Allow disabling ad-hoc socket spawning

We'll need this for handling `IAudioProcessor` method calls in VST3. We
basically want a `Vst3MessageHandler` per `IAudioProcessor` instance,
but without the additional socket spawning or extra thread.

src/common/communication/common.h

@@ -447,8 +447,13 @@ class SocketHandler {
  *
  * @tparam Thread The thread implementation to use. On the Linux side this
  *   should be `std::jthread` and on the Wine side this should be `Win32Thread`.
+ * @tparam ad_hoc_sockets Whether new sockets should be created on demand to be
+ *   able to handle multiple function calls at the same time. If this is set to
+ *   false, then simultaneous `send_message()` calls will have to wait for the
+ *   earlier call to finish. This also means that the listening side does not
+ *   have to spawn a thread to constantly listen for new connections.
  */
-template <typename Thread>
+template <typename Thread, bool ad_hoc_sockets>
 class AdHocSocketHandler {
    protected:
     /**
@@ -530,11 +535,15 @@ class AdHocSocketHandler {
      */
     template <typename T, typename F>
     T send(F callback) {
+        // When the ad-hoc socket spawning bheaviour is disabled we always want
+        // to acquire the lock, waiting if necessary.
         // XXX: Maybe at some point we should benchmark how often this
         //      ad hoc socket spawning mechanism gets used. If some hosts
         //      for instance consistently and repeatedly trigger this then
         //      we might be able to do some optimizations there.
-        std::unique_lock lock(write_mutex, std::try_to_lock);
+        std::unique_lock lock =
+            ad_hoc_sockets ? std::unique_lock(write_mutex, std::try_to_lock)
+                           : std::unique_lock(write_mutex);
         if (lock.owns_lock()) {
             // This was used to always block when sending the first message,
             // because the other side may not be listening for additional
@@ -602,71 +611,90 @@ class AdHocSocketHandler {
     void receive_multi(std::optional<std::reference_wrapper<Logger>> logger,
                        F primary_callback,
                        G secondary_callback) {
-        // As described above we'll handle incoming requests for `socket` on
-        // this thread. We'll also listen for incoming connections on `endpoint`
-        // on another thread. For any incoming connection we'll spawn a new
-        // thread to handle the request. When `socket` closes and this loop
-        // breaks, the listener and any still active threads will be cleaned up
-        // before this function exits.
-        boost::asio::io_context secondary_context{};
+        if constexpr (ad_hoc_sockets) {
+            // As described above we'll handle incoming requests for `socket` on
+            // this thread. We'll also listen for incoming connections on
+            // `endpoint` on another thread. For any incoming connection we'll
+            // spawn a new thread to handle the request. When `socket` closes
+            // and this loop breaks, the listener and any still active threads
+            // will be cleaned up before this function exits.
+            boost::asio::io_context secondary_context{};
 
-        // The previous acceptor has already been shut down by
-        // `AdHocSocketHandler::connect()`
-        acceptor.emplace(secondary_context, endpoint);
+            // The previous acceptor has already been shut down by
+            // `AdHocSocketHandler::connect()`
+            acceptor.emplace(secondary_context, endpoint);
 
-        // This works the exact same was as `active_plugins` and
-        // `next_plugin_id` in `GroupBridge`
-        std::map<size_t, Thread> active_secondary_requests{};
-        std::atomic_size_t next_request_id{};
-        std::mutex active_secondary_requests_mutex{};
-        accept_requests(
-            *acceptor, logger,
-            [&](boost::asio::local::stream_protocol::socket secondary_socket) {
-                const size_t request_id = next_request_id.fetch_add(1);
+            // This works the exact same was as `active_plugins` and
+            // `next_plugin_id` in `GroupBridge`
+            std::map<size_t, Thread> active_secondary_requests{};
+            std::atomic_size_t next_request_id{};
+            std::mutex active_secondary_requests_mutex{};
+            accept_requests(
+                *acceptor, logger,
+                [&](boost::asio::local::stream_protocol::socket
+                        secondary_socket) {
+                    const size_t request_id = next_request_id.fetch_add(1);
 
-                // We have to make sure to keep moving these sockets into the
-                // threads that will handle them
-                std::lock_guard lock(active_secondary_requests_mutex);
-                active_secondary_requests[request_id] = Thread(
-                    [&, request_id](boost::asio::local::stream_protocol::socket
-                                        secondary_socket) {
-                        secondary_callback(secondary_socket);
+                    // We have to make sure to keep moving these sockets into
+                    // the threads that will handle them
+                    std::lock_guard lock(active_secondary_requests_mutex);
+                    active_secondary_requests[request_id] = Thread(
+                        [&,
+                         request_id](boost::asio::local::stream_protocol::socket
+                                         secondary_socket) {
+                            secondary_callback(secondary_socket);
 
-                        // When we have processed this request, we'll join the
-                        // thread again with the thread that's handling
-                        // `secondary_context`
-                        boost::asio::post(secondary_context, [&, request_id]() {
-                            std::lock_guard lock(
-                                active_secondary_requests_mutex);
+                            // When we have processed this request, we'll join
+                            // the thread again with the thread that's handling
+                            // `secondary_context`
+                            boost::asio::post(
+                                secondary_context, [&, request_id]() {
+                                    std::lock_guard lock(
+                                        active_secondary_requests_mutex);
 
