[core] Rationalize error handling for resource loading

* Standardize on std::exception_ptr as the error representation
  (fixes #2854).
* Don't format textual strings at the error source; pass on the
  constituent data via observer method parameters instead.
* Use the null object pattern to simplify observer notification code.
* Further refactoring for ResourceLoading tests.

22 files changed, 157 insertions, 291 deletions

diff --git a/src/mbgl/map/map_context.cpp b/src/mbgl/map/map_context.cpp
index 75ca188fbb..f2c17824c3 100644
--- a/src/mbgl/map/map_context.cpp
+++ b/src/mbgl/map/map_context.cpp
@@ -311,16 +311,13 @@ void MapContext::onLowMemory() {
     style->onLowMemory();
     asyncInvalidate.send();
 }
-    
-void MapContext::onTileDataChanged() {
-    assert(util::ThreadContext::currentlyOn(util::ThreadType::Map));
+
+void MapContext::onResourceLoaded() {
     updateFlags |= Update::Repaint;
     asyncUpdate.send();
 }
 
-void MapContext::onResourceLoadingFailed(std::exception_ptr error) {
-    assert(util::ThreadContext::currentlyOn(util::ThreadType::Map));
-
+void MapContext::onResourceError(std::exception_ptr error) {
     if (data.mode == MapMode::Still && callback) {
         callback(error, {});
         callback = nullptr;
diff --git a/src/mbgl/map/map_context.hpp b/src/mbgl/map/map_context.hpp
index 24459f270a..012da785c9 100644
--- a/src/mbgl/map/map_context.hpp
+++ b/src/mbgl/map/map_context.hpp
@@ -65,14 +65,12 @@ public:
     void onLowMemory();
 
     void cleanup();
-
-    // Style::Observer implementation.
-    void onTileDataChanged() override;
-    void onResourceLoadingFailed(std::exception_ptr error) override;
-
     void dumpDebugLogs() const;
 
 private:
+    void onResourceLoaded() override;
+    void onResourceError(std::exception_ptr) override;
+
     // Update the state indicated by the accumulated Update flags, then render.
     void update();
 
diff --git a/src/mbgl/map/raster_tile_data.cpp b/src/mbgl/map/raster_tile_data.cpp
index 7ef13301e8..5c85cbd034 100644
--- a/src/mbgl/map/raster_tile_data.cpp
+++ b/src/mbgl/map/raster_tile_data.cpp
@@ -6,8 +6,6 @@
 #include <mbgl/util/worker.hpp>
 #include <mbgl/util/work_request.hpp>
 
-#include <sstream>
-
 using namespace mbgl;
 
 RasterTileData::RasterTileData(const TileID& id_,
@@ -41,9 +39,7 @@ void RasterTileData::request(float pixelRatio,
             if (res.error->reason == Response::Error::Reason::NotFound) {
                 state = State::parsed;
             } else {
-                std::stringstream message;
-                message <<  "Failed to load [" << url << "]: " << res.error->message;
-                error = message.str();
+                error = std::make_exception_ptr(std::runtime_error(res.error->message));
                 state = State::obsolete;
             }
             callback();
@@ -68,9 +64,7 @@ void RasterTileData::request(float pixelRatio,
                 state = State::parsed;
                 bucket = std::move(result.get<std::unique_ptr<Bucket>>());
             } else {
-                std::stringstream message;
-                message << "Failed to parse [" << std::string(id) << "]: " << result.get<std::string>();
-                error = message.str();
+                error = result.get<std::exception_ptr>();
                 state = State::obsolete;
             }
 
diff --git a/src/mbgl/map/source.cpp b/src/mbgl/map/source.cpp
index f09d6db85a..a814e484ff 100644
--- a/src/mbgl/map/source.cpp
+++ b/src/mbgl/map/source.cpp
@@ -31,6 +31,7 @@
 #include <rapidjson/error/en.h>
 
 #include <algorithm>
+#include <sstream>
 
 namespace mbgl {
 
@@ -75,9 +76,7 @@ void Source::load() {
         req = nullptr;
 
         if (res.error) {
-            std::stringstream message;
-            message <<  "Failed to load [" << info.url << "]: " << res.error->message;
-            emitSourceLoadingFailed(message.str());
+            observer->onSourceError(*this, std::make_exception_ptr(std::runtime_error(res.error->message)));
             return;
         }
 
@@ -86,8 +85,8 @@ void Source::load() {
 
         if (d.HasParseError()) {
             std::stringstream message;
-            message << "Failed to parse [" << info.url << "]: " << d.GetErrorOffset() << " - " << rapidjson::GetParseError_En(d.GetParseError());
-            emitSourceLoadingFailed(message.str());
+            message << d.GetErrorOffset() << " - " << rapidjson::GetParseError_En(d.GetParseError());
+            observer->onSourceError(*this, std::make_exception_ptr(std::runtime_error(message.str())));
             return;
         }
 
@@ -98,8 +97,7 @@ void Source::load() {
         }
 
         loaded = true;
-
-        emitSourceLoaded();
+        observer->onSourceLoaded(*this);
     });
 }
 
