Spawn new alacritty processes in CWD on macOS

On macOS we can use 'proc_pidinfo' to determine the working
directory of the terminal foreground process.

Fixes #1979.

6 files changed, 178 insertions, 8 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a350cff1..f8d615b7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -31,7 +31,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ### Changed
 
 - Cursors are now inverted when their fixed color is similar to the cell's background
-- Use working directory of active process instead of shell for SpawnNewInstance action
+- Use the working directory of the terminal foreground process, instead of the shell's working
+    directory, for `SpawnNewInstance` action
 - Fallback to normal underline for unsupported underline types in `CSI 4 : ? m` escapes
 - The user's background color is now used as the foreground for the render timer
 - Use yellow/red from the config for error and warning messages instead of fixed colors
@@ -39,6 +40,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Wayland's Client side decorations now use the search bar colors
 - Reduce memory usage by up to at least 30% with a full scrollback buffer
 - The number of zerowidth characters per cell is no longer limited to 5
+- `SpawnNewInstance` is now using the working directory of the terminal foreground process on macOS
 
 ### Fixed
 
diff --git a/alacritty/src/event.rs b/alacritty/src/event.rs
index 0f9c24a5..20f087c3 100644
--- a/alacritty/src/event.rs
+++ b/alacritty/src/event.rs
@@ -4,7 +4,7 @@ use std::borrow::Cow;
 use std::cmp::{max, min};
 use std::env;
 use std::fmt::Debug;
-#[cfg(unix)]
+#[cfg(not(any(target_os = "macos", windows)))]
 use std::fs;
 use std::fs::File;
 use std::io::Write;
@@ -45,6 +45,8 @@ use crate::config::Config;
 use crate::daemon::start_daemon;
 use crate::display::{Display, DisplayUpdate};
 use crate::input::{self, ActionContext as _, FONT_SIZE_STEP};
+#[cfg(target_os = "macos")]
+use crate::macos;
 use crate::message_bar::{Message, MessageBuffer};
 use crate::scheduler::{Scheduler, TimerId};
 use crate::url::{Url, Urls};
@@ -309,15 +311,17 @@ impl<'a, N: Notify + 'a, T: EventListener> input::ActionContext<T> for ActionCon
                 pid = tty::child_pid();
             }
 
-            #[cfg(not(target_os = "freebsd"))]
+            #[cfg(not(any(target_os = "macos", target_os = "freebsd")))]
             let link_path = format!("/proc/{}/cwd", pid);
             #[cfg(target_os = "freebsd")]
             let link_path = format!("/compat/linux/proc/{}/cwd", pid);
+            #[cfg(not(target_os = "macos"))]
+            let cwd = fs::read_link(link_path);
+            #[cfg(target_os = "macos")]
+            let cwd = macos::proc::cwd(pid);
 
             // Add the current working directory as parameter.
-            fs::read_link(link_path)
-                .map(|path| vec!["--working-directory".into(), path])
-                .unwrap_or_default()
+            cwd.map(|path| vec!["--working-directory".into(), path]).unwrap_or_default()
         };
 
