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mux: enable ssh agent forwarding
This is done with a wezterm twist: not only is the auth sock forwarded, but the mux on the remote end will automatically maintain a symlink to point to the auth sock of the most recently active mux client, and set SSH_AUTH_SOCK to that symlink so that your remote panes should always be referencing something sane. refs: https://github.com/wez/wezterm/issues/1647 refs: https://github.com/wez/wezterm/discussions/988
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@ -377,6 +377,9 @@ pub struct Config {
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#[dynamic(default = "default_mux_output_parser_buffer_size")]
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pub mux_output_parser_buffer_size: usize,
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#[dynamic(default = "default_true")]
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pub mux_enable_ssh_agent: bool,
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/// How many ms to delay after reading a chunk of output
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/// in order to try to coalesce fragmented writes into
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/// a single bigger chunk of output and reduce the chances
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@ -39,6 +39,11 @@ As features stabilize some brief notes about them will accumulate here.
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* [wezterm.serde](config/lua/wezterm.serde/index.md) module for serialization
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and deserialization of JSON, TOML and YAML. Thanks to @expnn! #4969
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* `wezterm ssh` now supports agent forwarding. Thanks to @Riatre! #5345
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* SSH multiplexer domains now support agent forwarding, and will automatically
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maintain `SSH_AUTH_SOCK` to an appropriate value on the destination host,
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depending on the value of the new
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[mux_enable_ssh_agent](config/lua/config/mux_enable_ssh_agent.md) option.
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?988 #1647
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#### Fixed
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* Race condition when very quickly adjusting font scale, and other improvements
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26
docs/config/lua/config/mux_enable_ssh_agent.md
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26
docs/config/lua/config/mux_enable_ssh_agent.md
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@ -0,0 +1,26 @@
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---
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tags:
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- multiplexing
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- ssh
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---
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# `mux_enable_ssh_agent = true`
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{{since('nightly')}}
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When set to `true` (the default), wezterm will configure the `SSH_AUTH_SOCK`
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environment variable for panes spawned in the `local` domain.
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The auth sock will point to a symbolic link that will in turn be pointed to the
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authentication socket associated with the most recently active multiplexer
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client.
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You can review the authentication socket that will be used for various clients
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by running `wezterm cli list-clients` and inspecting the `SSH_AUTH_SOCK`
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column.
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The symlink is updated within (at the time of writing this documentation) 100ms
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of the active Mux client changing.
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You can set `mux_enable_ssh_agent = false` to prevent wezterm from assigning
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`SSH_AUTH_SOCK` or updating the symlink.
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@ -468,6 +468,9 @@ impl LocalDomain {
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cmd.env("WEZTERM_UNIX_SOCKET", sock);
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}
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cmd.env("WEZTERM_PANE", pane_id.to_string());
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if let Some(agent) = Mux::get().agent.as_ref() {
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cmd.env("SSH_AUTH_SOCK", agent.path());
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}
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self.fixup_command(&mut cmd).await?;
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Ok(cmd)
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}
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@ -1,5 +1,6 @@
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use crate::client::{ClientId, ClientInfo};
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use crate::pane::{CachePolicy, Pane, PaneId};
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use crate::ssh_agent::AgentProxy;
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use crate::tab::{SplitRequest, Tab, TabId};
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use crate::window::{Window, WindowId};
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use anyhow::{anyhow, Context, Error};
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@ -38,6 +39,7 @@ pub mod localpane;
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pub mod pane;
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pub mod renderable;
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pub mod ssh;
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pub mod ssh_agent;
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pub mod tab;
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pub mod termwiztermtab;
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pub mod tmux;
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@ -108,6 +110,7 @@ pub struct Mux {
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identity: RwLock<Option<Arc<ClientId>>>,
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num_panes_by_workspace: RwLock<HashMap<String, usize>>,
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main_thread_id: std::thread::ThreadId,
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agent: Option<AgentProxy>,
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}
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const BUFSIZE: usize = 1024 * 1024;
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@ -421,6 +424,12 @@ impl Mux {
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);
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}
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let agent = if config::configuration().mux_enable_ssh_agent {
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Some(AgentProxy::new())
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} else {
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None
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};
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Self {
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tabs: RwLock::new(HashMap::new()),
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panes: RwLock::new(HashMap::new()),
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@ -434,6 +443,7 @@ impl Mux {
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identity: RwLock::new(None),
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num_panes_by_workspace: RwLock::new(HashMap::new()),
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main_thread_id: std::thread::current().id(),
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agent,
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}
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}
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@ -471,6 +481,9 @@ impl Mux {
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if let Some(info) = self.clients.write().get_mut(client_id) {
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info.update_last_input();
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}
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if let Some(agent) = &self.agent {
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agent.update_target();
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}
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}
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pub fn record_input_for_current_identity(&self) {
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209
mux/src/ssh_agent.rs
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209
mux/src/ssh_agent.rs
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@ -0,0 +1,209 @@
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use crate::{ClientId, Mux};
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use anyhow::Context;
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use chrono::{DateTime, Duration, Utc};
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use parking_lot::RwLock;
