[core] Add Resource::StoragePolicy and consider it in resource loader

include/mbgl/storage/resource.hpp

@@ -33,6 +33,8 @@ public:
         Offline
     };
 
+    enum class StoragePolicy : bool { Permanent, Volatile };
+
     struct TileData {
         std::string urlTemplate;
         uint8_t pixelRatio;
@@ -96,6 +98,7 @@ public:
     optional<std::string> priorEtag = {};
     std::shared_ptr<const std::string> priorData;
     Duration minimumUpdateInterval{Duration::zero()};
+    StoragePolicy storagePolicy{StoragePolicy::Permanent};
 };
 
 inline bool Resource::hasLoadingMethod(Resource::LoadingMethod method) const {

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

@@ -179,6 +179,8 @@ std::unique_ptr<AsyncRequest> DatabaseFileSource::request(const Resource& resour
 }
 
 void DatabaseFileSource::forward(const Resource& res, const Response& response, std::function<void()> callback) {
+    if (res.storagePolicy == Resource::StoragePolicy::Volatile) return;
+
     std::function<void()> wrapper;
     if (callback) {
         wrapper = Scheduler::GetCurrent()->bindOnce(std::move(callback));

test/storage/main_resource_loader.test.cpp

@@ -52,6 +52,33 @@ TEST(MainResourceLoader, TEST_REQUIRES_SERVER(CacheResponse)) {
     loop.run();
 }
 
+TEST(MainResourceLoader, TEST_REQUIRES_SERVER(VolatileStoragePolicy)) {
+    util::RunLoop loop;
+    MainResourceLoader fs(ResourceOptions{});
+
+    Resource resource{Resource::Unknown, "http://127.0.0.1:3000/cache"};
+    resource.storagePolicy = Resource::StoragePolicy::Volatile;
+
+    std::unique_ptr<AsyncRequest> req;
+    req = fs.request(resource, [&](Response res1) {
+        EXPECT_EQ(nullptr, res1.error);
+        ASSERT_TRUE(res1.data);
+        std::string firstData = *res1.data;
+
+        // Volatile resources are not stored in cache,
+        // so we always get new data from the server ("Response N+1").
+        req = fs.request(resource, [&](Response res2) {
+            req.reset();
+            EXPECT_EQ(nullptr, res2.error);
+            ASSERT_TRUE(res2.data);
+            EXPECT_NE(firstData, *res2.data);
+            loop.stop();
+        });
+    });
+
+    loop.run();
+}
+
 TEST(MainResourceLoader, TEST_REQUIRES_SERVER(CacheRevalidateSame)) {
     util::RunLoop loop;
     MainResourceLoader fs(ResourceOptions{});