         #[cfg(not(unix))]
diff --git a/alacritty/src/locale.rs b/alacritty/src/macos/locale.rs
index 6c02c12c..6c02c12c 100644
--- a/alacritty/src/locale.rs
+++ b/alacritty/src/macos/locale.rs
diff --git a/alacritty/src/macos/mod.rs b/alacritty/src/macos/mod.rs
new file mode 100644
index 00000000..1d630730
--- /dev/null
+++ b/alacritty/src/macos/mod.rs
@@ -0,0 +1,2 @@
+pub mod locale;
+pub mod proc;
diff --git a/alacritty/src/macos/proc.rs b/alacritty/src/macos/proc.rs
new file mode 100644
index 00000000..be7e4891
--- /dev/null
+++ b/alacritty/src/macos/proc.rs
@@ -0,0 +1,160 @@
+use std::ffi::{CStr, CString, IntoStringError};
+use std::fmt::{self, Display, Formatter};
+use std::io;
+use std::mem::{self, MaybeUninit};
+use std::os::raw::{c_int, c_void};
+use std::path::PathBuf;
+
+/// Error during working directory retrieval.
+#[derive(Debug)]
+pub enum Error {
+    Io(io::Error),
+
+    /// Error converting into utf8 string.
+    IntoString(IntoStringError),
+
+    /// Expected return size didn't match libproc's.
+    InvalidSize,
+}
+
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            Error::InvalidSize => None,
+            Error::Io(err) => err.source(),
+            Error::IntoString(err) => err.source(),
+        }
+    }
+}
+
+impl Display for Error {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        match self {
+            Error::InvalidSize => write!(f, "Invalid proc_pidinfo return size"),
+            Error::Io(err) => write!(f, "Error getting current working directory: {}", err),
+            Error::IntoString(err) => {
+                write!(f, "Error when parsing current working directory: {}", err)
+            },
+        }
+    }
+}
+
+impl From<io::Error> for Error {
+    fn from(val: io::Error) -> Self {
+        Error::Io(val)
+    }
+}
+
+impl From<IntoStringError> for Error {
+    fn from(val: IntoStringError) -> Self {
+        Error::IntoString(val)
+    }
+}
+
+pub fn cwd(pid: c_int) -> Result<PathBuf, Error> {
+    let mut info = MaybeUninit::<sys::proc_vnodepathinfo>::uninit();
+    let info_ptr = info.as_mut_ptr() as *mut c_void;
+    let size = mem::size_of::<sys::proc_vnodepathinfo>() as c_int;
+
+    let c_str = unsafe {
+        let pidinfo_size = sys::proc_pidinfo(pid, sys::PROC_PIDVNODEPATHINFO, 0, info_ptr, size);
+        match pidinfo_size {
+            c if c < 0 => return Err(io::Error::last_os_error().into()),
+            s if s != size => return Err(Error::InvalidSize),
+            _ => CStr::from_ptr(info.assume_init().pvi_cdir.vip_path.as_ptr()),
+        }
+    };
+
+    Ok(CString::from(c_str).into_string().map(PathBuf::from)?)
+}
+
+/// Bindings for libproc.
+#[allow(non_camel_case_types)]
+mod sys {
+    use std::os::raw::{c_char, c_int, c_longlong, c_void};
+
+    pub const PROC_PIDVNODEPATHINFO: c_int = 9;
+
+    type gid_t = c_int;
+    type off_t = c_longlong;
+    type uid_t = c_int;
+    type fsid_t = fsid;
+
+    #[repr(C)]
+    #[derive(Debug, Copy, Clone)]
+    pub struct fsid {
+        pub val: [i32; 2usize],
+    }
+
+    #[repr(C)]
+    #[derive(Debug, Copy, Clone)]
+    pub struct vinfo_stat {
+        pub vst_dev: u32,
+        pub vst_mode: u16,
+        pub vst_nlink: u16,
+        pub vst_ino: u64,
+        pub vst_uid: uid_t,
+        pub vst_gid: gid_t,
+        pub vst_atime: i64,
+        pub vst_atimensec: i64,
+        pub vst_mtime: i64,
+        pub vst_mtimensec: i64,
+        pub vst_ctime: i64,
+        pub vst_ctimensec: i64,
+        pub vst_birthtime: i64,
+        pub vst_birthtimensec: i64,
+        pub vst_size: off_t,
+        pub vst_blocks: i64,
+        pub vst_blksize: i32,
+        pub vst_flags: u32,
+        pub vst_gen: u32,
+        pub vst_rdev: u32,
+        pub vst_qspare: [i64; 2usize],
+    }
+
+    #[repr(C)]
+    #[derive(Debug, Copy, Clone)]
+    pub struct vnode_info {
+        pub vi_stat: vinfo_stat,
+        pub vi_type: c_int,
+        pub vi_pad: c_int,
+        pub vi_fsid: fsid_t,
+    }
+
+    #[repr(C)]
+    #[derive(Copy, Clone)]
+    pub struct vnode_info_path {
+        pub vip_vi: vnode_info,
+        pub vip_path: [c_char; 1024usize],
+    }
+
+    #[repr(C)]
+    #[derive(Copy, Clone)]
+    pub struct proc_vnodepathinfo {
+        pub pvi_cdir: vnode_info_path,
+        pub pvi_rdir: vnode_info_path,
+    }
+
+    extern "C" {
+        pub fn proc_pidinfo(
+            pid: c_int,
+            flavor: c_int,
+            arg: u64,
+            buffer: *mut c_void,
+            buffersize: c_int,
+        ) -> c_int;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use std::env;
+    use std::process;
+
+    #[test]
+    fn cwd_matches_current_dir() {
+        assert_eq!(cwd(process::id() as i32).ok(), env::current_dir().ok());
+    }
+}
diff --git a/alacritty/src/main.rs b/alacritty/src/main.rs
index dc316c79..98fe2d05 100644
--- a/alacritty/src/main.rs
+++ b/alacritty/src/main.rs
@@ -38,9 +38,9 @@ mod daemon;
 mod display;
 mod event;
 mod input;
-#[cfg(target_os = "macos")]
-mod locale;
 mod logging;
+#[cfg(target_os = "macos")]
+mod macos;
 mod message_bar;
 mod meter;
 #[cfg(windows)]
@@ -63,6 +63,8 @@ use crate::config::monitor;
 use crate::config::Config;
 use crate::display::Display;
 use crate::event::{Event, EventProxy, Processor};
+#[cfg(target_os = "macos")]
+use crate::macos::locale;
 use crate::message_bar::MessageBuffer;
 
 fn main() {