@@ -166,8 +164,8 @@ bool Source::handlePartialTile(const TileID& id, Worker&) {
         return true;
     }
 
-    return tileData->parsePending([this]() {
-        emitTileLoaded(false);
+    return tileData->parsePending([this, id]() {
+        observer->onTileLoaded(*this, id, false);
     });
 }
 
@@ -464,12 +462,12 @@ void Source::onLowMemory() {
     cache.clear();
 }
 
-void Source::setObserver(Observer* observer) {
-    observer_ = observer;
+void Source::setObserver(Observer* observer_) {
+    observer = observer_;
 }
 
-void Source::tileLoadingCompleteCallback(const TileID& normalized_id, const TransformState& transformState, bool collisionDebug) {
-    auto it = tileDataMap.find(normalized_id);
+void Source::tileLoadingCompleteCallback(const TileID& id, const TransformState& transformState, bool collisionDebug) {
+    auto it = tileDataMap.find(id);
     if (it == tileDataMap.end()) {
         return;
     }
@@ -479,43 +477,13 @@ void Source::tileLoadingCompleteCallback(const TileID& normalized_id, const Tran
         return;
     }
 
-    if (tileData->getState() == TileData::State::obsolete && !tileData->getError().empty()) {
-        emitTileLoadingFailed(tileData->getError());
+    if (tileData->getState() == TileData::State::obsolete && tileData->getError()) {
+        observer->onTileError(*this, id, tileData->getError());
         return;
     }
 
     tileData->redoPlacement({ transformState.getAngle(), transformState.getPitch(), collisionDebug });
-    emitTileLoaded(true);
-}
-
-void Source::emitSourceLoaded() {
-    if (observer_) {
-        observer_->onSourceLoaded();
-    }
-}
-
-void Source::emitSourceLoadingFailed(const std::string& message) {
-    if (!observer_) {
-        return;
-    }
-
-    auto error = std::make_exception_ptr(util::SourceLoadingException(message));
-    observer_->onSourceLoadingFailed(error);
-}
-
-void Source::emitTileLoaded(bool isNewTile) {
-    if (observer_) {
-        observer_->onTileLoaded(isNewTile);
-    }
-}
-
-void Source::emitTileLoadingFailed(const std::string& message) {
-    if (!observer_) {
-        return;
-    }
-
-    auto error = std::make_exception_ptr(util::TileLoadingException(message));
-    observer_->onTileLoadingFailed(error);
+    observer->onTileLoaded(*this, id, true);
 }
 
 void Source::dumpDebugLogs() const {
diff --git a/src/mbgl/map/source.hpp b/src/mbgl/map/source.hpp
index dc939113b2..36613c2052 100644
--- a/src/mbgl/map/source.hpp
+++ b/src/mbgl/map/source.hpp
@@ -25,11 +25,11 @@ public:
     public:
         virtual ~Observer() = default;
 
-        virtual void onSourceLoaded() = 0;
-        virtual void onSourceLoadingFailed(std::exception_ptr error) = 0;
+        virtual void onSourceLoaded(Source&) {};
+        virtual void onSourceError(Source&, std::exception_ptr) {};
 
-        virtual void onTileLoaded(bool isNewTile) = 0;
-        virtual void onTileLoadingFailed(std::exception_ptr error) = 0;
+        virtual void onTileLoaded(Source&, const TileID&, bool /* isNewTile */) {};
+        virtual void onTileError(Source&, const TileID&, std::exception_ptr) {};
     };
 
     Source();
@@ -64,12 +64,6 @@ public:
 
 private:
     void tileLoadingCompleteCallback(const TileID&, const TransformState&, bool collisionDebug);
-
-    void emitSourceLoaded();
-    void emitSourceLoadingFailed(const std::string& message);
-    void emitTileLoaded(bool isNewTile);
-    void emitTileLoadingFailed(const std::string& message);
-
     bool handlePartialTile(const TileID &id, Worker &worker);
     bool findLoadedChildren(const TileID& id, int32_t maxCoveringZoom, std::forward_list<TileID>& retain);
     void findLoadedParent(const TileID& id, int32_t minCoveringZoom, std::forward_list<TileID>& retain);
@@ -82,7 +76,6 @@ private:
 
     double getZoom(const TransformState &state) const;
 
-
     // Stores the time when this source was most recently updated.
     TimePoint updated = TimePoint::min();
 
@@ -92,7 +85,9 @@ private:
     TileCache cache;
 
     std::unique_ptr<FileRequest> req;
-    Observer* observer_ = nullptr;
+
+    Observer nullObserver;
+    Observer* observer = &nullObserver;
 };
 
 } // namespace mbgl
diff --git a/src/mbgl/map/tile_data.cpp b/src/mbgl/map/tile_data.cpp
index bba7d7fe22..d9fd3f0047 100644
--- a/src/mbgl/map/tile_data.cpp
+++ b/src/mbgl/map/tile_data.cpp
@@ -1,5 +1,6 @@
 #include <mbgl/map/tile_data.hpp>
 #include <mbgl/renderer/debug_bucket.hpp>
+#include <mbgl/util/string.hpp>
 
 namespace mbgl {
 
@@ -26,7 +27,10 @@ const char* TileData::StateToString(const State state) {
 void TileData::dumpDebugLogs() const {
     Log::Info(Event::General, "TileData::id: %s", std::string(id).c_str());
     Log::Info(Event::General, "TileData::state: %s", TileData::StateToString(state));
-    Log::Info(Event::General, "TileData::error: %s", error.c_str());
+
+    if (error) {
+        Log::Info(Event::General, "TileData::error: %s", util::toString(error).c_str());
+    }
 }
 
