Fix test for races with multiple parallel requests (#3891)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

include/mbgl/style/sources/geojson_source.hpp

@@ -82,6 +82,7 @@ protected:
 private:
     std::optional<std::string> url;
     std::unique_ptr<AsyncRequest> req;
+    std::atomic<uint64_t> requestGeneration{0};
     std::shared_ptr<Scheduler> sequencedScheduler;
     mapbox::base::WeakPtrFactory<Source> weakFactory{this};
     // Do not add members here, see `WeakPtrFactory`

src/mbgl/style/sources/geojson_source.cpp

@@ -99,10 +99,12 @@ void GeoJSONSource::loadDescription(FileSource& fileSource) {
                     return makeMutable<Impl>(current, std::move(geoJSONData));
                 },
                 /* onImplReady */
-                [this, self = makeWeakPtr(), capturedReq = req.get()](Immutable<Source::Impl> newImpl) {
-                    assert(capturedReq);
+                [this, self = makeWeakPtr(), capturedReqGeneration = ++requestGeneration](
+                    Immutable<Source::Impl> newImpl) {
+                    assert(capturedReqGeneration);
                     if (auto guard = self.lock(); self) {
-                        if (capturedReq == req.get()) { // If a new request is being processed, ignore this impl.
+                        if (capturedReqGeneration ==
+                            requestGeneration) { // If a new request is being processed, ignore this impl.
                             baseImpl = std::move(newImpl);
                             loaded = true;
                             observer->onSourceLoaded(*this);