Implement CLAP callback requests

src/wine-host/bridges/clap-impls/host-proxy.cpp

@@ -65,5 +65,28 @@ clap_host_proxy::host_request_process(const struct clap_host* host) {
 
 void CLAP_ABI
 clap_host_proxy::host_request_callback(const struct clap_host* host) {
-    // TODO: Implement
+    assert(host && host->host_data);
+    auto self = static_cast<clap_host_proxy*>(host->host_data);
+
+    // TODO: Log
+
+    // Only schedule a `clap_plugin::on_main_thread()` call if we don't already
+    // have a pending one. This limits the number of unnecessarily stacked
+    // calls.
+    bool expected = false;
+    if (self->has_pending_host_callbacks_.compare_exchange_strong(expected,
+                                                                  true)) {
+        // We're acquiring a lock on the instance and then move it into the task
+        // to prevent this instance from being removed before this callback has
+        // been run
+        auto instance_lock =
+            self->bridge_.get_instance(self->owner_instance_id());
+        self->bridge_.main_context_.schedule_task(
+            [self, instance_lock = std::move(instance_lock)]() {
+                const auto& [instance, _] = instance_lock;
+                self->has_pending_host_callbacks_.store(false);
+
+                instance.plugin->on_main_thread(instance.plugin.get());
+            });
+    }
 }

src/wine-host/bridges/clap-impls/host-proxy.h

@@ -16,6 +16,8 @@
 
 #pragma once
 
+#include <atomic>
+
 #include <clap/host.h>
 
 #include "../../common/serialization/clap/plugin-factory.h"
@@ -78,4 +80,12 @@ class clap_host_proxy {
      * the start of the struct and directly casting the `clap_host_t*`.
      */
     const clap_host_t host_vtable_;
+
+    /**
+     * Keeps track of whether there are pending host callbacks. Used to prevent
+     * calling `clap_plugin::on_main_thread()` multiple times in a row when the
+     * plugin calls `clap_host::request_callback()` multiple times before
+     * `clap_plugin::on_main_thread()` is called.
+     */
+    std::atomic_bool has_pending_host_callbacks_ = false;
 };