 } // namespace mbgl
diff --git a/src/mbgl/map/tile_data.hpp b/src/mbgl/map/tile_data.hpp
index ed3426cf80..2bb11054a6 100644
--- a/src/mbgl/map/tile_data.hpp
+++ b/src/mbgl/map/tile_data.hpp
@@ -88,7 +88,7 @@ public:
         return state;
     }
 
-    std::string getError() const {
+    std::exception_ptr getError() const {
         return error;
     }
 
@@ -103,7 +103,7 @@ public:
 
 protected:
     std::atomic<State> state;
-    std::string error;
+    std::exception_ptr error;
 };
 
 } // namespace mbgl
diff --git a/src/mbgl/map/tile_worker.hpp b/src/mbgl/map/tile_worker.hpp
index 14f996c1b0..d25b6e50f2 100644
--- a/src/mbgl/map/tile_worker.hpp
+++ b/src/mbgl/map/tile_worker.hpp
@@ -33,8 +33,9 @@ public:
     std::unordered_map<std::string, std::unique_ptr<Bucket>> buckets;
 };
 
-using TileParseResult = mapbox::util::variant<TileParseResultBuckets, // success
-                                              std::string>;           // error
+using TileParseResult = mapbox::util::variant<
+    TileParseResultBuckets, // success
+    std::exception_ptr>;    // error
 
 class TileWorker : public util::noncopyable {
 public:
diff --git a/src/mbgl/map/vector_tile.cpp b/src/mbgl/map/vector_tile.cpp
index 00307f04a9..39e31da0d4 100644
--- a/src/mbgl/map/vector_tile.cpp
+++ b/src/mbgl/map/vector_tile.cpp
@@ -5,7 +5,6 @@
 #include <mbgl/storage/file_source.hpp>
 #include <mbgl/util/thread_context.hpp>
 
-#include <sstream>
 #include <utility>
 
 namespace mbgl {
@@ -194,9 +193,7 @@ std::unique_ptr<FileRequest> VectorTileMonitor::monitorTile(const GeometryTileMo
                 callback(nullptr, nullptr, res.modified, res.expires);
                 return;
             } else {
-                std::stringstream message;
-                message << "Failed to load [" << url << "]: " << res.error->message;
-                callback(std::make_exception_ptr(std::runtime_error(message.str())), nullptr, res.modified, res.expires);
+                callback(std::make_exception_ptr(std::runtime_error(res.error->message)), nullptr, res.modified, res.expires);
                 return;
             }
         }
diff --git a/src/mbgl/map/vector_tile_data.cpp b/src/mbgl/map/vector_tile_data.cpp
index 708ded02cc..3fe423b502 100644
--- a/src/mbgl/map/vector_tile_data.cpp
+++ b/src/mbgl/map/vector_tile_data.cpp
@@ -6,8 +6,6 @@
 #include <mbgl/style/style.hpp>
 #include <mbgl/storage/file_source.hpp>
 
-#include <sstream>
-
 namespace mbgl {
 
 VectorTileData::VectorTileData(const TileID& id_,
@@ -32,14 +30,10 @@ VectorTileData::VectorTileData(const TileID& id_,
                                                         Seconds modified_,
                                                         Seconds expires_) {
         if (err) {
-            try {
-                std::rethrow_exception(err);
-            } catch (const std::exception& e) {
-                error = e.what();
-                state = State::obsolete;
-                callback();
-                return;
-            }
+            error = err;
+            state = State::obsolete;
+            callback();
+            return;
         }
 
         if (!tile) {
@@ -86,9 +80,7 @@ VectorTileData::VectorTileData(const TileID& id_,
                     redoPlacement();
                 }
             } else {
-                std::stringstream message;
-                message << "Failed to parse [" << std::string(id) << "]: " << result.get<std::string>();
-                error = message.str();
+                error = result.get<std::exception_ptr>();
                 state = State::obsolete;
             }
 
@@ -130,9 +122,7 @@ bool VectorTileData::parsePending(std::function<void()> callback) {
                 redoPlacement();
             }
         } else {
-            std::stringstream message;
-            message << "Failed to parse [" << std::string(id) << "]: " << result.get<std::string>();
-            error = message.str();
+            error = result.get<std::exception_ptr>();
             state = State::obsolete;
         }
 
diff --git a/src/mbgl/sprite/sprite_parser.cpp b/src/mbgl/sprite/sprite_parser.cpp
index 59dce93241..1aa95310c5 100644
--- a/src/mbgl/sprite/sprite_parser.cpp
+++ b/src/mbgl/sprite/sprite_parser.cpp
@@ -106,7 +106,7 @@ SpriteParseResult parseSprite(const std::string& image, const std::string& json)
     try {
         raster = decodeImage(image);
     } catch (...) {
-        return std::string("Could not parse sprite image");
+        return std::current_exception();
     }
 
