fix(storage): more strict parsing for HttpResponse (#7707)

Author: coryan

google/cloud/storage/internal/http_response.cc

@@ -173,7 +173,7 @@ Status AsStatus(HttpResponse const& http_response) {
   // structured and useful error Status. If the payload fails to parse as JSON,
   // we simply attach the full error payload as the Status's message string.
   auto json = nlohmann::json::parse(http_response.payload, nullptr, false);
-  if (json.is_discarded()) return Status(status_code, http_response.payload);
+  if (!json.is_object()) return Status(status_code, http_response.payload);
 
   // We expect JSON that looks like the following:
   //   {

google/cloud/storage/internal/http_response_test.cc

@@ -132,6 +132,13 @@ TEST(HttpResponseTest, ErrorInfoInvalidJson) {
   EXPECT_EQ(AsStatus(HttpResponse{400, kJsonPayload, {}}), expected);
 }
 
+TEST(HttpResponseTest, ErrorInfoInvalidOnlyString) {
+  // Some valid json, but not what we're looking for.
+  auto constexpr kJsonPayload = R"("uh-oh some error here")";
+  Status expected{StatusCode::kInvalidArgument, kJsonPayload};
+  EXPECT_EQ(AsStatus(HttpResponse{400, kJsonPayload, {}}), expected);
+}
+
 }  // namespace
 }  // namespace internal
 GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_END