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#[cfg(unix)]
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use std::os::unix::fs::symlink as symlink_file;
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#[cfg(windows)]
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use std::os::windows::fs::symlink_file;
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use std::path::{Path, PathBuf};
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use std::sync::mpsc::{sync_channel, Receiver, SyncSender};
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use std::sync::Arc;
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/// AgentProxy manages an agent.PID symlink in the wezterm runtime
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/// directory.
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/// The intent is to maintain the symlink and have it point to the
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/// appropriate ssh agent socket path for the most recently active
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/// mux client.
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///
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/// Why symlink rather than running an agent proxy socket of our own?
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/// Some agent implementations use low level unix socket operations
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/// to decide whether the client process is allowed to consume
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/// the agent or not, and us sitting in the middle breaks that.
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///
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/// As a further complication, when a wezterm proxy client is
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/// present, both the proxy and the mux instance inside a gui
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/// tend to be updated together, with the gui often being
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/// touched last.
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///
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/// To deal with that we de-bounce input events and weight
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/// proxy clients higher so that we can avoid thrashing
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/// between gui and proxy.
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///
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/// The consequence of this is that there is 100ms of artificial
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/// latency to detect a change in the active client.
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/// This number was selected because it is unlike for a human
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/// to be able to switch devices that quickly.
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///
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/// How is this used? The Mux::client_had_input function
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/// will call AgentProxy::update_target to signal when
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/// the active client may have changed.
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pub struct AgentProxy {
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sock_path: PathBuf,
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current_target: RwLock<Option<Arc<ClientId>>>,
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sender: SyncSender<()>,
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}
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impl Drop for AgentProxy {
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fn drop(&mut self) {
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std::fs::remove_file(&self.sock_path).ok();
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}
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}
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fn update_symlink<P: AsRef<Path>, Q: AsRef<Path>>(original: P, link: Q) -> anyhow::Result<()> {
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let original = original.as_ref();
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let link = link.as_ref();
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match symlink_file(original, link) {
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Ok(()) => Ok(()),
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Err(err) => {
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if err.kind() == std::io::ErrorKind::AlreadyExists {
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std::fs::remove_file(link)
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.with_context(|| format!("failed to remove {}", link.display()))?;
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symlink_file(original, link).with_context(|| {
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format!(
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"failed to create symlink {} -> {}: {err:#}",
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link.display(),
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original.display()
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)
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})
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} else {
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anyhow::bail!(
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"failed to create symlink {} -> {}: {err:#}",
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link.display(),
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original.display()
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);
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}
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}
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}
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}
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impl AgentProxy {
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pub fn new() -> Self {
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let pid = unsafe { libc::getpid() };
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let sock_path = config::RUNTIME_DIR.join(format!("agent.{pid}"));
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if let Ok(inherited) = std::env::var("SSH_AUTH_SOCK") {
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if let Err(err) = update_symlink(&inherited, &sock_path) {
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log::error!("failed to set {sock_path:?} to initial inherited SSH_AUTH_SOCK value of {inherited:?}: {err:#}");
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}
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}
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let (sender, receiver) = sync_channel(16);
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std::thread::spawn(move || Self::process_updates(receiver));
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Self {
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sock_path,
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current_target: RwLock::new(None),
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sender,
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}
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}
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pub fn path(&self) -> &Path {
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&self.sock_path
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}
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pub fn update_target(&self) {
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// If the send fails, the channel is most likely
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// full, which means that the updater thread is
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// going to observe the now-current state when
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// it wakes up, so we needn't try any harder
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self.sender.try_send(()).ok();
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}
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fn process_updates(receiver: Receiver<()>) {
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while let Ok(_) = receiver.recv() {
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// De-bounce multiple input events so that we don't quickly
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// thrash between the host and proxy value
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std::thread::sleep(std::time::Duration::from_millis(100));
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while receiver.try_recv().is_ok() {}
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if let Some(agent) = &Mux::get().agent {
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agent.update_now();
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}
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}
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}
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fn update_now(&self) {
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// Get list of clients from mux
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// Order by most recent activity
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// Take first one with auth sock -> that's the path
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// If we find none, then we print an error and drop
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// this stream.