     JSDocument doc;
@@ -115,9 +115,9 @@ SpriteParseResult parseSprite(const std::string& image, const std::string& json)
     if (doc.HasParseError()) {
         std::stringstream message;
         message << "Failed to parse JSON: " << rapidjson::GetParseError_En(doc.GetParseError()) << " at offset " << doc.GetErrorOffset();
-        return message.str();
+        return std::make_exception_ptr(std::runtime_error(message.str()));
     } else if (!doc.IsObject()) {
-        return std::string("Sprite JSON root must be an object");
+        return std::make_exception_ptr(std::runtime_error("Sprite JSON root must be an object"));
     } else {
         for (JSValue::ConstMemberIterator itr = doc.MemberBegin(); itr != doc.MemberEnd(); ++itr) {
             const std::string name = { itr->name.GetString(), itr->name.GetStringLength() };
diff --git a/src/mbgl/sprite/sprite_parser.hpp b/src/mbgl/sprite/sprite_parser.hpp
index 468ba13c53..36e8947992 100644
--- a/src/mbgl/sprite/sprite_parser.hpp
+++ b/src/mbgl/sprite/sprite_parser.hpp
@@ -30,8 +30,8 @@ using Sprites = std::map<std::string, SpriteImagePtr>;
 
 
 using SpriteParseResult = mapbox::util::variant<
-    Sprites,      // success
-    std::string>; // error
+    Sprites,             // success
+    std::exception_ptr>; // error
 
 // Parses an image and an associated JSON file and returns the sprite objects.
 SpriteParseResult parseSprite(const std::string& image, const std::string& json);
diff --git a/src/mbgl/sprite/sprite_store.cpp b/src/mbgl/sprite/sprite_store.cpp
index f54f3f18fd..0c3a7d3ba0 100644
--- a/src/mbgl/sprite/sprite_store.cpp
+++ b/src/mbgl/sprite/sprite_store.cpp
@@ -9,7 +9,6 @@
 
 #include <cassert>
 #include <string>
-#include <sstream>
 
 namespace mbgl {
 
@@ -48,14 +47,11 @@ void SpriteStore::setURL(const std::string& url) {
         loader->jsonRequest = nullptr;
 
         if (res.error) {
-            std::stringstream message;
-            message << "Failed to load [" << jsonURL << "]: " << res.error->message;
-            emitSpriteLoadingFailed(message.str());
-            return;
+            observer->onSpriteError(std::make_exception_ptr(std::runtime_error(res.error->message)));
         } else {
             loader->json = res.data;
+            emitSpriteLoadedIfComplete();
         }
-        emitSpriteLoadedIfComplete();
     });
 
     loader->spriteRequest =
@@ -68,14 +64,11 @@ void SpriteStore::setURL(const std::string& url) {
             loader->spriteRequest = nullptr;
 
             if (res.error) {
-                std::stringstream message;
-                message << "Failed to load [" << spriteURL << "]: " << res.error->message;
-                emitSpriteLoadingFailed(message.str());
-                return;
+                observer->onSpriteError(std::make_exception_ptr(std::runtime_error(res.error->message)));
             } else {
                 loader->image = res.data;
+                emitSpriteLoadedIfComplete();
             }
-            emitSpriteLoadedIfComplete();
         });
 }
 
@@ -91,23 +84,12 @@ void SpriteStore::emitSpriteLoadedIfComplete() {
     if (result.is<Sprites>()) {
         loaded = true;
         setSprites(result.get<Sprites>());
-        if (observer) {
-            observer->onSpriteLoaded();
-        }
+        observer->onSpriteLoaded();
     } else {
-        emitSpriteLoadingFailed(result.get<std::string>());
+        observer->onSpriteError(result.get<std::exception_ptr>());
     }
 }
 
-void SpriteStore::emitSpriteLoadingFailed(const std::string& message) {
-    if (!observer) {
-        return;
-    }
-
-    auto error = std::make_exception_ptr(util::SpriteLoadingException(message));
-    observer->onSpriteLoadingFailed(error);
-}
-
 void SpriteStore::setObserver(Observer* observer_) {
     observer = observer_;
 }
diff --git a/src/mbgl/sprite/sprite_store.hpp b/src/mbgl/sprite/sprite_store.hpp
index 98b43ac8e4..7c5fe71802 100644
--- a/src/mbgl/sprite/sprite_store.hpp
+++ b/src/mbgl/sprite/sprite_store.hpp
@@ -18,8 +18,8 @@ public:
     public:
         virtual ~Observer() = default;
 
-        virtual void onSpriteLoaded() = 0;
-        virtual void onSpriteLoadingFailed(std::exception_ptr) = 0;
+        virtual void onSpriteLoaded() {};
+        virtual void onSpriteError(std::exception_ptr) {};
     };
 
