[WIP] feat(multi-client): Multi-client support for the application

This implements only the bare minimum to allow the multi-client support
to be tested. To test it open three terminal windows.

- In window 1 run: ./s
- In window 2 run: ./c
- In window 3 run: ./c

You will see the UI appear in windows 2 and 3. You can open the same file
in both windows and observe changes in one window appear in the other.

TODO:
- Support more than two clients.
- Support dynamically attaching and detaching clients.
- Prevent reading from terminal while client is suspended.
- Support sending current directory, stdin from the client to the server.
- Wait until Helix switches to Termina (helix-editor#13307), and then reimplement this
  on top of Termina. For now this depends on a forked version of Crossterm which
  is available at: https://github.com/pcc/crossterm/tree/client-server-rebase
  From a brief look at the Termina code it seems easier to extend
  Termina to support this than it was to extend Crossterm.

Author: pcc

Cargo.lock

@@ -57,3 +57,6 @@ repository = "https://github.com/helix-editor/helix"
 homepage = "https://helix-editor.com"
 license = "MPL-2.0"
 rust-version = "1.82"
+
+[patch.crates-io]
+crossterm = { path = 'crossterm' }

Cargo.toml

@@ -0,0 +1,3 @@
+#!/bin/sh
+export HELIX_RUNTIME=$PWD/runtime
+target/release/hx --client /tmp/hx "$@"

c

@@ -4,6 +4,8 @@ use std::{
     ops::{Add, AddAssign, Sub, SubAssign},
 };
 
+use serde::{Deserialize, Serialize};
+
 use helix_stdx::rope::RopeSliceExt;
 
 use crate::{
@@ -16,7 +18,7 @@ use crate::{
 };
 
 /// Represents a single point in a text buffer. Zero indexed.
-#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
 pub struct Position {
     pub row: usize,
     pub col: usize,

helix-core/src/position.rs

@@ -21,12 +21,13 @@ bitflags.workspace = true
 once_cell = "1.21"
 regex-automata = "0.4.9"
 unicode-segmentation.workspace = true
+tokio = { version = "1.38.0", features = ["net"] }
 
 [target.'cfg(windows)'.dependencies]
 windows-sys = { version = "0.59", features = ["Win32_Foundation", "Win32_Security", "Win32_Security_Authorization", "Win32_Storage_FileSystem", "Win32_System_Threading"] }
 
 [target.'cfg(unix)'.dependencies]
-rustix = { version = "1.0", features = ["fs"] }
+rustix = { version = "1.0", features = ["fs", "net"] }
 
 [dev-dependencies]
 tempfile.workspace = true

helix-stdx/Cargo.toml

@@ -3,5 +3,6 @@ pub mod faccess;
 pub mod path;
 pub mod range;
 pub mod rope;
+pub mod socket;
 
 pub use range::Range;

helix-stdx/src/lib.rs

@@ -0,0 +1,36 @@
+use rustix::fd::AsFd;
+use rustix::net::{
+    recvmsg, sendmsg, RecvAncillaryBuffer, RecvAncillaryMessage, RecvFlags, SendAncillaryBuffer,
+    SendAncillaryMessage, SendFlags,
+};
+use std::fs::File;
+use std::io::IoSlice;
+use std::io::{self, IoSliceMut};
+use std::mem::MaybeUninit;
+
+pub fn write_fd<Fd: AsFd>(socket: Fd, file: &File) -> io::Result<()> {
+    let mut space = [MaybeUninit::uninit(); rustix::cmsg_space!(ScmRights(1))];
+    let mut buf = SendAncillaryBuffer::new(&mut space);
+    let fd_arr = [file.as_fd()];
+    buf.push(SendAncillaryMessage::ScmRights(&fd_arr));
+    sendmsg(socket, &[IoSlice::new(&[0])], &mut buf, SendFlags::empty())?;
+    Ok(())
+}
+
+pub fn read_fd<Fd: AsFd>(socket: Fd) -> io::Result<File> {
+    let mut space = [MaybeUninit::uninit(); rustix::cmsg_space!(ScmRights(1))];
+    let mut buf = RecvAncillaryBuffer::new(&mut space);
+    let mut recv_buf = [0];
+    recvmsg(
+        socket,
+        &mut [IoSliceMut::new(&mut recv_buf)],
+        &mut buf,
+        RecvFlags::empty(),
+    )?;
+    if let Some(RecvAncillaryMessage::ScmRights(mut fd)) = buf.drain().next() {
+        if let Some(fd) = fd.next() {
+            return Ok(fd.into());
+        }
+    }
+    Err(io::Error::new(io::ErrorKind::Other, "did not receive fd"))
+}

helix-stdx/src/socket.rs

@@ -90,6 +90,8 @@ serde = { version = "1.0", features = ["derive"] }
 # ripgrep for global search
 grep-regex = "0.1.13"
 grep-searcher = "0.1.14"
+rmp-serde = "1.3.0"
+tokio-util = { version = "0.7.15", features = ["io", "io-util"] }
 
 [target.'cfg(not(windows))'.dependencies]  # https://github.com/vorner/signal-hook/issues/100
 signal-hook-tokio = { version = "0.3", features = ["futures-v0_3"] }