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let mut clients = Mux::get().iter_clients();
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clients.retain(|info| info.client_id.ssh_auth_sock.is_some());
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clients.sort_by(|a, b| {
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// The biggest last_input time is most recent, so it sorts sooner.
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// However, when using a proxy into a gui mux, both the proxy and the
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// gui will update around the same time, with the gui often being
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// updated fractionally after the proxy.
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// In this situation we want the proxy to be selected, so we weight
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// proxy entries slightly higher by adding a small Duration to
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// the actual observed value.
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// `via proxy pid` is coupled with the Pdu::SetClientId logic
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// in wezterm-mux-server-impl/src/sessionhandler.rs
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const PROXY_MARKER: &str = "via proxy pid";
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let a_proxy = a.client_id.hostname.contains(PROXY_MARKER);
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let b_proxy = b.client_id.hostname.contains(PROXY_MARKER);
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fn adjust_for_proxy(time: DateTime<Utc>, is_proxy: bool) -> DateTime<Utc> {
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if is_proxy {
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time + Duration::milliseconds(100)
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} else {
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time
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}
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}
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let a_time = adjust_for_proxy(a.last_input, a_proxy);
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let b_time = adjust_for_proxy(b.last_input, b_proxy);
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b_time.cmp(&a_time)
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});
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log::trace!("filtered to {clients:#?}");
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match clients.get(0) {
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Some(info) => {
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let current = self.current_target.read().clone();
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let needs_update = match (current, &info.client_id) {
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(None, _) => true,
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(Some(prior), current) => prior != *current,
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};
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if needs_update {
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let ssh_auth_sock = info
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.client_id
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.ssh_auth_sock
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.as_ref()
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.expect("we checked in the retain above");
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log::trace!(
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"Will update {} -> {ssh_auth_sock}",
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self.sock_path.display(),
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);
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self.current_target.write().replace(info.client_id.clone());
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if let Err(err) = update_symlink(ssh_auth_sock, &self.sock_path) {
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log::error!(
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"Problem updating {} -> {ssh_auth_sock}: {err:#}",
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self.sock_path.display(),
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);
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}
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}
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}
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None => {
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if self.current_target.write().take().is_some() {
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log::trace!("Updating agent to be bogus");
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if let Err(err) = update_symlink(".", &self.sock_path) {
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log::error!(
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"Problem updating {} -> .: {err:#}",
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self.sock_path.display()
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);
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}
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}
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}
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}
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}
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}
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@ -311,6 +311,8 @@ impl SessionHandler {
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// on from the `wezterm cli list-clients` information
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if let Some(proxy_id) = &self.proxy_client_id {
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client_id.ssh_auth_sock = proxy_id.ssh_auth_sock.clone();
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// Note that this `via proxy pid` string is coupled
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// with the logic in mux/src/ssh_agent
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client_id.hostname =
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format!("{} (via proxy pid {})", client_id.hostname, proxy_id.pid);
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}
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