     SpriteStore(float pixelRatio);
@@ -54,16 +54,15 @@ public:
 
 private:
     void _setSprite(const std::string&, const std::shared_ptr<const SpriteImage>& = nullptr);
-
     void emitSpriteLoadedIfComplete();
-    void emitSpriteLoadingFailed(const std::string& message);
 
     struct Loader;
     std::unique_ptr<Loader> loader;
 
     bool loaded = false;
 
-    Observer* observer = nullptr;
+    Observer nullObserver;
+    Observer* observer = &nullObserver;
 
     // Lock for sprites and dirty maps.
     std::mutex mutex;
diff --git a/src/mbgl/style/style.cpp b/src/mbgl/style/style.cpp
index 77a544714e..b1c867a692 100644
--- a/src/mbgl/style/style.cpp
+++ b/src/mbgl/style/style.cpp
@@ -17,6 +17,7 @@
 #include <mbgl/geometry/glyph_atlas.hpp>
 #include <mbgl/geometry/line_atlas.hpp>
 #include <mbgl/util/constants.hpp>
+#include <mbgl/util/string.hpp>
 #include <mbgl/platform/log.hpp>
 #include <mbgl/layer/background_layer.hpp>
 
@@ -304,73 +305,64 @@ void Style::onLowMemory() {
 
 void Style::setObserver(Observer* observer_) {
     assert(util::ThreadContext::currentlyOn(util::ThreadType::Map));
-    assert(!observer);
-
     observer = observer_;
 }
 
-void Style::onGlyphRangeLoaded() {
+void Style::onGlyphsLoaded(const std::string& fontStack, const GlyphRange& glyphRange) {
     shouldReparsePartialTiles = true;
-
-    emitTileDataChanged();
+    observer->onGlyphsLoaded(fontStack, glyphRange);
+    observer->onResourceLoaded();
 }
 
-void Style::onGlyphRangeLoadingFailed(std::exception_ptr error) {
-    emitResourceLoadingFailed(error);
+void Style::onGlyphsError(const std::string& fontStack, const GlyphRange& glyphRange, std::exception_ptr error) {
+    lastError = error;
+    Log::Error(Event::Style, "Failed to load glyph range %d-%d for font stack %s: %s",
+               glyphRange.first, glyphRange.second, fontStack.c_str(), util::toString(error).c_str());
+    observer->onGlyphsError(fontStack, glyphRange, error);
+    observer->onResourceError(error);
 }
 
-void Style::onSourceLoaded() {
-    emitTileDataChanged();
+void Style::onSourceLoaded(Source& source) {
+    observer->onSourceLoaded(source);
+    observer->onResourceLoaded();
 }
 
-void Style::onSourceLoadingFailed(std::exception_ptr error) {
-    emitResourceLoadingFailed(error);
+void Style::onSourceError(Source& source, std::exception_ptr error) {
+    lastError = error;
+    Log::Error(Event::Style, "Failed to load source %s: %s",
+               source.info.source_id.c_str(), util::toString(error).c_str());
+    observer->onSourceError(source, error);
+    observer->onResourceError(error);
 }
 
-void Style::onTileLoaded(bool isNewTile) {
+void Style::onTileLoaded(Source& source, const TileID& tileID, bool isNewTile) {
     if (isNewTile) {
         shouldReparsePartialTiles = true;
     }
 
-    emitTileDataChanged();
+    observer->onTileLoaded(source, tileID, isNewTile);
+    observer->onResourceLoaded();
 }
 
-void Style::onTileLoadingFailed(std::exception_ptr error) {
-    emitResourceLoadingFailed(error);
+void Style::onTileError(Source& source, const TileID& tileID, std::exception_ptr error) {
+    lastError = error;
+    Log::Error(Event::Style, "Failed to load tile %s for source %s: %s",
+               std::string(tileID).c_str(), source.info.source_id.c_str(), util::toString(error).c_str());
+    observer->onTileError(source, tileID, error);
+    observer->onResourceError(error);
 }
 
 void Style::onSpriteLoaded() {
     shouldReparsePartialTiles = true;
-    emitTileDataChanged();
-}
-
-void Style::onSpriteLoadingFailed(std::exception_ptr error) {
-    emitResourceLoadingFailed(error);
+    observer->onSpriteLoaded();
+    observer->onResourceLoaded();
 }
 
-void Style::emitTileDataChanged() {
-    assert(util::ThreadContext::currentlyOn(util::ThreadType::Map));
-
-    if (observer) {
-        observer->onTileDataChanged();
-    }
-}
-
-void Style::emitResourceLoadingFailed(std::exception_ptr error) {
-    assert(util::ThreadContext::currentlyOn(util::ThreadType::Map));
-
-    try {
-        if (error) {
-            lastError = error;
-            std::rethrow_exception(error);
-        }
-    } catch(const std::exception& e) {
-        Log::Error(Event::Style, "%s", e.what());
-    }
-
-    if (observer) {
-        observer->onResourceLoadingFailed(error);
-    }
+void Style::onSpriteError(std::exception_ptr error) {
+    lastError = error;
+    Log::Error(Event::Style, "Failed to load sprite: %s", util::toString(error).c_str());
+    observer->onSpriteError(error);
+    observer->onResourceError(error);
 }
 
 void Style::dumpDebugLogs() const {
diff --git a/src/mbgl/style/style.hpp b/src/mbgl/style/style.hpp
index ed5df04256..ba5da7435a 100644
--- a/src/mbgl/style/style.hpp
+++ b/src/mbgl/style/style.hpp
@@ -58,12 +58,18 @@ public:
     Style(MapData&);
     ~Style();
 
