nushell/crates/nu-system/src/foreground.rs

use std::process::{Child, Command};

/// A simple wrapper for `std::process::Command`
///
/// ## spawn behavior
/// ### Unix
/// When invoke `spawn`, current process will block `SIGTSTP`, `SIGTTOU`, `SIGTTIN`, `SIGCHLD`
///
/// spawned child process will get it's own process group id, and it's going to foreground(by making stdin belong's to child's process group).
///
/// When child is to over, unblock `SIGTSTP`, `SIGTTOU`, `SIGTTIN`, `SIGCHLD`, foreground process is back to callers' process.
/// It bahaves something like `SignalHandler` in ion(https://gitlab.redox-os.org/redox-os/ion/-/tree/master/).
///
/// ### Windows
/// It does nothing special on windows system, `spawn` is the same as [std::process::Command::spawn](std::process::Command::spawn)
pub struct ForegroundProcess {
    inner: Command,
}

/// A simple wrapper for `std::process::Child`
///
/// It can only be created by `ForegroundProcess::spawn`.
pub struct ForegroundChild {
    inner: Child,
}

impl ForegroundProcess {
    pub fn new(cmd: Command) -> Self {
        Self { inner: cmd }
    }

    pub fn spawn(&mut self) -> std::io::Result<ForegroundChild> {
        fg_process_setup::prepare_to_foreground(&mut self.inner);
        self.inner.spawn().map(|child| {
            fg_process_setup::set_foreground(&child);
            ForegroundChild { inner: child }
        })
    }
}

impl AsMut<Child> for ForegroundChild {
    fn as_mut(&mut self) -> &mut Child {
        &mut self.inner
    }
}

impl Drop for ForegroundChild {
    fn drop(&mut self) {
        // It's ok to use here because we have called `set_foreground` during creation.
        unsafe { fg_process_setup::reset_foreground_id() }
    }
}

// It's a simpler version of fish shell's external process handling.
#[cfg(target_family = "unix")]
mod fg_process_setup {
    use crate::signal::{block, unblock};
    use nix::unistd::{self, Pid};
    use std::os::unix::prelude::CommandExt;

    pub(super) fn prepare_to_foreground(external_command: &mut std::process::Command) {
        unsafe {
            block();
            // Safety:
            // POSIX only allows async-signal-safe functions to be called.
            // And `setpgid` is async-signal-safe function according to:
            // https://manpages.ubuntu.com/manpages/bionic/man7/signal-safety.7.html
            // So we're ok to invoke `libc::setpgid` inside `pre_exec`.
            external_command.pre_exec(|| {
                // make the command startup with new process group.
                // The process group id must be the same as external commands' pid.
                // Or else we'll failed to set it as foreground process.
                // For more information, check `fork_child_for_process` function:
                // https://github.com/fish-shell/fish-shell/blob/023042098396aa450d2c4ea1eb9341312de23126/src/exec.cpp#L398
                if let Err(e) = unistd::setpgid(Pid::from_raw(0), Pid::from_raw(0)) {
                    println!("ERROR: setpgid for external failed, result: {e:?}");
                }
                Ok(())
            });
        }
    }

    // If `prepare_to_foreground` function is not called, the function will fail with silence and do nothing.
    pub(super) fn set_foreground(process: &std::process::Child) {
        if atty::is(atty::Stream::Stdin) {
            if let Err(e) =
                nix::unistd::tcsetpgrp(nix::libc::STDIN_FILENO, Pid::from_raw(process.id() as i32))
            {
                println!("ERROR: set foreground id failed, tcsetpgrp result: {e:?}");
            }
        }
    }

    /// Reset foreground to current process, unblock `SIGTSTP`, `SIGTTOU`, `SIGTTIN`, `SIGCHLD`
    ///
    /// ## Safety
    /// It can only be called when you have called `set_foreground`, or results in undefined behavior.
    pub(super) unsafe fn reset_foreground_id() {
        if atty::is(atty::Stream::Stdin) {
            if let Err(e) = nix::unistd::tcsetpgrp(nix::libc::STDIN_FILENO, unistd::getpgrp()) {
                println!("ERROR: reset foreground id failed, tcsetpgrp result: {e:?}");
            }
        }
        unblock()
    }
}

// TODO: investigate if we can set foreground process through windows system call.
#[cfg(target_family = "windows")]
mod fg_process_setup {

    pub(super) fn prepare_to_foreground(_external_command: &mut std::process::Command) {}

    pub(super) fn set_foreground(_process: &std::process::Child) {}

    pub(super) unsafe fn reset_foreground_id() {}
}