Add background forwarding mode

src/bin/dosh-client.rs

@@ -27,7 +27,10 @@ use std::collections::HashMap;
 use std::collections::HashSet;
 use std::fs;
 use std::io::{Read, Write};
-use std::net::{Ipv4Addr, Ipv6Addr, SocketAddr, ToSocketAddrs};
+use std::net::{
+    Ipv4Addr, Ipv6Addr, SocketAddr, TcpListener as StdTcpListener, TcpStream as StdTcpStream,
+    ToSocketAddrs,
+};
 use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};
 use std::sync::Arc;
@@ -70,6 +73,8 @@ struct Args {
     dynamic_forward: Vec<String>,
     #[arg(short = 'N', long)]
     forward_only: bool,
+    #[arg(short = 'f', long)]
+    background: bool,
     #[arg(long)]
     predict: bool,
     #[arg(long)]
@@ -157,7 +162,7 @@ async fn main() -> Result<()> {
     if args.server.as_deref() == Some("trust") {
         return run_trust_command(&config, &args);
     }
-    let requested_server = args.server.unwrap_or_else(|| config.server.clone());
+    let requested_server = args.server.clone().unwrap_or_else(|| config.server.clone());
     let hosts = load_hosts_config(None).unwrap_or_default();
     let host = hosts
         .hosts
@@ -216,6 +221,9 @@ async fn main() -> Result<()> {
     if forwarding_requested && !auth_allows(&auth_preference, "native") {
         return Err(anyhow!("port forwarding requires --auth native or auto"));
     }
+    if args.background && std::env::var_os("DOSH_BACKGROUND_CHILD").is_none() {
+        return spawn_background_forwarder(&args, forwarding_requested);
+    }
     let allow_cache = !args.no_cache && !forwarding_requested;
 
     let started = Instant::now();
@@ -539,6 +547,79 @@ fn startup_command_from_string(command: &str) -> Vec<u8> {
     command.into_bytes()
 }
 
+fn spawn_background_forwarder(args: &Args, forwarding_requested: bool) -> Result<()> {
+    if !forwarding_requested {
+        return Err(anyhow!(
+            "-f requires at least one -L, -R, or -D forwarding request"
+        ));
+    }
+    if !args.forward_only {
+        return Err(anyhow!("-f currently requires -N for forward-only mode"));
+    }
+    let listener =
+        StdTcpListener::bind("127.0.0.1:0").context("bind background readiness socket")?;
+    listener
+        .set_nonblocking(true)
+        .context("configure background readiness socket")?;
+    let readiness_addr = listener.local_addr()?;
+    let token = URL_SAFE_NO_PAD.encode(crypto::random_16());
+    let exe = std::env::current_exe().context("locate current dosh-client executable")?;
+    let mut child = Command::new(exe)
+        .args(std::env::args_os().skip(1))
+        .env("DOSH_BACKGROUND_CHILD", "1")
+        .env("DOSH_BACKGROUND_READY_ADDR", readiness_addr.to_string())
+        .env("DOSH_BACKGROUND_READY_TOKEN", &token)
+        .stdin(Stdio::null())
+        .stdout(Stdio::null())
+        .stderr(Stdio::null())
+        .spawn()
+        .context("spawn background dosh forwarder")?;
+    if let Some(path) = std::env::var_os("DOSH_BACKGROUND_PID_FILE") {
+        fs::write(path, child.id().to_string()).context("write background pid file")?;
+    }
+
+    let deadline = Instant::now() + Duration::from_secs(30);
+    loop {
+        match listener.accept() {
+            Ok((mut stream, _)) => {
+                let mut raw = String::new();
+                stream.read_to_string(&mut raw).ok();
+                if raw.trim() == token {
+                    eprintln!("dosh background forwarding ready");
+                    return Ok(());
+                }
+                return Err(anyhow!("background forwarder sent invalid readiness token"));
+            }
+            Err(err) if err.kind() == std::io::ErrorKind::WouldBlock => {}
+            Err(err) => return Err(err).context("wait for background readiness"),
+        }
+        if let Some(status) = child.try_wait()? {
+            return Err(anyhow!(
+                "background dosh forwarder exited before readiness: {status}"
+            ));
+        }
+        if Instant::now() >= deadline {
+            let _ = child.kill();
+            let _ = child.wait();
+            return Err(anyhow!("background dosh forwarder did not become ready"));
+        }
+        std::thread::sleep(Duration::from_millis(20));
+    }
+}
+
+fn signal_background_ready() {
+    let Some(addr) = std::env::var_os("DOSH_BACKGROUND_READY_ADDR") else {
+        return;
+    };
+    let Some(token) = std::env::var_os("DOSH_BACKGROUND_READY_TOKEN") else {
+        return;
+    };
+    if let Ok(mut stream) = StdTcpStream::connect(addr.to_string_lossy().as_ref()) {
+        let _ = stream.write_all(token.to_string_lossy().as_bytes());
+        let _ = stream.shutdown(std::net::Shutdown::Write);
+    }
+}
+
 fn parse_local_forwards(raw: &[String]) -> Result<Vec<LocalForward>> {
     raw.iter().map(|value| parse_local_forward(value)).collect()
 }
@@ -1513,6 +1594,7 @@ async fn run_terminal(
     let stream_ids = Arc::new(AtomicU64::new(1));
     start_local_forwards(&local_forwards, forward_tx.clone(), Arc::clone(&stream_ids)).await?;
     start_dynamic_forwards(&dynamic_forwards, forward_tx.clone(), stream_ids).await?;
+    signal_background_ready();
     let mut stream_writers: HashMap<u64, tokio::net::tcp::OwnedWriteHalf> = HashMap::new();
     let mut pending_socks_replies: HashSet<u64> = HashSet::new();
     if let Some(frame) = first_frame {