config_file: refuse modifying included variables

Modifying variables pulled in by an included file currently succeeds,
but it doesn't actually do what one would expect, as refreshing the
configuration will cause the values to reappear. As we are currently not
really able to support this use case, we will instead just return an
error for deleting and setting variables which were included via an
include.

src/config_file.c

@@ -490,6 +490,12 @@ static int config_set(git_config_backend *cfg, const char *name, const char *val
 			goto out;
 		}
 
+		if (existing->included) {
+			giterr_set(GITERR_CONFIG, "modifying included variable is not supported");
+			ret = -1;
+			goto out;
+		}
+
 		/* don't update if old and new values already match */
 		if ((!existing->entry->value && !value) ||
 			(existing->entry->value && value &&
@@ -624,6 +630,11 @@ static int config_delete(git_config_backend *cfg, const char *name)
 	var = git_strmap_value_at(values, pos);
 	refcounted_strmap_free(map);
 
+	if (var->included) {
+		giterr_set(GITERR_CONFIG, "cannot delete included variable");
+		return -1;
+	}
+
 	if (var->next != NULL) {
 		giterr_set(GITERR_CONFIG, "cannot delete multivar with a single delete");
 		return -1;

tests/config/include.c

@@ -146,3 +146,21 @@ void test_config_include__rewriting_include_refreshes_values(void)
 	cl_git_pass(git_config_get_string_buf(&buf, cfg, "first.other"));
 	cl_assert_equal_s(buf.ptr, "value");
 }
+
+void test_config_include__included_variables_cannot_be_deleted(void)
+{
+	cl_git_mkfile("top-level", "[include]\npath = included\n");
+	cl_git_mkfile("included", "[foo]\nbar = value");
+
+	cl_git_pass(git_config_open_ondisk(&cfg, "top-level"));
+	cl_git_fail(git_config_delete_entry(cfg, "foo.bar"));
+}
+
+void test_config_include__included_variables_cannot_be_modified(void)
+{
+	cl_git_mkfile("top-level", "[include]\npath = included\n");
+	cl_git_mkfile("included", "[foo]\nbar = value");
+
+	cl_git_pass(git_config_open_ondisk(&cfg, "top-level"));
+	cl_git_fail(git_config_set_string(cfg, "foo.bar", "other-value"));
+}