-                            // The join is implicit because we're using
-                            // `std::jthread`/`Win32Thread`
-                            active_secondary_requests.erase(request_id);
-                        });
-                    },
-                    std::move(secondary_socket));
-            });
+                                    // The join is implicit because we're using
+                                    // `std::jthread`/`Win32Thread`
+                                    active_secondary_requests.erase(request_id);
+                                });
+                        },
+                        std::move(secondary_socket));
+                });
 
-        Thread secondary_requests_handler([&]() { secondary_context.run(); });
+            Thread secondary_requests_handler(
+                [&]() { secondary_context.run(); });
 
-        // Now we'll handle reads on the primary socket in a loop until the
-        // socket shuts down
-        while (true) {
-            try {
-                primary_callback(socket);
-            } catch (const boost::system::system_error&) {
-                // This happens when the sockets got closed because the plugin
-                // is being shut down
-                break;
+            // Now we'll handle reads on the primary socket in a loop until the
+            // socket shuts down
+            while (true) {
+                try {
+                    primary_callback(socket);
+                } catch (const boost::system::system_error&) {
+                    // This happens when the sockets got closed because the
+                    // plugin is being shut down
+                    break;
+                }
+            }
+
+            // After the primary socket gets terminated (during shutdown) we'll
+            // make sure all outstanding jobs have been processed and then drop
+            // all work from the IO context
+            std::lock_guard lock(active_secondary_requests_mutex);
+            secondary_context.stop();
+            acceptor.reset();
+        } else {
+            // If ad-hoc sockets are disabled, then we only care about the
+            // primary socket loop (e.g. when handing calls to
+            // `IAudioProcessor`, where we want the exact same behaviour as when
+            // handling normal VST3 messages but we don't want to spawn
+            // additional threads for perofrmance considerations)
+            while (true) {
+                try {
+                    primary_callback(socket);
+                } catch (const boost::system::system_error&) {
+                    break;
+                }
             }
         }
-
-        // After the primary socket gets terminated (during shutdown) we'll make
-        // sure all outstanding jobs have been processed and then drop all work
-        // from the IO context
-        std::lock_guard lock(active_secondary_requests_mutex);
-        secondary_context.stop();
-        acceptor.reset();
     }
 
     /**

src/common/communication/vst2.h

@@ -106,7 +106,7 @@ class DefaultDataConverter {
  *   should be `std::jthread` and on the Wine side this should be `Win32Thread`.
  */
 template <typename Thread>
-class EventHandler : public AdHocSocketHandler<Thread> {
+class EventHandler : public AdHocSocketHandler<Thread, true> {
    public:
     /**
      * Sets up a single main socket for this type of events. The sockets won't
@@ -125,7 +125,7 @@ class EventHandler : public AdHocSocketHandler<Thread> {
     EventHandler(boost::asio::io_context& io_context,
                  boost::asio::local::stream_protocol::endpoint endpoint,
                  bool listen)
-        : AdHocSocketHandler<Thread>(io_context, endpoint, listen) {}
+        : AdHocSocketHandler<Thread, true>(io_context, endpoint, listen) {}
 
     /**
      * Serialize and send an event over a socket. This is used for both the host

src/common/communication/vst3.h

@@ -38,9 +38,14 @@
  * @tparam Thread The thread implementation to use. On the Linux side this
  *   should be `std::jthread` and on the Wine side this should be `Win32Thread`.
  * @tparam Request Either `ControlRequest` or `CallbackRequest`.
+ * @tparam ad_hoc_sockets Whether new sockets should be created on demand to be
+ *   able to handle multiple function calls at the same time. If this is set to
+ *   false, then simultaneous `send_message()` calls will have to wait for the
+ *   earlier call to finish. This also means that the listening side does not
+ *   have to spawn a thread to constantly listen for new connections.
  */
-template <typename Thread, typename Request>
-class Vst3MessageHandler : public AdHocSocketHandler<Thread> {
+template <typename Thread, typename Request, bool ad_hoc_sockets>
+class Vst3MessageHandler : public AdHocSocketHandler<Thread, ad_hoc_sockets> {
    public:
     /**
      * Sets up a single main socket for this type of events. The sockets won't
@@ -59,7 +64,9 @@ class Vst3MessageHandler : public AdHocSocketHandler<Thread> {
     Vst3MessageHandler(boost::asio::io_context& io_context,
                        boost::asio::local::stream_protocol::endpoint endpoint,
                        bool listen)
-        : AdHocSocketHandler<Thread>(io_context, endpoint, listen) {}
+        : AdHocSocketHandler<Thread, ad_hoc_sockets>(io_context,
+                                                     endpoint,
+                                                     listen) {}
 
     /**
      * Serialize and send an event over a socket and return the appropriate
@@ -272,12 +279,12 @@ class Vst3Sockets : public Sockets {
      * This will be listened on by the Wine plugin host when it calls
      * `receive_multi()`.
      */
-    Vst3MessageHandler<Thread, ControlRequest> host_vst_control;
+    Vst3MessageHandler<Thread, ControlRequest, true> host_vst_control;
 
     /**
      * For sending callbacks from the plugin back to the host. After we have a
      * better idea of what our communication model looks like we'll probably
      * want to provide an abstraction similar to `EventHandler`.
      */
-    Vst3MessageHandler<Thread, CallbackRequest> vst_host_callback;
+    Vst3MessageHandler<Thread, CallbackRequest, true> vst_host_callback;
 };