Implement the event loop for CLAP

It cannot yet handle any tasks though.

src/wrapper/clap/plugin.rs

@@ -2,16 +2,18 @@ use clap_sys::host::clap_host;
 use clap_sys::plugin::clap_plugin;
 use clap_sys::process::{clap_process, clap_process_status};
 use crossbeam::atomic::AtomicCell;
+use crossbeam::queue::ArrayQueue;
 use parking_lot::RwLock;
 use std::collections::VecDeque;
 use std::ffi::c_void;
 use std::os::raw::c_char;
 use std::ptr;
 use std::sync::atomic::AtomicU32;
+use std::thread::{self, ThreadId};
 
 use super::context::WrapperProcessContext;
 use super::descriptor::PluginDescriptor;
-use crate::event_loop::{EventLoop, MainThreadExecutor};
+use crate::event_loop::{EventLoop, MainThreadExecutor, TASK_QUEUE_CAPACITY};
 use crate::plugin::{BufferConfig, BusConfig, ClapPlugin};
 use crate::NoteEvent;
 
@@ -45,6 +47,18 @@ pub struct Plugin<P: ClapPlugin> {
     /// Needs to be boxed because the plugin object is supposed to contain a static reference to
     /// this.
     plugin_descriptor: Box<PluginDescriptor<P>>,
+
+    /// A queue of tasks that still need to be performed. Because CLAP lets the plugin request a
+    /// host callback directly, we don't need to use the OsEventLoop we use in our other plugin
+    /// implementations. Instead, we'll post tasks to this queue, ask the host to call
+    /// [Self::on_main_thread] on the main thread, and then continue to pop tasks off this queue
+    /// there until it is empty.
+    tasks: ArrayQueue<Task>,
+    /// The ID of the main thread. In practice this is the ID of the thread that created this
+    /// object.
+    ///
+    /// TODO: If the host supports the ThreadCheck extension, we should use that instead.
+    main_thread_id: ThreadId,
 }
 
 /// Send+Sync wrapper around clap_host.
@@ -62,22 +76,35 @@ pub enum Task {
 /// Because CLAP has this [clap_host::request_host_callback()] function, we don't need to use
 /// `OsEventLoop` and can instead just request a main thread callback directly.
 impl<P: ClapPlugin> EventLoop<Task, Plugin<P>> for Plugin<P> {
-    fn new_and_spawn(executor: std::sync::Weak<Self>) -> Self {
+    fn new_and_spawn(_executor: std::sync::Weak<Self>) -> Self {
         panic!("What are you doing");
     }
 
     fn do_maybe_async(&self, task: Task) -> bool {
-        todo!()
+        if self.is_main_thread() {
+            unsafe { self.execute(task) };
+            true
+        } else {
+            let success = self.tasks.push(task).is_ok();
+            if success {
+                // CLAP lets us use the host's event loop instead of having to implement our own
+                let host = self.host_callback.0;
+                unsafe { ((*host).request_callback)(host) };
+            }
+
+            success
+        }
     }
 
     fn is_main_thread(&self) -> bool {
-        todo!()
+        // TODO: Use the `thread_check::is_main_thread` extension method if that's available
+        thread::current().id() == self.main_thread_id
     }
 }
 
 impl<P: ClapPlugin> MainThreadExecutor<Task> for Plugin<P> {
     unsafe fn execute(&self, task: Task) {
-        todo!()
+        todo!("Implement latency changes for CLAP")
     }
 }
 
@@ -120,6 +147,9 @@ impl<P: ClapPlugin> Plugin<P> {
 
             host_callback: HostCallback(host_callback),
             plugin_descriptor,
+
+            tasks: ArrayQueue::new(TASK_QUEUE_CAPACITY),
+            main_thread_id: thread::current().id(),
         }
     }
 
@@ -142,7 +172,7 @@ impl<P: ClapPlugin> Plugin<P> {
     unsafe extern "C" fn activate(
         plugin: *const clap_plugin,
         sample_rate: f64,
-        min_frames_count: u32,
+        _min_frames_count: u32,
         max_frames_count: u32,
     ) -> bool {
         let plugin = &*(plugin as *const Self);
@@ -199,6 +229,12 @@ impl<P: ClapPlugin> Plugin<P> {
     }
 
     unsafe extern "C" fn on_main_thread(plugin: *const clap_plugin) {
-        todo!();
+        let plugin = &*(plugin as *const Self);
+
+        // [Self::do_maybe_async] posts a task to the queue and asks the host to call this function
+        // on the main thread, so once that's done we can just handle all requests here
+        while let Some(task) = plugin.tasks.pop() {
+            plugin.execute(task);
+        }
     }
 }