[core] use stale sprite data

This is a naïve implementation that essentially merges updated data into existing data. It will *not* remove icons from the stale sprite if they aren't present in the fresh sprite (we aren't tracking the source of a sprite, and the user could have changed it as well). Similarly, it will not update icons that have changed in dimension. This is a rare edge case and probably not worth implementing.

src/mbgl/sprite/sprite_store.cpp

@@ -40,15 +40,17 @@ void SpriteStore::setURL(const std::string& url) {
     FileSource* fs = util::ThreadContext::getFileSource();
     loader->jsonRequest = fs->request({ Resource::Kind::SpriteJSON, jsonURL },
                                       [this, jsonURL](Response res) {
-        if (res.stale) {
-            // Only handle fresh responses.
-            return;
-        }
-        loader->jsonRequest = nullptr;
-
         if (res.error) {
             observer->onSpriteError(std::make_exception_ptr(std::runtime_error(res.error->message)));
-        } else {
+        }
+
+        if (res.notModified) {
+            // We got the same data back as last time. Abort early.
+            return;
+        }
+
+        if (!loader->json || *loader->json != *res.data) {
+            // Only trigger a sprite loaded event we got new data.
             loader->json = res.data;
             emitSpriteLoadedIfComplete();
         }
@@ -57,15 +59,16 @@ void SpriteStore::setURL(const std::string& url) {
     loader->spriteRequest =
         fs->request({ Resource::Kind::SpriteImage, spriteURL },
                     [this, spriteURL](Response res) {
-            if (res.stale) {
-                // Only handle fresh responses.
-                return;
-            }
-            loader->spriteRequest = nullptr;
-
             if (res.error) {
                 observer->onSpriteError(std::make_exception_ptr(std::runtime_error(res.error->message)));
-            } else {
+            }
+
+            if (res.notModified) {
+                // We got the same data back as last time. Abort early.
+                return;
+            }
+
+            if (!loader->image || *loader->image != *res.data) {
                 loader->image = res.data;
                 emitSpriteLoadedIfComplete();
             }
@@ -79,8 +82,7 @@ void SpriteStore::emitSpriteLoadedIfComplete() {
         return;
     }
 
-    auto local = std::move(loader);
-    auto result = parseSprite(*local->image, *local->json);
+    auto result = parseSprite(*loader->image, *loader->json);
     if (result.is<Sprites>()) {
         loaded = true;
         setSprites(result.get<Sprites>());

test/fixtures/stale/sprite.json

@@ -0,0 +1,10 @@
+{
+  "noise": {
+    "x": 0,
+    "y": 0,
+    "width": 32,
+    "height": 32,
+    "pixelRatio": 1,
+    "sdf": false
+  }
+}

test/fixtures/stale/style_and_sprite.json

@@ -0,0 +1,26 @@
+{
+  "version": 8,
+  "name": "Water",
+  "sources": {
+    "mapbox": {
+      "type": "vector",
+      "url": "asset://test/fixtures/stale/streets.json"
+    }
+  },
+  "sprite": "http://127.0.0.1:3000/stale/sprite",
+  "layers": [{
+    "id": "background",
+    "type": "background",
+    "paint": {
+      "background-color": "red"
+    }
+  }, {
+    "id": "water",
+    "type": "fill",
+    "source": "mapbox",
+    "source-layer": "water",
+    "paint": {
+      "fill-pattern": "noise"
+    }
+  }]
+}

test/storage/cache_stale.cpp

@@ -72,3 +72,31 @@ TEST_F(Storage, CacheStaleStyleAndTileJSON) {
 
     checkRendering(map, "stale_style_and_tilejson", 1000ms);
 }
+
+TEST_F(Storage, CacheStaleStyleAndSprite) {
+    HeadlessView view(display, 1);
+
+    auto cache = SQLiteCache::getShared(":memory:");
+
+    // Rig the cache with an expired stylesheet.
+    const std::string stylePath = "stale/style_and_sprite.json";
+    const Resource styleResource{ Resource::Kind::Style, prefix + "/" + stylePath };
+    cache->put(styleResource, expiredItem(stylePath));
+
+    // Rig the cache with an expired sprite JSON.
+    const std::string spritejsonPath = "stale/sprite.json";
+    const Resource spritejsonResource{ Resource::Kind::SpriteJSON, prefix + "/" + spritejsonPath };
+    cache->put(spritejsonResource, expiredItem(spritejsonPath));
+
+    // Rig the cache with an expired sprite JSON.
+    const std::string spriteimagePath = "stale/sprite.png";
+    const Resource spriteimageResource{ Resource::Kind::SpriteImage, prefix + "/" + spriteimagePath };
+    cache->put(spriteimageResource, expiredItem(spriteimagePath));
+
+    DefaultFileSource fileSource(":memory:", ".");
+
+    Map map(view, fileSource, MapMode::Still);
+    map.setStyleURL(styleResource.url);
+
+    checkRendering(map, "stale_style_and_sprite", 1000ms);
+}