-    class Observer {
+    class Observer : public GlyphStore::Observer,
+                     public SpriteStore::Observer,
+                     public Source::Observer {
     public:
-        virtual ~Observer() = default;
-
-        virtual void onTileDataChanged() = 0;
-        virtual void onResourceLoadingFailed(std::exception_ptr error) = 0;
+        /**
+         * In addition to the individual glyph, sprite, and source events, the
+         * following "rollup" events are provided for convenience. They are
+         * strictly additive; e.g. when a source is loaded, both `onSourceLoaded`
+         * and `onResourceLoaded` will be called.
+         */
+         virtual void onResourceLoaded() {};
+         virtual void onResourceError(std::exception_ptr) {};
     };
 
     void setJSON(const std::string& data, const std::string& base);
@@ -115,25 +121,23 @@ private:
     std::vector<std::unique_ptr<StyleLayer>>::const_iterator findLayer(const std::string& layerID) const;
 
     // GlyphStore::Observer implementation.
-    void onGlyphRangeLoaded() override;
-    void onGlyphRangeLoadingFailed(std::exception_ptr error) override;
+    void onGlyphsLoaded(const std::string& fontStack, const GlyphRange&) override;
+    void onGlyphsError(const std::string& fontStack, const GlyphRange&, std::exception_ptr) override;
 
     // SpriteStore::Observer implementation.
     void onSpriteLoaded() override;
-    void onSpriteLoadingFailed(std::exception_ptr error) override;
+    void onSpriteError(std::exception_ptr) override;
 
     // Source::Observer implementation.
-    void onSourceLoaded() override;
-    void onSourceLoadingFailed(std::exception_ptr error) override;
-    void onTileLoaded(bool isNewTile) override;
-    void onTileLoadingFailed(std::exception_ptr error) override;
-
-    void emitTileDataChanged();
-    void emitResourceLoadingFailed(std::exception_ptr error);
+    void onSourceLoaded(Source&) override;
+    void onSourceError(Source&, std::exception_ptr) override;
+    void onTileLoaded(Source&, const TileID&, bool isNewTile) override;
+    void onTileError(Source&, const TileID&, std::exception_ptr) override;
 
     bool shouldReparsePartialTiles = false;
 
-    Observer* observer = nullptr;
+    Observer nullObserver;
+    Observer* observer = &nullObserver;
 
     std::exception_ptr lastError;
 
diff --git a/src/mbgl/text/glyph_pbf.cpp b/src/mbgl/text/glyph_pbf.cpp
index c14f52de7a..0c0626c0de 100644
--- a/src/mbgl/text/glyph_pbf.cpp
+++ b/src/mbgl/text/glyph_pbf.cpp
@@ -12,8 +12,6 @@
 #include <mbgl/util/token.hpp>
 #include <mbgl/util/url.hpp>
 
-#include <sstream>
-
 namespace {
 
 void parseGlyphPBF(mbgl::FontStack& stack, const std::string& data) {
@@ -65,8 +63,10 @@ namespace mbgl {
 
 GlyphPBF::GlyphPBF(GlyphStore* store,
                    const std::string& fontStack,
-                   const GlyphRange& glyphRange)
-    : parsed(false) {
+                   const GlyphRange& glyphRange,
+                   GlyphStore::Observer* observer_)
+    : parsed(false),
+      observer(observer_) {
     // Load the glyph set URL
     std::string url = util::replaceTokens(store->getURL(), [&](const std::string &name) -> std::string {
         if (name == "fontstack") return util::percentEncode(fontStack);
@@ -74,7 +74,7 @@ GlyphPBF::GlyphPBF(GlyphStore* store,
         return "";
     });
 
-    auto requestCallback = [this, store, fontStack, url](Response res) {
+    auto requestCallback = [this, store, fontStack, glyphRange](Response res) {
         if (res.stale) {
             // Only handle fresh responses.
             return;
@@ -82,12 +82,10 @@ GlyphPBF::GlyphPBF(GlyphStore* store,
         req = nullptr;
 
         if (res.error) {
-            std::stringstream message;
-            message <<  "Failed to load [" << url << "]: " << res.error->message;
-            emitGlyphPBFLoadingFailed(message.str());
+            observer->onGlyphsError(fontStack, glyphRange, std::make_exception_ptr(std::runtime_error(res.error->message)));
         } else {
             data = res.data;
-            parse(store, fontStack, url);
+            parse(store, fontStack, glyphRange);
         }
     };
 
@@ -97,7 +95,7 @@ GlyphPBF::GlyphPBF(GlyphStore* store,
 
 GlyphPBF::~GlyphPBF() = default;
 
