Add framework for separate printer control thread

1 files changed, 81 insertions, 0 deletions

diff --git a/crates/control/src/lib.rs b/crates/control/src/lib.rs
new file mode 100644
index 0000000..5691cf2
--- /dev/null
+++ b/crates/control/src/lib.rs
@@ -0,0 +1,81 @@
+//! Printer control related code
+//!
+//! To ensure the volume of web requests doesn't mess with printing, communication with the printer is done
+//! in a separate thread, on its own async executor.
+//!
+//! The main server spawns this thread by creating a [`Handle`] and communicates by sending [messages](crate::Message) over an MPSC channel.
+
+use std::{
+    mem::ManuallyDrop,
+    thread::{self, JoinHandle},
+};
+use tokio::{
+    runtime,
+    sync::mpsc::{self, Receiver, Sender},
+};
+use tracing::info;
+
+/// The maximum number of messages to buffer before 'blocking' new ones
+/// This provides a form of backpressure
+const CHANNEL_CAPACITY: usize = 100;
+
+/// A control message sent from the API to the printer control thread
+pub enum Message {
+    Quit,
+}
+
+/// A handle to the printer control thread, allowing for communication.
+pub struct Handle {
+    cmd_send: Sender<Message>,
+    thread_handle: ManuallyDrop<JoinHandle<()>>,
+}
+
+impl Handle {
+    /// Spawn a printer control thread, returning a handle
+    pub fn spawn() -> Self {
+        let (send, recv) = mpsc::channel(CHANNEL_CAPACITY);
+
+        let state = State { cmd_recv: recv };
+        let thread_handle = thread::spawn(move || {
+            let rt = runtime::Builder::new_current_thread()
+                .build()
+                .expect("error spawning printer control thread");
+
+            rt.block_on(async move { state.entrypoint().await });
+        });
+
+        Self {
+            cmd_send: send,
+            thread_handle: ManuallyDrop::new(thread_handle),
+        }
+    }
+}
+
+impl Drop for Handle {
+    fn drop(&mut self) {
+        let _ = self.cmd_send.blocking_send(Message::Quit);
+        unsafe {
+            info!("waiting for printer thread to exit...");
+            ManuallyDrop::take(&mut self.thread_handle)
+                .join()
+                .expect("error joining printer thread")
+        };
+    }
+}
+
+/// The state held by the printer control thread
+pub struct State {
+    cmd_recv: Receiver<Message>,
+}
+
+impl State {
+    /// The entrypoint of the printer control thread
+    pub async fn entrypoint(mut self) {
+        info!("listening for events");
+        while let Some(msg) = self.cmd_recv.recv().await {
+            match msg {
+                Message::Quit => break,
+            }
+        }
+    }
+}