";
+	cl_git_pass(git_index_add(index, &entry));
+
+	/* the read-tree fills the tree cache */
+	cl_git_pass(git_index_write_tree(&tree_id, index));
+	cl_git_pass(git_tree_lookup(&tree, g_repo, &tree_id));
+	cl_git_pass(git_index_read_tree(index, tree));
+	git_tree_free(tree);
+	cl_git_pass(git_index_write(index));
+
+	/* we now must have cache entries for "" and "subdir" */
+	cl_assert(index->tree);
+	cl_assert(git_tree_cache_get(index->tree, "subdir"));
+
+	cl_git_pass(git_index_read(index, true));
+	/* we must still have cache entries for "" and "subdir", since we wrote it out */
+	cl_assert(index->tree);
+	cl_assert(git_tree_cache_get(index->tree, "subdir"));
+
+	entry.path = "top-level.txt";
+	cl_git_pass(git_oid_fromstr(&entry.id, "3697d64be941a53d4ae8f6a271e4e3fa56b022cc"));
+	cl_git_pass(git_index_add(index, &entry));
+
+	/* writ out the index after we invalidate the root */
+	cl_git_pass(git_index_write(index));
+	cl_git_pass(git_index_read(index, true));
+
+	/* the cache for the subtree must still be valid, even if the root isn't */
+	cl_assert(index->tree);
+	cl_assert_equal_i(-1, index->tree->entry_count);
+	cl_assert_equal_i(1, index->tree->children_count);
+	tree_cache = git_tree_cache_get(index->tree, "subdir");
+	cl_assert(tree_cache);
+	cl_assert_equal_i(0, tree_cache->entry_count);
+}
+
 void test_index_cache__read_tree_children(void)
 {
 	git_index *index;