-void GlyphPBF::parse(GlyphStore* store, const std::string& fontStack, const std::string& url) {
+void GlyphPBF::parse(GlyphStore* store, const std::string& fontStack, const GlyphRange& glyphRange) {
     assert(data);
     if (data->empty()) {
         // If there is no data, this means we either haven't
@@ -107,35 +105,13 @@ void GlyphPBF::parse(GlyphStore* store, const std::string& fontStack, const std:
 
     try {
         parseGlyphPBF(**store->getFontStack(fontStack), *data);
-    } catch (const std::exception& ex) {
-        std::stringstream message;
-        message <<  "Failed to parse [" << url << "]: " << ex.what();
-        emitGlyphPBFLoadingFailed(message.str());
+    } catch (...) {
+        observer->onGlyphsError(fontStack, glyphRange, std::current_exception());
         return;
     }
 
     parsed = true;
-
-    emitGlyphPBFLoaded();
-}
-
-void GlyphPBF::setObserver(Observer* observer_) {
-    observer = observer_;
-}
-
-void GlyphPBF::emitGlyphPBFLoaded() {
-    if (observer) {
-        observer->onGlyphPBFLoaded();
-    }
-}
-
-void GlyphPBF::emitGlyphPBFLoadingFailed(const std::string& message) {
-    if (!observer) {
-        return;
-    }
-
-    auto error = std::make_exception_ptr(util::GlyphRangeLoadingException(message));
-    observer->onGlyphPBFLoadingFailed(error);
+    observer->onGlyphsLoaded(fontStack, glyphRange);
 }
 
 } // namespace mbgl
diff --git a/src/mbgl/text/glyph_pbf.hpp b/src/mbgl/text/glyph_pbf.hpp
index d595298516..e0f65e7789 100644
--- a/src/mbgl/text/glyph_pbf.hpp
+++ b/src/mbgl/text/glyph_pbf.hpp
@@ -2,6 +2,7 @@
 #define MBGL_TEXT_GLYPH_PBF
 
 #include <mbgl/text/glyph.hpp>
+#include <mbgl/text/glyph_store.hpp>
 #include <mbgl/util/noncopyable.hpp>
 
 #include <atomic>
@@ -11,43 +12,30 @@
 
 namespace mbgl {
 
-class GlyphStore;
 class FontStack;
 class FileRequest;
 
 class GlyphPBF : private util::noncopyable {
 public:
-    class Observer {
-    public:
-        virtual ~Observer() = default;
-
-        virtual void onGlyphPBFLoaded() = 0;
-        virtual void onGlyphPBFLoadingFailed(std::exception_ptr error) = 0;
-    };
-
     GlyphPBF(GlyphStore* store,
              const std::string& fontStack,
-             const GlyphRange& glyphRange);
-    virtual ~GlyphPBF();
+             const GlyphRange&,
+             GlyphStore::Observer*);
+    ~GlyphPBF();
 
     bool isParsed() const {
         return parsed;
-    };
-
-    void setObserver(Observer* observer);
+    }
 
 private:
-    void emitGlyphPBFLoaded();
-    void emitGlyphPBFLoadingFailed(const std::string& message);
-
-    void parse(GlyphStore* store, const std::string& fontStack, const std::string& url);
+    void parse(GlyphStore*, const std::string& fontStack, const GlyphRange&);
 
     std::shared_ptr<const std::string> data;
     std::atomic<bool> parsed;
 
     std::unique_ptr<FileRequest> req;
 
-    Observer* observer = nullptr;
+    GlyphStore::Observer* observer = nullptr;
 };
 
 } // namespace mbgl
diff --git a/src/mbgl/text/glyph_store.cpp b/src/mbgl/text/glyph_store.cpp
index 047db17b7d..a3d1530e3d 100644
--- a/src/mbgl/text/glyph_store.cpp
+++ b/src/mbgl/text/glyph_store.cpp
@@ -7,6 +7,9 @@
 
 namespace mbgl {
 
+GlyphStore::GlyphStore() = default;
+GlyphStore::~GlyphStore() = default;
+
 void GlyphStore::requestGlyphRange(const std::string& fontStackName, const GlyphRange& range) {
     assert(util::ThreadContext::currentlyOn(util::ThreadType::Map));
 
@@ -18,10 +21,8 @@ void GlyphStore::requestGlyphRange(const std::string& fontStackName, const Glyph
         return;
     }
 
-    auto glyphPBF = std::make_unique<GlyphPBF>(this, fontStackName, range);
-    glyphPBF->setObserver(this);
-
-    rangeSets.emplace(range, std::move(glyphPBF));
+    rangeSets.emplace(range,
+        std::make_unique<GlyphPBF>(this, fontStackName, range, observer));
 }
 
 
@@ -66,18 +67,6 @@ util::exclusive<FontStack> GlyphStore::getFontStack(const std::string& fontStack
     return { it->second.get(), std::move(lock) };
 }
 
