[core] Database file source does not return stored tiles for volatile…

… resources

The `DatabaseFileSource.VolatileResource` test is included.

platform/default/src/mbgl/storage/database_file_source.cpp

@@ -27,7 +27,7 @@ class DatabaseFileSourceThread {
             offlineResponse->noContent = true;
             offlineResponse->error =
                 std::make_unique<Response::Error>(Response::Error::Reason::NotFound, "Not found in offline database");
-        } else if (!offlineResponse->isUsable()) {
+        } else if (resource.storagePolicy == Resource::StoragePolicy::Volatile || !offlineResponse->isUsable()) {
             offlineResponse->error =
                 std::make_unique<Response::Error>(Response::Error::Reason::NotFound, "Cached resource is unusable");
         }

test/storage/database_file_source.test.cpp

@@ -22,5 +22,34 @@ TEST(DatabaseFileSource, PauseResume) {
     util::Timer resumeTimer;
     resumeTimer.start(Milliseconds(5), Duration::zero(), [dbfs] { dbfs->resume(); });
 
+    loop.run();
+}
+
+TEST(DatabaseFileSource, VolatileResource) {
+    util::RunLoop loop;
+
+    std::shared_ptr<FileSource> dbfs =
+        FileSourceManager::get()->getFileSource(FileSourceType::Database, ResourceOptions{});
+
+    Resource resource{Resource::Unknown, "http://127.0.0.1:3000/test", {}, Resource::LoadingMethod::CacheOnly};
+    Response response{};
+    response.data = std::make_shared<std::string>("Cached value");
+    std::unique_ptr<mbgl::AsyncRequest> req;
+
+    dbfs->forward(resource, response, [&] {
+        req = dbfs->request(resource, [&](Response res1) {
+            EXPECT_EQ(nullptr, res1.error);
+            ASSERT_TRUE(res1.data.get());
+            EXPECT_EQ("Cached value", *res1.data);
+            resource.storagePolicy = Resource::StoragePolicy::Volatile;
+            req = dbfs->request(resource, [&](Response res2) {
+                req.reset();
+                ASSERT_TRUE(res2.error.get());
+                EXPECT_EQ(Response::Error::Reason::NotFound, res2.error->reason);
+                EXPECT_EQ("Cached resource is unusable", res2.error->message);
+                loop.stop();
+            });
+        });
+    });
     loop.run();
 }