fix(pty): use async io to avoid polling

This patch fixes zellij-org#509 by using async read instead of polling a
non-blocking fd. This reduces CPU usage when the ptys are idle.

src/tests/fakes.rs

@@ -10,9 +10,10 @@ use zellij_utils::{nix, zellij_tile};
 use crate::tests::possible_tty_inputs::{get_possible_tty_inputs, Bytes};
 use crate::tests::utils::commands::{QUIT, SLEEP};
 use zellij_client::os_input_output::ClientOsApi;
-use zellij_server::os_input_output::{Pid, ServerOsApi};
+use zellij_server::os_input_output::{async_trait, AsyncReader, Pid, ServerOsApi};
 use zellij_tile::data::Palette;
 use zellij_utils::{
+    async_std,
     channels::{ChannelWithContext, SenderType, SenderWithContext},
     errors::ErrorContext,
     interprocess::local_socket::LocalSocketStream,
@@ -227,6 +228,33 @@ impl ClientOsApi for FakeInputOutput {
     }
 }
 
+struct FakeAsyncReader {
+    fd: RawFd,
+    os_api: Box<dyn ServerOsApi>,
+}
+
+#[async_trait]
+impl AsyncReader for FakeAsyncReader {
+    async fn read(&mut self, buf: &mut [u8]) -> Result<usize, std::io::Error> {
+        // simulates async semantics: EAGAIN is not propagated to caller
+        loop {
+            let res = self.os_api.read_from_tty_stdout(self.fd, buf);
+            match res {
+                Err(nix::Error::Sys(nix::errno::Errno::EAGAIN)) => {
+                    async_std::task::sleep(Duration::from_millis(10)).await;
+                    continue;
+                }
+                Err(e) => {
+                    break Err(std::io::Error::from_raw_os_error(
+                        e.as_errno().unwrap() as i32
+                    ))
+                }
+                Ok(n_bytes) => break Ok(n_bytes),
+            }
+        }
+    }
+}
+
 impl ServerOsApi for FakeInputOutput {
     fn set_terminal_size_using_fd(&self, pid: RawFd, cols: u16, rows: u16) {
         let terminal_input = self
@@ -277,6 +305,12 @@ impl ServerOsApi for FakeInputOutput {
             None => Err(nix::Error::Sys(nix::errno::Errno::EAGAIN)),
         }
     }
+    fn async_file_reader(&self, fd: RawFd) -> Box<dyn AsyncReader> {
+        Box::new(FakeAsyncReader {
+            fd,
+            os_api: ServerOsApi::box_clone(self),
+        })
+    }
     fn tcdrain(&self, pid: RawFd) -> Result<(), nix::Error> {
         self.io_events.lock().unwrap().push(IoEvent::TcDrain(pid));
         Ok(())

zellij-server/Cargo.toml

@@ -10,6 +10,7 @@ license = "MIT"
 
 [dependencies]
 ansi_term = "0.12.1"
+async-trait = "0.1.50"
 daemonize = "0.4.1"
 serde_json = "1.0"
 unicode-width = "0.1.8"

zellij-server/src/os_input_output.rs

@@ -4,10 +4,11 @@ use std::path::PathBuf;
 use std::process::{Child, Command};
 use std::sync::{Arc, Mutex};
 
-use zellij_utils::{interprocess, libc, nix, signal_hook, zellij_tile};
+use zellij_utils::{async_std, interprocess, libc, nix, signal_hook, zellij_tile};
 
+use async_std::fs::File as AsyncFile;
+use async_std::os::unix::io::FromRawFd;
 use interprocess::local_socket::LocalSocketStream;
-use nix::fcntl::{fcntl, FcntlArg, OFlag};
 use nix::pty::{forkpty, Winsize};
 use nix::sys::signal::{kill, Signal};
 use nix::sys::termios;
@@ -21,7 +22,11 @@ use zellij_utils::{
     shared::default_palette,
 };
 
+use async_std::io::ReadExt;
+pub use async_trait::async_trait;
+
 pub use nix::unistd::Pid;
+
 pub(crate) fn set_terminal_size_using_fd(fd: RawFd, columns: u16, rows: u16) {
     // TODO: do this with the nix ioctl
     use libc::ioctl;
@@ -88,8 +93,6 @@ fn spawn_terminal(file_to_open: Option<PathBuf>, orig_termios: termios::Termios)
                 let pid_secondary = match fork_pty_res.fork_result {
                     ForkResult::Parent { child } => {
                         // fcntl(pid_primary, FcntlArg::F_SETFL(OFlag::empty())).expect("could not fcntl");
-                        fcntl(pid_primary, FcntlArg::F_SETFL(OFlag::O_NONBLOCK))
-                            .expect("could not fcntl");
                         child
                     }
                     ForkResult::Child => match file_to_open {
@@ -133,6 +136,34 @@ pub struct ServerOsInputOutput {
     send_instructions_to_client: Arc<Mutex<Option<IpcSenderWithContext<ServerToClientMsg>>>>,
 }
 
