Cancel pending style request when loading style JSON (#3709)

Co-authored-by: Tim Sylvester <tim@wetdogweather.com>

test/src/mbgl/test/delayed_file_source.cpp

@@ -0,0 +1,32 @@
+#include <mbgl/test/delayed_file_source.hpp>
+
+namespace mbgl {
+
+DelayedFileSource::AsyncRequestImpl::AsyncRequestImpl(DelayedFileSource& fs, Callback callback_)
+    : fileSource(fs),
+      callback(std::move(callback_)) {}
+
+void DelayedFileSource::AsyncRequestImpl::respond() {
+    if (callback) {
+        callback(*fileSource.response);
+    }
+}
+
+DelayedFileSource::DelayedFileSource()
+    : StubFileSource(StubFileSource::ResponseType::Synchronous) {}
+
+std::unique_ptr<AsyncRequest> DelayedFileSource::request(const Resource&, Callback callback) {
+    auto req = std::make_unique<AsyncRequestImpl>(*this, std::move(callback));
+    pendingRequest = req.get();
+    return req;
+}
+
+void DelayedFileSource::respondToRequest(const std::string& data) {
+    response = Response();
+    response->data = std::make_shared<std::string>(data);
+    if (pendingRequest) {
+        pendingRequest->respond();
+    }
+}
+
+} // namespace mbgl

test/src/mbgl/test/delayed_file_source.hpp

@@ -0,0 +1,43 @@
+#pragma once
+
+#include <mbgl/storage/file_source.hpp>
+#include <mbgl/util/async_request.hpp>
+#include <mbgl/test/stub_file_source.hpp>
+#include <memory>
+#include <string>
+#include <optional>
+
+namespace mbgl {
+
+/*
+  This is a file source that allows control of when a request is responded to.
+*/
+class DelayedFileSource : public StubFileSource {
+public:
+    DelayedFileSource();
+    ~DelayedFileSource() override = default;
+
+    std::unique_ptr<AsyncRequest> request(const Resource&, Callback) override;
+
+    // Custom method to respond to pending request
+    void respondToRequest(const std::string& data);
+
+private:
+    class AsyncRequestImpl : public AsyncRequest {
+    public:
+        using Callback = std::function<void(const Response&)>;
+
+        AsyncRequestImpl(DelayedFileSource& fs, Callback callback_);
+        void respond();
+
+    private:
+        DelayedFileSource& fileSource;
+        Callback callback;
+    };
+
+    friend class AsyncRequestImpl;
+    std::optional<Response> response;
+    AsyncRequestImpl* pendingRequest = nullptr;
+};
+
+} // namespace mbgl