[core] Schedule invocation of onStyleImageMissing completion callback on the same thread

Before this change, ImageManger's 'done' callback for onStyleImageMissing observer
notification that was created on renderer thread, could be called from different thread,
therefore, is not thread safe. For example, on Android platform, callback was invoked
from UI thread. This change makes callback to be scheduled on originating thread.

1 files changed, 20 insertions, 8 deletions

diff --git a/src/mbgl/renderer/image_manager.cpp b/src/mbgl/renderer/image_manager.cpp
index 8e584ffd34..860de487e7 100644
--- a/src/mbgl/renderer/image_manager.cpp
+++ b/src/mbgl/renderer/image_manager.cpp
@@ -1,4 +1,6 @@
 #include <mbgl/renderer/image_manager.hpp>
+#include <mbgl/actor/actor.hpp>
+#include <mbgl/actor/scheduler.hpp>
 #include <mbgl/util/logging.hpp>
 #include <mbgl/gfx/context.hpp>
 #include <mbgl/renderer/image_manager_observer.hpp>
@@ -128,7 +130,7 @@ void ImageManager::removeRequestor(ImageRequestor& requestor) {
 
 void ImageManager::notifyIfMissingImageAdded() {
     for (auto it = missingImageRequestors.begin(); it != missingImageRequestors.end();) {
-        if (it->second.callbacksRemaining == 0) {
+        if (it->second.callbacks.empty()) {
             notify(*it->first, it->second.pair);
             it = missingImageRequestors.erase(it);
         } else {
@@ -162,19 +164,29 @@ void ImageManager::checkMissingAndNotify(ImageRequestor& requestor, const ImageR
     if (missing > 0) {
         ImageRequestor* requestorPtr = &requestor;
 
-        missingImageRequestors.emplace(requestorPtr, MissingImageRequestPair { std::move(pair), missing });
+        auto emplaced = missingImageRequestors.emplace(requestorPtr, MissingImageRequestPair { pair, {} });
+        assert(emplaced.second);
 
         for (const auto& dependency : pair.first) {
             auto it = images.find(dependency.first);
             if (it == images.end()) {
                 assert(observer != nullptr);
-                observer->onStyleImageMissing(dependency.first, [this, requestorPtr]() {
-                    auto requestorIt = missingImageRequestors.find(requestorPtr);
-                    if (requestorIt != missingImageRequestors.end()) {
-                        assert(requestorIt->second.callbacksRemaining > 0);
-                        requestorIt->second.callbacksRemaining--;
-                    }
+                auto callback = std::make_unique<ActorCallback>(
+                        *Scheduler::GetCurrent(),
+                        [this, requestorPtr, imageId = dependency.first] {
+                            auto requestorIt = missingImageRequestors.find(requestorPtr);
+                            if (requestorIt != missingImageRequestors.end()) {
+                                assert(requestorIt->second.callbacks.find(imageId) != requestorIt->second.callbacks.end());
+                                requestorIt->second.callbacks.erase(imageId);
+                            }
+                        });
+
+                auto actorRef = callback->self();
+                emplaced.first->second.callbacks.emplace(dependency.first, std::move(callback));
+                observer->onStyleImageMissing(dependency.first, [actorRef]() mutable {
+                    actorRef.invoke(&Callback::operator());
                 });
+
             }
         }