ssh remoting: Daemonize server process properly by forking and closing stdio (#19544)

This fixes the `ssh` proxy process not being notified when the proxy
process dies. Turns out that the server would have stdout/stderr/stdin
connected to the grand-parent ssh process connected to it and as long as
the server kept running (even once it was daemonized into the
background) the grand-parent ssh process wouldn't exit.

That in turn meant that the Zed client wasn't notified when the proxy
process died.

Release Notes:

- N/A

---------

Co-authored-by: Bennet <bennet@zed.dev>

crates/remote_server/src/unix.rs

@@ -27,6 +27,7 @@ use smol::io::AsyncReadExt;
 
 use smol::Async;
 use smol::{net::unix::UnixListener, stream::StreamExt as _};
+use std::ops::ControlFlow;
 use std::{
     io::Write,
     mem,
@@ -305,10 +306,15 @@ pub fn execute_run(
     stdout_socket: PathBuf,
     stderr_socket: PathBuf,
 ) -> Result<()> {
-    let log_rx = init_logging_server(log_file)?;
-    init_panic_hook();
     init_paths()?;
 
+    match daemonize()? {
+        ControlFlow::Break(_) => return Ok(()),
+        ControlFlow::Continue(_) => {}
+    }
+
+    init_panic_hook();
+    let log_rx = init_logging_server(log_file)?;
     log::info!(
         "starting up. pid_file: {:?}, stdin_socket: {:?}, stdout_socket: {:?}, stderr_socket: {:?}",
         pid_file,
@@ -322,8 +328,6 @@ pub fn execute_run(
 
     let listeners = ServerListeners::new(stdin_socket, stdout_socket, stderr_socket)?;
 
-    log::info!("starting headless gpui app");
-
     let git_hosting_provider_registry = Arc::new(GitHostingProviderRegistry::new());
     gpui::App::headless().run(move |cx| {
         settings::init(cx);
@@ -523,7 +527,8 @@ fn spawn_server(paths: &ServerPaths) -> Result<()> {
     }
 
     let binary_name = std::env::current_exe()?;
-    let server_process = smol::process::Command::new(binary_name)
+    let mut server_process = std::process::Command::new(binary_name);
+    server_process
         .arg("run")
         .arg("--log-file")
         .arg(&paths.log_file)
@@ -534,12 +539,14 @@ fn spawn_server(paths: &ServerPaths) -> Result<()> {
         .arg("--stdout-socket")
         .arg(&paths.stdout_socket)
         .arg("--stderr-socket")
-        .arg(&paths.stderr_socket)
-        .spawn()?;
+        .arg(&paths.stderr_socket);
 
-    log::info!(
-        "proxy spawned server process. PID: {:?}",
-        server_process.id()
+    let status = server_process
+        .status()
+        .context("failed to launch server process")?;
+    anyhow::ensure!(
+        status.success(),
+        "failed to launch and detach server process"
     );
 
     let mut total_time_waited = std::time::Duration::from_secs(0);
@@ -745,3 +752,43 @@ fn read_proxy_settings(cx: &mut ModelContext<'_, HeadlessProject>) -> Option<Uri
         .or_else(read_proxy_from_env);
     proxy_url
 }
+
+fn daemonize() -> Result<ControlFlow<()>> {
+    match fork::fork().map_err(|e| anyhow::anyhow!("failed to call fork with error code {}", e))? {
+        fork::Fork::Parent(_) => {
+            return Ok(ControlFlow::Break(()));
+        }
+        fork::Fork::Child => {}
+    }
+
+    // Once we've detached from the parent, we want to close stdout/stderr/stdin
+    // so that the outer SSH process is not attached to us in any way anymore.
+    unsafe { redirect_standard_streams() }?;
+
+    Ok(ControlFlow::Continue(()))
+}
+
+unsafe fn redirect_standard_streams() -> Result<()> {
+    let devnull_fd = libc::open(b"/dev/null\0" as *const [u8; 10] as _, libc::O_RDWR);
+    anyhow::ensure!(devnull_fd != -1, "failed to open /dev/null");
+
+    let process_stdio = |name, fd| {
+        let reopened_fd = libc::dup2(devnull_fd, fd);
+        anyhow::ensure!(
+            reopened_fd != -1,
+            format!("failed to redirect {} to /dev/null", name)
+        );
+        Ok(())
+    };
+
+    process_stdio("stdin", libc::STDIN_FILENO)?;
+    process_stdio("stdout", libc::STDOUT_FILENO)?;
+    process_stdio("stderr", libc::STDERR_FILENO)?;
+
+    anyhow::ensure!(
+        libc::close(devnull_fd) != -1,
+        "failed to close /dev/null fd after redirecting"
+    );
+
+    Ok(())
+}