[core] Offline download must clear batch buffer at load interrupt

Otherwise, the stale buffer data are processed again at the
repeated load. It is causing various problems like missing load
completion notification or crashes (if the previously used observer
instance has been deleted).

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

@@ -379,6 +379,7 @@ void OfflineDownload::deactivateDownload() {
     requiredSourceURLs.clear();
     resourcesRemaining.clear();
     requests.clear();
+    buffer.clear();
 }
 
 void OfflineDownload::queueResource(Resource&& resource) {

test/storage/offline_download.test.cpp

@@ -757,7 +757,6 @@ TEST(OfflineDownload, Deactivate) {
     test.loop.run();
 }
 
-
 TEST(OfflineDownload, AllOfflineRequestsHaveLowPriorityAndOfflineUsage) {
     OfflineTest test;
     auto region = test.createRegion();
@@ -947,3 +946,85 @@ TEST(OfflineDownload, ResourceOfflineUsageUnset) {
     test.loop.run();
 }
 #endif // __QT__
+
+// Test verifies that resource batch buffer is cleared at loading interrupt.
+TEST(OfflineDownload, InterruptAndResume) {
+    OfflineTest test;
+    auto region = test.createRegion();
+    ASSERT_TRUE(region);
+    OfflineDownload download(region->getID(),
+                             OfflineTilePyramidRegionDefinition(
+                                 "http://127.0.0.1:3000/style.json", LatLngBounds::world(), 0.0, 0.0, 1.0, true),
+                             test.db,
+                             test.fileSource);
+
+    test.fileSource.styleResponse = [&](const Resource& resource) {
+        EXPECT_EQ("http://127.0.0.1:3000/style.json", resource.url);
+        return test.response("style.json");
+    };
+
+    test.fileSource.spriteImageResponse = [&](const Resource& resource) {
+        EXPECT_TRUE(resource.url == "http://127.0.0.1:3000/sprite.png" ||
+                    resource.url == "http://127.0.0.1:3000/sprite@2x.png");
+        return test.response("sprite.png");
+    };
+
+    test.fileSource.imageResponse = [&](const Resource& resource) {
+        EXPECT_EQ("http://127.0.0.1:3000/radar.gif", resource.url);
+        return test.response("radar.gif");
+    };
+
+    test.fileSource.spriteJSONResponse = [&](const Resource& resource) {
+        EXPECT_TRUE(resource.url == "http://127.0.0.1:3000/sprite.json" ||
+                    resource.url == "http://127.0.0.1:3000/sprite@2x.json");
+        return test.response("sprite.json");
+    };
+
+    test.fileSource.glyphsResponse = [&](const Resource&) { return test.response("glyph.pbf"); };
+
+    test.fileSource.sourceResponse = [&](const Resource& resource) {
+        EXPECT_EQ("http://127.0.0.1:3000/streets.json", resource.url);
+        return test.response("streets.json");
+    };
+
+    test.fileSource.tileResponse = [&](const Resource& resource) {
+        const Resource::TileData& tile = *resource.tileData;
+        EXPECT_EQ("http://127.0.0.1:3000/{z}-{x}-{y}.vector.pbf", tile.urlTemplate);
+        EXPECT_EQ(1, tile.pixelRatio);
+        EXPECT_EQ(0, tile.x);
+        EXPECT_EQ(0, tile.y);
+        EXPECT_EQ(0, tile.z);
+        return test.response("0-0-0.vector.pbf");
+    };
+
+    auto observer = std::make_unique<MockObserver>();
+    bool interrupted = false;
+    observer->statusChangedFn = [&](OfflineRegionStatus status) {
+        if (!interrupted && status.completedResourceCount > 0) {
+            interrupted = true;
+            assert(!status.complete());
+            test.loop.schedule([&]() {
+                download.setState(OfflineRegionDownloadState::Inactive);
+                test.loop.stop();
+            });
+        }
+    };
+
+    download.setObserver(std::move(observer));
+    download.setState(OfflineRegionDownloadState::Active);
+    test.loop.run();
+
+    auto newObserver = std::make_unique<MockObserver>();
+    newObserver->statusChangedFn = [&](OfflineRegionStatus status) {
+        if (status.complete()) {
+            EXPECT_EQ(status.completedTileCount, status.requiredTileCount);
+            EXPECT_EQ(264u, status.completedResourceCount); // 256 glyphs, 2 sprite images, 2 sprite jsons, 1 tile, 1
+                                                            // style, source, image
+            test.loop.stop();
+        }
+    };
+
+    download.setObserver(std::move(newObserver));
+    download.setState(OfflineRegionDownloadState::Active);
+    test.loop.run();
+}