Make AppClass::transport_generator initialized at const-init time (#3665

)

This fixes the "bad_function_call" issue where it was invoked while null
(probably actually uninitialized). I'm not sure why it wasn't
initialized yet, since the compiler should have ensured that it was.
There is likely a compiler bug there but I'll need to dig in more. To
work around that, I made it a simple function pointer, which lambdas can
implicitly convert to, and all of that should be possible at const-init
time (basically, it should be burned into the binary image with nothing
happening at runtime). Unfortunately due to a different compiler bug,
(https://developercommunity.visualstudio.com/t/const-init-of-function-pointers-from-lambdas/1383098),
this requires putting a `+` in front of the lambda to explicitly decay
it to a function pointer, rather than relying on the implicit conversion.

src/js_app.hpp

@@ -59,7 +59,7 @@ class AppClass : public ClassDefinition<T, SharedApp> {
      * Generates instances of GenericNetworkTransport, eventually allowing Realm Object Store to perform network requests.
      * Exposed to allow other components (ex the RPCServer) to override the underlying implementation.
      */
-    static inline NetworkTransportFactory transport_generator = [] (ContextType ctx, typename NetworkTransport::Dispatcher eld) {
+    static inline NetworkTransportFactory transport_generator = +[] (ContextType ctx, typename NetworkTransport::Dispatcher eld) -> std::unique_ptr<app::GenericNetworkTransport> {
         return std::make_unique<NetworkTransport>(ctx, std::move(eld));
     };
 

src/js_network_transport.hpp

@@ -179,7 +179,9 @@ struct JavaScriptNetworkTransport : public app::GenericNetworkTransport {
     }
 
     using SendRequestHandler = void(ContextType m_ctx, const app::Request request, std::function<void(const app::Response)> completion_callback);
-    using NetworkTransportFactory = std::function<std::unique_ptr<app::GenericNetworkTransport>(ContextType, Dispatcher)>;
+
+    // This needs to be a plain function pointer to ensure that AppClass::transport_factory is correctly initialized on MSVC builds.
+    using NetworkTransportFactory = std::unique_ptr<app::GenericNetworkTransport>(*)(ContextType, Dispatcher);
 
     JavaScriptNetworkTransport(ContextType ctx, Dispatcher eld) : m_ctx(ctx), m_dispatcher(std::move(eld)) {};
 

src/jsc/rpc.cpp

@@ -46,7 +46,8 @@ using json = nlohmann::json;
 
 using RPCObjectID = u_int64_t;
 using RPCRequest = std::function<json(const json)>;
-using NetworkTransportFactory = typename js::JavaScriptNetworkTransport<jsc::Types>::NetworkTransportFactory;
+using NetworkTransport = js::JavaScriptNetworkTransport<jsc::Types>;
+using NetworkTransportFactory = typename NetworkTransport::NetworkTransportFactory;
 
 using Value = js::Value<jsc::Types>;
 using Accessor = realm::js::NativeAccessor<jsc::Types>;
@@ -347,7 +348,7 @@ RPCServerImpl::RPCServerImpl() {
 
     // Make the App use the RPC Network Transport from now on
     previous_transport_generator = AppClass::transport_generator;
-    AppClass::transport_generator = [] (jsc::Types::Context ctx, auto&& dispatcher) {
+    AppClass::transport_generator = [] (jsc::Types::Context ctx, NetworkTransport::Dispatcher dispatcher) -> std::unique_ptr<app::GenericNetworkTransport> {
         (void)dispatcher; // We don't need to use the dispatcher because JSC separately guarantees thread-safety.
         return std::make_unique<RPCNetworkTransport>(ctx);
     };