[core] Add a custom deleter for the TileData

We cannot simply delete the TileData at will because
it might block. The custom deleter makes sure that
it will only delete the object when it is non-blocking.

src/mbgl/map/source.cpp

@@ -286,12 +286,14 @@ TileData::State Source::addTile(const TileID& tileID, const StyleUpdateParameter
                 return TileData::State::invalid;
             }
 
-            newTile->data = std::make_shared<VectorTileData>(normalizedID,
-                                                             std::move(monitor),
-                                                             id,
-                                                             parameters.style,
-                                                             parameters.mode,
-                                                             callback);
+            newTile->data = std::shared_ptr<TileData>(new VectorTileData{
+                normalizedID,
+                std::move(monitor),
+                id,
+                parameters.style,
+                parameters.mode,
+                callback
+            }, [this](TileData* data) { tileDataDeleter.add(data); });
         }
 
         tileDataMap.emplace(newTile->data->id, newTile->data);

src/mbgl/map/source.hpp

@@ -2,6 +2,7 @@
 #define MBGL_MAP_SOURCE
 
 #include <mbgl/map/tile_cache.hpp>
+#include <mbgl/map/tile_data_deleter.hpp>
 #include <mbgl/map/source_info.hpp>
 
 #include <mbgl/util/mat4.hpp>
@@ -111,6 +112,8 @@ class Source : private util::noncopyable {
 
     Observer nullObserver;
     Observer* observer = &nullObserver;
+
+    TileDataDeleter tileDataDeleter;
 };
 
 } // namespace mbgl

src/mbgl/map/tile_data_deleter.cpp

@@ -0,0 +1,34 @@
+#include <mbgl/map/tile_data_deleter.hpp>
+
+#include <mbgl/map/tile_data.hpp>
+
+namespace mbgl {
+
+TileDataDeleter::~TileDataDeleter() {
+    // Force the canceling of all pending TileData
+    // before destroying. We have assert()s on Debug
+    // mode on TileData to make sure it doesn't block
+    // at the destructor unless explicitly canceled.
+    for (auto& entry : tileDataMap) {
+        entry.first->tryCancel(true);
+    }
+}
+
+void TileDataDeleter::add(TileData* data) {
+    std::unique_ptr<TileData> tileData(data);
+
+    // This custom deleter will try to cancel the task if it is
+    // non-blocking, otherwise we keep it and subscribe to the
+    // event when the work is completed. When the event triggers
+    // we delete the TileData because this time it won't block.
+    if (!tileData->tryCancel()) {
+        data->setObserver(this);
+        tileDataMap.emplace(data, std::move(tileData));
+    }
+}
+
+void TileDataDeleter::onTileDataWorkCompleted(TileData* data) {
+    tileDataMap.erase(data);
+}
+
+} // namespace mbgl

src/mbgl/map/tile_data_deleter.hpp

@@ -0,0 +1,29 @@
+#ifndef MBGL_MAP_TILE_DATA_DELETER
+#define MBGL_MAP_TILE_DATA_DELETER
+
+#include <mbgl/map/tile_data.hpp>
+#include <mbgl/util/noncopyable.hpp>
+
+#include <memory>
+#include <unordered_map>
+
+namespace mbgl {
+
+class TileData;
+
+class TileDataDeleter : public TileData::Observer, private util::noncopyable {
+public:
+    virtual ~TileDataDeleter();
+
+    void add(TileData*);
+
+    // TileData::Observer implementation.
+    void onTileDataWorkCompleted(TileData*) override;
+
+private:
+    std::unordered_map<TileData*, std::unique_ptr<TileData>> tileDataMap;
+};
+
+} // namespace mbgl
+
+#endif