+// async fn in traits is not supported by rust, so dtolnay's excellent async_trait macro is being
+// used. See https://smallcultfollowing.com/babysteps/blog/2019/10/26/async-fn-in-traits-are-hard/
+#[async_trait]
+pub trait AsyncReader: Send + Sync {
+    async fn read(&mut self, buf: &mut [u8]) -> Result<usize, std::io::Error>;
+}
+
+/// An `AsyncReader` that wraps a `RawFd`
+struct RawFdAsyncReader {
+    fd: async_std::fs::File,
+}
+
+impl RawFdAsyncReader {
+    fn new(fd: RawFd) -> RawFdAsyncReader {
+        RawFdAsyncReader {
+            /// The supplied `RawFd` is consumed by the created `RawFdAsyncReader`, closing it when dropped
+            fd: unsafe { AsyncFile::from_raw_fd(fd) },
+        }
+    }
+}
+
+#[async_trait]
+impl AsyncReader for RawFdAsyncReader {
+    async fn read(&mut self, buf: &mut [u8]) -> Result<usize, std::io::Error> {
+        self.fd.read(buf).await
+    }
+}
+
 /// The `ServerOsApi` trait represents an abstract interface to the features of an operating system that
 /// Zellij server requires.
 pub trait ServerOsApi: Send + Sync {
@@ -142,6 +173,8 @@ pub trait ServerOsApi: Send + Sync {
     fn spawn_terminal(&self, file_to_open: Option<PathBuf>) -> (RawFd, Pid);
     /// Read bytes from the standard output of the virtual terminal referred to by `fd`.
     fn read_from_tty_stdout(&self, fd: RawFd, buf: &mut [u8]) -> Result<usize, nix::Error>;
+    /// Creates an `AsyncReader` that can be used to read from `fd` in an async context
+    fn async_file_reader(&self, fd: RawFd) -> Box<dyn AsyncReader>;
     /// Write bytes to the standard input of the virtual terminal referred to by `fd`.
     fn write_to_tty_stdin(&self, fd: RawFd, buf: &[u8]) -> Result<usize, nix::Error>;
     /// Wait until all output written to the object referred to by `fd` has been transmitted.
@@ -172,6 +205,9 @@ impl ServerOsApi for ServerOsInputOutput {
     fn read_from_tty_stdout(&self, fd: RawFd, buf: &mut [u8]) -> Result<usize, nix::Error> {
         unistd::read(fd, buf)
     }
+    fn async_file_reader(&self, fd: RawFd) -> Box<dyn AsyncReader> {
+        Box::new(RawFdAsyncReader::new(fd))
+    }
     fn write_to_tty_stdin(&self, fd: RawFd, buf: &[u8]) -> Result<usize, nix::Error> {
         unistd::write(fd, buf)
     }

zellij-server/src/pty.rs

@@ -1,16 +1,14 @@
-use zellij_utils::{async_std, nix};
+use zellij_utils::async_std;
 
-use async_std::stream::*;
-use async_std::task;
-use async_std::task::*;
+use async_std::future::timeout as async_timeout;
+use async_std::task::{self, JoinHandle};
 use std::collections::HashMap;
 use std::os::unix::io::RawFd;
 use std::path::PathBuf;
-use std::pin::*;
 use std::time::{Duration, Instant};
 
 use crate::{
-    os_input_output::{Pid, ServerOsApi},
+    os_input_output::{AsyncReader, Pid, ServerOsApi},
     panes::PaneId,
     screen::ScreenInstruction,
     thread_bus::{Bus, ThreadSenders},
@@ -23,52 +21,6 @@ use zellij_utils::{
     logging::debug_to_file,
 };
 
-pub struct ReadFromPid {
-    pid: RawFd,
-    os_input: Box<dyn ServerOsApi>,
-}
-
-impl ReadFromPid {
-    pub fn new(pid: &RawFd, os_input: Box<dyn ServerOsApi>) -> ReadFromPid {
-        ReadFromPid {
-            pid: *pid,
-            os_input,
-        }
-    }
-}
-
-impl Stream for ReadFromPid {
-    type Item = Vec<u8>;
-    fn poll_next(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
-        let mut read_buffer = [0; 65535];
-        let pid = self.pid;
-        let read_result = &self.os_input.read_from_tty_stdout(pid, &mut read_buffer);
-        match read_result {
-            Ok(res) => {
-                if *res == 0 {
-                    // indicates end of file
-                    Poll::Ready(None)
-                } else {
-                    let res = Some(read_buffer[..*res].to_vec());
-                    Poll::Ready(res)
-                }
-            }
-            Err(e) => {
-                match e {
-                    nix::Error::Sys(errno) => {
-                        if *errno == nix::errno::Errno::EAGAIN {
-                            Poll::Ready(Some(vec![])) // TODO: better with timeout waker somehow
-                        } else {
-                            Poll::Ready(None)
-                        }
-                    }
-                    _ => Poll::Ready(None),
-                }
-            }
-        }
-    }
-}
-
 pub type VteBytes = Vec<u8>;
 