-void GlyphStore::onGlyphPBFLoaded() {
-    if (observer) {
-        observer->onGlyphRangeLoaded();
-    }
-}
-
-void GlyphStore::onGlyphPBFLoadingFailed(std::exception_ptr error) {
-    if (observer) {
-        observer->onGlyphRangeLoadingFailed(error);
-    }
-}
-
 void GlyphStore::setObserver(Observer* observer_) {
     observer = observer_;
 }
diff --git a/src/mbgl/text/glyph_store.hpp b/src/mbgl/text/glyph_store.hpp
index 4e665087ea..2236bce38c 100644
--- a/src/mbgl/text/glyph_store.hpp
+++ b/src/mbgl/text/glyph_store.hpp
@@ -3,7 +3,6 @@
 
 #include <mbgl/text/font_stack.hpp>
 #include <mbgl/text/glyph.hpp>
-#include <mbgl/text/glyph_pbf.hpp>
 #include <mbgl/util/exclusive.hpp>
 #include <mbgl/util/noncopyable.hpp>
 #include <mbgl/util/work_queue.hpp>
@@ -15,21 +14,23 @@
 
 namespace mbgl {
 
+class GlyphPBF;
+
 // The GlyphStore manages the loading and storage of Glyphs
 // and creation of FontStack objects. The GlyphStore lives
 // on the MapThread but can be queried from any thread.
-class GlyphStore : public GlyphPBF::Observer, private util::noncopyable {
+class GlyphStore : private util::noncopyable {
 public:
     class Observer {
     public:
         virtual ~Observer() = default;
 
-        virtual void onGlyphRangeLoaded() = 0;
-        virtual void onGlyphRangeLoadingFailed(std::exception_ptr error) = 0;
+        virtual void onGlyphsLoaded(const std::string& /* fontStack */, const GlyphRange&) {};
+        virtual void onGlyphsError(const std::string& /* fontStack */, const GlyphRange&, std::exception_ptr) {};
     };
 
-    GlyphStore() = default;
-    virtual ~GlyphStore() = default;
+    GlyphStore();
+    ~GlyphStore();
 
     util::exclusive<FontStack> getFontStack(const std::string& fontStack);
 
@@ -38,7 +39,7 @@ public:
     // made and when the glyph if finally parsed, it gets added to the respective
     // FontStack and a signal is emitted to notify the observers. This method
     // can be called from any thread.
-    bool hasGlyphRanges(const std::string& fontStackName, const std::set<GlyphRange>& glyphRanges);
+    bool hasGlyphRanges(const std::string& fontStack, const std::set<GlyphRange>& glyphRanges);
 
     void setURL(const std::string &url) {
         glyphURL = url;
@@ -48,10 +49,6 @@ public:
         return glyphURL;
     }
 
-    // GlyphPBF::Observer implementation.
-    void onGlyphPBFLoaded() override;
-    void onGlyphPBFLoadingFailed(std::exception_ptr error) override;
-
     void setObserver(Observer* observer);
 
 private:
@@ -67,7 +64,8 @@ private:
 
     util::WorkQueue workQueue;
 
-    Observer* observer = nullptr;
+    Observer nullObserver;
+    Observer* observer = &nullObserver;
 };
 
 } // namespace mbgl
diff --git a/src/mbgl/util/worker.cpp b/src/mbgl/util/worker.cpp
index d23831b440..49d1c2bc5b 100644
--- a/src/mbgl/util/worker.cpp
+++ b/src/mbgl/util/worker.cpp
@@ -18,18 +18,12 @@ public:
     void parseRasterTile(std::unique_ptr<RasterBucket> bucket,
                          const std::shared_ptr<const std::string> data,
                          std::function<void(RasterTileParseResult)> callback) {
-        PremultipliedImage image;
-
         try {
-            image = decodeImage(*data);
+            bucket->setImage(decodeImage(*data));
+            callback(RasterTileParseResult(std::move(bucket)));
         } catch (...) {
-            callback(RasterTileParseResult("error parsing raster image"));
-            return;
+            callback(std::current_exception());
         }
-
-        bucket->setImage(std::move(image));
-
-        callback(RasterTileParseResult(std::move(bucket)));
     }
 
     void parseGeometryTile(TileWorker* worker,
@@ -39,8 +33,8 @@ public:
                            std::function<void(TileParseResult)> callback) {
         try {
             callback(worker->parseAllLayers(std::move(layers), *tile, config));
-        } catch (const std::exception& ex) {
-            callback(TileParseResult(ex.what()));
+        } catch (...) {
+            callback(std::current_exception());
         }
     }
 
@@ -48,8 +42,8 @@ public:
                                         std::function<void(TileParseResult)> callback) {
         try {
             callback(worker->parsePendingLayers());
-        } catch (const std::exception& ex) {
-            callback(TileParseResult(ex.what()));
+        } catch (...) {
+            callback(std::current_exception());
         }
     }
 
diff --git a/src/mbgl/util/worker.hpp b/src/mbgl/util/worker.hpp
index 5ba477491d..d4660d9e7c 100644
--- a/src/mbgl/util/worker.hpp
+++ b/src/mbgl/util/worker.hpp
@@ -16,7 +16,7 @@ class GeometryTileLoader;
 
 using RasterTileParseResult = mapbox::util::variant<
     std::unique_ptr<Bucket>, // success
-    std::string>;            // error
+    std::exception_ptr>;     // error
 
 class Worker : public mbgl::util::noncopyable {
 public: