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",

i18n/en/cosmic_term.ftl

@@ -60,6 +60,8 @@ focus-follow-mouse = Typing focus follows mouse
 advanced = Advanced
 show-headerbar = Show header
 show-header-description = Reveal the header from the right-click menu.
+open-in-cwd = Use parent CWD
+open-in-cwd-description = Start new terms using the focused tab's working directory.
 
 # Find
 find-placeholder = Find...

src/config.rs

@@ -229,6 +229,8 @@ pub struct Config {
     pub font_stretch: u16,
     pub font_size_zoom_step_mul_100: u16,
     pub opacity: u8,
+    /// Open new terminal with the current working directory of the focused term
+    pub open_in_cwd: bool,
     pub profiles: BTreeMap<ProfileId, Profile>,
     pub show_headerbar: bool,
     pub use_bright_bold: bool,
@@ -253,6 +255,7 @@ impl Default for Config {
             font_stretch: Stretch::Normal.to_number(),
             font_weight: Weight::NORMAL.0,
             opacity: 100,
+            open_in_cwd: true,
             profiles: BTreeMap::new(),
             show_headerbar: true,
             syntax_theme_dark: COSMIC_THEME_DARK.to_string(),

src/main.rs

@@ -335,6 +335,7 @@ pub enum Message {
     ProfileSyntaxTheme(ProfileId, ColorSchemeKind, usize),
     ProfileTabTitle(ProfileId, String),
     SelectAll(Option<segmented_button::Entity>),
+    SetOpenInCWD(bool),
     ShowAdvancedFontSettings(bool),
     ShowHeaderBar(bool),
     SyntaxTheme(ColorSchemeKind, usize),
@@ -1196,11 +1197,17 @@ impl App {
                 .toggler(self.config.focus_follow_mouse, Message::FocusFollowMouse),
         );
 
-        let advanced_section = widget::settings::view_section(fl!("advanced")).add(
-            widget::settings::item::builder(fl!("show-headerbar"))
-                .description(fl!("show-header-description"))
-                .toggler(self.config.show_headerbar, Message::ShowHeaderBar),
-        );
+        let advanced_section = widget::settings::view_section(fl!("advanced"))
+            .add(
+                widget::settings::item::builder(fl!("show-headerbar"))
+                    .description(fl!("show-header-description"))
+                    .toggler(self.config.show_headerbar, Message::ShowHeaderBar),
+            )
+            .add(
+                widget::settings::item::builder(fl!("open-in-cwd"))
+                    .description(fl!("open-in-cwd-description"))
+                    .toggler(self.config.open_in_cwd, Message::SetOpenInCWD),
+            );
 
         widget::settings::view_column(vec![
             appearance_section.into(),
@@ -1238,6 +1245,22 @@ impl App {
                     Some(colors) => {
                         let current_pane = self.pane_model.focus;
                         if let Some(tab_model) = self.pane_model.active_mut() {
+                            // Current working directory of the selected tab/terminal
+                            #[cfg(not(windows))]
+                            let cwd = self
+                                .config
+                                .open_in_cwd
+                                .then(|| {
+                                    tab_model.active_data::<Mutex<Terminal>>().and_then(
+                                        |terminal| {
+                                            terminal.lock().unwrap().current_working_directory()
+                                        },
+                                    )
+                                })
+                                .flatten();
+                            #[cfg(windows)]
+                            let cwd: Option<std::path::PathBuf> = None;
+
                             // 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))
@@ -1250,8 +1273,8 @@ 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,
@@ -1266,7 +1289,12 @@ 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()
@@ -2198,6 +2226,12 @@ impl Application for App {
                 }
                 return self.update_focus();
             }
+            Message::SetOpenInCWD(open_in_cwd) => {
+                if open_in_cwd != self.config.open_in_cwd {
+                    self.config.open_in_cwd = open_in_cwd;
+                    return self.update_config();
+                }
+            }
             Message::ShowHeaderBar(show_headerbar) => {
                 if show_headerbar != self.config.show_headerbar {
                     config_set!(show_headerbar, show_headerbar);

src/terminal.rs

@@ -25,6 +25,8 @@ use cosmic_text::{
     Weight, Wrap,
 };
 use indexmap::IndexSet;
+#[cfg(not(windows))]
+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;
@@ -214,6 +218,10 @@ pub struct Terminal {
     size: Size,
     use_bright_bold: bool,
     zoom_adj: i8,
+    #[cfg(not(windows))]
+    master_fd: Option<rustix::fd::OwnedFd>,
+    #[cfg(not(windows))]
+    shell_pid: rustix::process::Pid,
 }
 
 impl Terminal {
@@ -283,6 +291,11 @@ 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();
@@ -306,6 +319,10 @@ impl Terminal {
             term,
             use_bright_bold,
             zoom_adj: Default::default(),
+            #[cfg(not(windows))]
+            master_fd,
+            #[cfg(not(windows))]
+            shell_pid,
         })
     }
 
@@ -924,6 +941,32 @@ 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 {