 /// Instructions related to PTYs (pseudoterminals).
@@ -160,6 +112,42 @@ pub(crate) fn pty_thread_main(mut pty: Pty, maybe_layout: Option<Layout>) {
     }
 }
 
+enum ReadResult {
+    Ok(usize),
+    Timeout,
+    Err(std::io::Error),
+}
+
+impl From<std::io::Result<usize>> for ReadResult {
+    fn from(e: std::io::Result<usize>) -> ReadResult {
+        match e {
+            Err(e) => ReadResult::Err(e),
+            Ok(n) => ReadResult::Ok(n),
+        }
+    }
+}
+
+async fn deadline_read(
+    reader: &mut dyn AsyncReader,
+    deadline: Option<Instant>,
+    buf: &mut [u8],
+) -> ReadResult {
+    if let Some(deadline) = deadline {
+        let timeout = deadline.checked_duration_since(Instant::now());
+        if let Some(timeout) = timeout {
+            match async_timeout(timeout, reader.read(buf)).await {
+                Ok(res) => res.into(),
+                _ => ReadResult::Timeout,
+            }
+        } else {
+            // deadline has already elapsed
+            ReadResult::Timeout
+        }
+    } else {
+        reader.read(buf).await.into()
+    }
+}
+
 fn stream_terminal_bytes(
     pid: RawFd,
     senders: ThreadSenders,
@@ -170,50 +158,42 @@ fn stream_terminal_bytes(
     task::spawn({
         async move {
             err_ctx.add_call(ContextType::AsyncTask);
-            let mut terminal_bytes = ReadFromPid::new(&pid, os_input);
 
-            let mut last_byte_receive_time: Option<Instant> = None;
-            let mut pending_render = false;
-            let max_render_pause = Duration::from_millis(30);
+            // After a successful read, we keep on reading additional data up to a duration of
+            // `render_pause`. This is in order to batch up PtyBytes before rendering them.
+            // Once `render_deadline` has elapsed, we send Render.
+            let render_pause = Duration::from_millis(30);
+            let mut render_deadline = None;
 
-            while let Some(bytes) = terminal_bytes.next().await {
-                let bytes_is_empty = bytes.is_empty();
-                if debug {
-                    debug_to_file(&bytes, pid).unwrap();
-                }
-                if !bytes_is_empty {
-                    let _ = senders.send_to_screen(ScreenInstruction::PtyBytes(pid, bytes));
-                    // for UX reasons, if we got something on the wire, we only send the render notice if:
-                    // 1. there aren't any more bytes on the wire afterwards
-                    // 2. a certain period (currently 30ms) has elapsed since the last render
-                    //    (otherwise if we get a large amount of data, the display would hang
-                    //    until it's done)
-                    // 3. the stream has ended, and so we render 1 last time
-                    match last_byte_receive_time.as_mut() {
-                        Some(receive_time) => {
-                            if receive_time.elapsed() > max_render_pause {
-                                pending_render = false;
-                                let _ = senders.send_to_screen(ScreenInstruction::Render);
-                                last_byte_receive_time = Some(Instant::now());
-                            } else {
-                                pending_render = true;
-                            }
-                        }
-                        None => {
-                            last_byte_receive_time = Some(Instant::now());
-                            pending_render = true;
-                        }
-                    };
-                } else {
-                    if pending_render {
-                        pending_render = false;
+            let mut buf = [0u8; 65536];
+            let mut async_reader = os_input.async_file_reader(pid);
+            loop {
+                match deadline_read(async_reader.as_mut(), render_deadline, &mut buf).await {
+                    ReadResult::Ok(0) | ReadResult::Err(_) => break, // EOF or error
+                    ReadResult::Timeout => {
                         let _ = senders.send_to_screen(ScreenInstruction::Render);
+                        // next read does not need a deadline as we just rendered everything
+                        render_deadline = None;
+
+                        // yield so Screen thread has some time to render before send additional
+                        // PtyBytes.
+                        task::sleep(Duration::from_millis(10)).await;
+                    }
+                    ReadResult::Ok(n_bytes) => {
+                        let bytes = &buf[..n_bytes];
+                        if debug {
+                            let _ = debug_to_file(bytes, pid);
+                        }
+                        let _ = senders
+                            .send_to_screen(ScreenInstruction::PtyBytes(pid, bytes.to_vec()));
+                        // if we already have a render_deadline we keep it, otherwise we set it
+                        // to the duration of `render_pause`.
+                        render_deadline.get_or_insert(Instant::now() + render_pause);
                     }
-                    last_byte_receive_time = None;
-                    task::sleep(::std::time::Duration::from_millis(10)).await;
                 }
             }
-            senders.send_to_screen(ScreenInstruction::Render).unwrap();
+            let _ = senders.send_to_screen(ScreenInstruction::Render);
+
             #[cfg(not(any(feature = "test", test)))]
             // this is a little hacky, and is because the tests end the file as soon as
             // we read everything, rather than hanging until there is new data