feat: Open new terms in focused term's CWD

Closes: #251

This patch implements an optional (but enabled by default) feature for
opening new terminals using the focused terminal's working directory.
The code to retrieve the CWD is largely based on Alacritty's
implementation of the same feature.

I added Rustix as a new direct dependency for the working directory
logic. Both libc and Rustix are transitive dependencies of COSMIC Term.
I opted for Rustix over libc to avoid an `unsafe` block as well as for
its stronger type guarantees.

References:
* https://github.com/alacritty/alacritty/blob/6bd1674bd80e73df0d41e4342ad4e34bb7d04f84/alacritty/src/daemon.rs#L85-L108

Cargo.toml

@@ -23,6 +23,7 @@ ron = "0.8"
 serde = { version = "1", features = ["serde_derive"] }
 shlex = "1"
 tokio = { version = "1", features = ["sync"] }
+rustix = { version = "0.38", features = ["termios"] }
 # Internationalization
 i18n-embed = { version = "0.14", features = [
     "fluent-system",

src/config.rs

@@ -236,6 +236,8 @@ pub struct Config {
     pub syntax_theme_light: String,
     pub focus_follow_mouse: bool,
     pub default_profile: Option<ProfileId>,
+    /// Open new terminal with the current working directory of the focused term
+    pub current_working_directory: bool,
 }
 
 impl Default for Config {
@@ -259,6 +261,7 @@ impl Default for Config {
             syntax_theme_light: COSMIC_THEME_LIGHT.to_string(),
             use_bright_bold: false,
             default_profile: None,
+            current_working_directory: true,
         }
     }
 }

src/main.rs

@@ -1202,6 +1202,18 @@ impl App {
                     Some(colors) => {
                         let current_pane = self.pane_model.focus;
                         if let Some(tab_model) = self.pane_model.active_mut() {
+                            let cwd = self
+                                .config
+                                .current_working_directory
+                                .then(|| {
+                                    tab_model.active_data::<Mutex<Terminal>>().and_then(
+                                        |terminal| {
+                                            terminal.lock().unwrap().current_working_directory()
+                                        },
+                                    )
+                                })
+                                .flatten();
+
                             // Use the profile options, startup options, or defaults
                             let (options, tab_title_override) = match profile_id_opt
                                 .and_then(|profile_id| self.config.profiles.get(&profile_id))
@@ -1214,8 +1226,8 @@ impl App {
                                             shell = Some(tty::Shell::new(command, args));
                                         }
                                     }
-                                    let working_directory = (!profile.working_directory.is_empty())
-                                        .then(|| profile.working_directory.clone().into());
+                                    let working_directory = cwd
+                                        .or_else(|| Some(profile.working_directory.clone().into()));
 
                                     let options = tty::Options {
                                         shell,
@@ -1230,7 +1242,12 @@ impl App {
                                     };
                                     (options, tab_title_override)
                                 }
-                                None => (self.startup_options.take().unwrap_or_default(), None),
+                                None => {
+                                    let mut options =
+                                        self.startup_options.take().unwrap_or_default();
+                                    options.working_directory = cwd;
+                                    (options, None)
+                                }
                             };
                             let entity = tab_model
                                 .insert()

src/terminal.rs

@@ -25,6 +25,8 @@ use cosmic_text::{
     Weight, Wrap,
 };
 use indexmap::IndexSet;
+#[cfg(not(any(windows, target_os = "macos")))]
+use rustix::fd::AsFd;
 use std::{
     borrow::Cow,
     collections::HashMap,
@@ -35,6 +37,8 @@ use std::{
     },
     time::Instant,
 };
+#[cfg(not(windows))]
+use std::{fs, path::PathBuf};
 use tokio::sync::mpsc;
 
 pub use alacritty_terminal::grid::Scroll as TerminalScroll;
@@ -212,6 +216,10 @@ pub struct Terminal {
     search_value: String,
     size: Size,
     use_bright_bold: bool,
+    #[cfg(not(windows))]
+    master_fd: Option<rustix::fd::OwnedFd>,
+    #[cfg(not(windows))]
+    shell_pid: rustix::process::Pid,
 }
 
 impl Terminal {
@@ -281,6 +289,11 @@ impl Terminal {
         let window_id = 0;
         let pty = tty::new(&options, size.into(), window_id)?;
 
+        #[cfg(not(windows))]
+        let master_fd = pty.file().as_fd().try_clone_to_owned().ok();
+        #[cfg(not(windows))]
+        let shell_pid = rustix::process::Pid::from_child(pty.child());
+
         let pty_event_loop = EventLoop::new(term.clone(), event_proxy, pty, options.hold, false)?;
         let notifier = Notifier(pty_event_loop.channel());
         let _pty_join_handle = pty_event_loop.spawn();
@@ -303,6 +316,10 @@ impl Terminal {
             tab_title_override,
             term,
             use_bright_bold,
+            #[cfg(not(any(windows, target_os = "macos")))]
+            master_fd,
+            #[cfg(not(any(windows, target_os = "macos")))]
+            shell_pid,
         })
     }
 
@@ -915,6 +932,32 @@ impl Terminal {
             );
         }
     }
+
+    /// Current working directory
+    #[cfg(not(windows))]
+    pub fn current_working_directory(&self) -> Option<PathBuf> {
+        // Largely based off of Alacritty
+        // https://github.com/alacritty/alacritty/blob/6bd1674bd80e73df0d41e4342ad4e34bb7d04f84/alacritty/src/daemon.rs#L85-L108
+        let pid = self
+            .master_fd
+            .as_ref()
+            .and_then(|pid| rustix::termios::tcgetpgrp(pid).ok())
+            .or(Some(self.shell_pid))?;
+
+        #[cfg(not(any(target_os = "freebsd", target_os = "macos")))]
+        let link_path = format!("/proc/{}/cwd", pid.as_raw_nonzero());
+        #[cfg(target_os = "freebsd")]
+        let link_path = format!("/compat/linux/proc/{}/cwd", pid.as_raw_nonzero());
+
+        #[cfg(not(target_os = "macos"))]
+        let cwd = fs::read_link(link_path).ok();
+
+        // TODO: macOS support
+        #[cfg(target_os = "macos")]
+        let cwd = None;
+
+        cwd
+    }
 }
 
 impl Drop for Terminal {