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//! 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,
}
}
}
}
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