Implement ProxyUseFDpass for ssh

This allows the ProxyUseFDpass ssh config option, to be
used on Linux and Mac OS.  This was requested in
enhancement issue #6093

Cargo.lock

@@ -3656,6 +3656,15 @@ dependencies = [
  "windows-targets 0.52.6",
 ]
 
+[[package]]
+name = "passfd"
+version = "0.1.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b332c50e4d07c0011fff51ea305374408319908908bc1dbed7a0ffaaf63a8151"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "paste"
 version = "1.0.15"
@@ -6479,6 +6488,7 @@ dependencies = [
  "libssh-rs",
  "log",
  "once_cell",
+ "passfd",
  "portable-pty",
  "predicates",
  "regex",

wezterm-ssh/Cargo.toml

@@ -40,6 +40,9 @@ wezterm-uds = { path = "../wezterm-uds" }
 # Not used directly, but is used to centralize the openssl vendor feature selection
 async_ossl = { path = "../async_ossl" }
 
+[target.'cfg(unix)'.dependencies]
+passfd = "0.1.6"
+
 [dev-dependencies]
 assert_fs = "1.0.4"
 clap = {version="4.0", features=["derive"]}

wezterm-ssh/src/sessioninner.rs

@@ -357,11 +357,16 @@ impl SessionInner {
 
                 #[cfg(unix)]
                 unsafe {
+                    use passfd::FdPassingExt;
                     use std::os::unix::io::{FromRawFd, IntoRawFd};
-                    return Ok((
-                        Socket::from_raw_fd(a.into_raw_fd()),
-                        Some(KillOnDropChild(child)),
-                    ));
+
+                    let raw = a.into_raw_fd();
+                    let dest = match self.config.get("proxyusefdpass").map(|s| s.as_str()) {
+                        Some("yes") => raw.recv_fd()?,
+                        _ => raw,
+                    };
+
+                    return Ok((Socket::from_raw_fd(dest), Some(KillOnDropChild(child))));
                 }
                 #[cfg(windows)]
                 unsafe {