[core] Don't allow style mutations to be overwritten by revalidation

* Once we get a fresh style, stop revalidating.
* If the style is mutated, stop revalidating and preserve the existing mutations.

test/src/mbgl/test/fake_file_source.hpp

@@ -0,0 +1,60 @@
+#pragma once
+
+#include <mbgl/storage/file_source.hpp>
+
+#include <list>
+
+namespace mbgl {
+
+/*
+   FakeFileSource is similar to StubFileSource, but it follows a post hoc, "push" model rather than
+   a pre hoc, "pull" model. You pass it to code that makes requests, it records what requests are
+   made, then you can examine them, make assertions about them, and respond as desired.
+
+   This is particularly useful if you want to simulate multiple responses, e.g. as part of a resource
+   revalidation flow. StubFileSource allows only a single response.
+*/
+class FakeFileSource : public FileSource {
+public:
+    class FakeFileRequest : public AsyncRequest {
+    public:
+        Resource resource;
+        Callback callback;
+
+        std::list<FakeFileRequest*>& list;
+        std::list<FakeFileRequest*>::iterator link;
+
+        FakeFileRequest(Resource resource_, Callback callback_, std::list<FakeFileRequest*>& list_)
+            : resource(std::move(resource_)),
+              callback(std::move(callback_)),
+              list(list_),
+              link((list.push_back(this), std::prev(list.end()))) {
+        }
+
+        ~FakeFileRequest() override {
+            list.erase(link);
+        }
+    };
+
+    std::unique_ptr<AsyncRequest> request(const Resource& resource, Callback callback) override {
+        return std::make_unique<FakeFileRequest>(resource, callback, requests);
+    }
+
+    bool respond(Resource::Kind kind, const Response& response) {
+        auto it = std::find_if(requests.begin(), requests.end(), [&] (FakeFileRequest* request) {
+            return request->resource.kind == kind;
+        });
+
+        if (it != requests.end()) {
+            // Copy the callback, in case calling it deallocates the AsyncRequest.
+            Callback callback_ = (*it)->callback;
+            callback_(response);
+        }
+
+        return it != requests.end();
+    }
+
+    std::list<FakeFileRequest*> requests;
+};
+
+} // namespace mbgl