Add a Linux-specific event loop

Implementations for Windows and macOS should be fairly similar, but
using the OS' message loop instad of a thread.

src/context.rs

@@ -0,0 +1,84 @@
+// nih-plug: plugins, but rewritten in Rust
+// Copyright (C) 2022 Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+//! Different contexts the plugin can use to make callbacks to the host in different...contexts.
+
+use std::sync::Arc;
+
+#[cfg(all(target_family = "unix", not(target_os = "macos")))]
+mod linux;
+
+#[cfg(all(target_family = "unix", not(target_os = "macos")))]
+pub(crate) use linux::LinuxEventLoop as OsEventLoop;
+
+// TODO: ProcessContext for parameter automation and sending events
+// TODO: GuiContext for GUI parameter automation and resizing
+
+pub(crate) const TASK_QUEUE_CAPACITY: usize = 512;
+
+/// General callbacks the plugin can make during its lifetime. This is passed to the plugin during
+/// [Plugin::initialize].
+///
+/// # Safety
+///
+/// This context is passed to be stored by the plugin, and it may thus not outlive the wrapper.
+/// Hence the use of reference counted smart pointers. The implementing wrapper needs to be able to
+/// handle concurrent requests, and it should perform the actual callback within
+/// [MainThreadQueue::do_maybe_async].
+pub unsafe trait ProcessContext {
+    /// Update the current latency of the plugin. If the plugin is currently processing audio, then
+    /// this may cause audio playback to be restarted.
+    fn set_latency_samples(self: &Arc<Self>, samples: u32);
+}
+
+/// A trait describing the functionality of the platform-specific event loop that can execute tasks
+/// of type `T` in executor `E`. Posting a task to the queue should be realtime safe. This thread
+/// queue should be created during the wrapper's initial initialization on the main thread.
+///
+/// This is never used generically, but having this as a trait will cause any missing functions on
+/// an implementation to show up as compiler errors even when using a different platform.
+///
+/// TODO: At some point rethink the design to make it possible to have a singleton message queue for
+///       all instances of a plugin.
+pub(crate) trait EventLoop<T, E>
+where
+    T: Send,
+    E: MainThreadExecutor<T>,
+{
+    /// Create a main thread tasks queue for the given executor. The thread this is called on will
+    /// be designated as the main thread, so this should be called when constructing the wrapper.
+    ///
+    /// TODO: Spawn, and update docs
+    fn new_and_spawn(executor: Arc<E>) -> Self;
+
+    /// Either post the function to a queue so it can be run later from the main thread using a
+    /// timer, or run the function directly if this is the main thread. This needs to be callable at
+    /// any time withotu blocking.
+    ///
+    /// If the task queue was full, then this will return false.
+    #[must_use]
+    fn do_maybe_async(&self, task: T) -> bool;
+
+    /// Whether the calling thread is the even loop's main thread. This is usually the thread the
+    /// event loop instance wel initialized on.
+    fn is_main_thread(&self) -> bool;
+}
+
+/// Something that can execute tasks of type `T`.
+pub(crate) trait MainThreadExecutor<T>: Send + Sync {
+    /// Execute a task on the current thread.
+    fn execute(&self, task: T);
+}

src/context/linux.rs

@@ -0,0 +1,127 @@
+// nih-plug: plugins, but rewritten in Rust
+// Copyright (C) 2022 Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+//! An event loop impelemntation for Linux. APIs on Linux are generally thread safe, so the context
+//! of a main thread does not exist there. Because of that, this mostly just serves as a way to
+//! delegate expensive processing to another thread.
+
+use crossbeam::channel;
+use std::sync::Arc;
+use std::thread::{self, JoinHandle, ThreadId};
+
+use crate::nih_log;
+
+use super::{EventLoop, MainThreadExecutor};
+
+/// See [super::EventLoop].
+pub(crate) struct LinuxEventLoop<T, E> {
+    /// The thing that ends up executing these tasks. The tasks are usually executed from the worker
+    /// thread, but if the current thread is the main thread then the task cna also be executed
+    /// directly.
+    executor: Arc<E>,
+
+    /// The ID of the main thread. In practice this is the ID of the thread that created this task
+    /// queue.
+    main_thread_id: ThreadId,
+
+    /// A thread that act as our worker thread. When [do_maybe_async] is called, this thread will be
+    /// woken up to execute the task on the executor. This is wrapped in an `Option` so the thread
+    /// can be taken out of it and joined when this struct gets dropped.
+    worker_thread: Option<JoinHandle<()>>,
+    /// A channel for waking up the worker thread and having it perform one of the tasks from
+    /// [Message].
+    worker_thread_channel: channel::Sender<Message<T>>,
+}
+
+/// A message for communicating with the worker thread.
+enum Message<T> {
+    /// A new task for the event loop to execute.
+    Task(T),
+    /// Shut down the worker thread.
+    Shutdown,
+}
+
+impl<T, E> EventLoop<T, E> for LinuxEventLoop<T, E>
+where
+    T: Send,
+    E: MainThreadExecutor<T>,
+{
+    fn new_and_spawn(executor: Arc<E>) -> Self {
+        let (sender, receiver) = channel::bounded(super::TASK_QUEUE_CAPACITY);
+
+        Self {
+            executor: executor.clone(),
+            main_thread_id: thread::current().id(),
+            // With our drop implementation we guarentee that this thread never outlives this struct
+            worker_thread: Some(unsafe {
+                thread::Builder::new()
+                    .name(String::from("worker"))
+                    // FIXME: Find another way to bind a thread lifetime to this struct without a
+                    //        nightly-only fature
+                    .spawn_unchecked(move || worker_thread(receiver, executor))
+                    .expect("Could not spawn worker thread")
+            }),
+            worker_thread_channel: sender,
+        }
+    }
+
+    fn do_maybe_async(&self, task: T) -> bool {
+        if self.is_main_thread() {
+            self.executor.execute(task);
+            true
+        } else {
+            self.worker_thread_channel
+                .try_send(Message::Task(task))
+                .is_ok()
+        }
+    }
+
+    fn is_main_thread(&self) -> bool {
+        thread::current().id() == self.main_thread_id
+    }
+}
+
+impl<T, E> Drop for LinuxEventLoop<T, E> {
+    fn drop(&mut self) {
+        self.worker_thread_channel
+            .send(Message::Shutdown)
+            .expect("Failed while sending worker thread shutdown request");
+        if let Some(join_handle) = self.worker_thread.take() {
+            join_handle.join().expect("Worker thread panicked");
+        }
+    }
+}
+
+/// The worker thread used in [EventLoop] that executes incmoing tasks on the event loop's executor.
+fn worker_thread<T, E>(receiver: channel::Receiver<Message<T>>, executor: Arc<E>)
+where
+    T: Send,
+    E: MainThreadExecutor<T>,
+{
+    loop {
+        match receiver.recv() {
+            Ok(Message::Task(task)) => executor.execute(task),
+            Ok(Message::Shutdown) => return,
+            Err(err) => {
+                nih_log!(
+                    "Worker thread got disconnected unexpectedly, shutting down: {}",
+                    err
+                );
+                return;
+            }
+        }
+    }
+}

src/lib.rs

@@ -16,6 +16,10 @@
 
 // TODO: Once everything is more fleshed out, document the basic usage of this library
 
+// FIXME: Find an alternative for this
+#![feature(thread_spawn_unchecked)]
+
+pub mod context;
 #[macro_use]
 pub mod debug;
 pub mod formatters;