Use double-fork when spawning subprocesses

Fixes https://github.com/YaLTeR/niri/issues/10

src/input.rs

@@ -1,5 +1,3 @@
-use std::process::Command;
-
 use smithay::backend::input::{
     AbsolutePositionEvent, Axis, AxisSource, ButtonState, Device, DeviceCapability, Event,
     GestureBeginEvent, GestureEndEvent, GesturePinchUpdateEvent as _, GestureSwipeUpdateEvent as _,
@@ -19,7 +17,7 @@ use smithay::wayland::tablet_manager::{TabletDescriptor, TabletSeatTrait};
 
 use crate::config::{Action, Config, Modifiers};
 use crate::niri::State;
-use crate::utils::get_monotonic_time;
+use crate::utils::{get_monotonic_time, spawn};
 
 pub enum CompositorMod {
     Super,
@@ -149,9 +147,7 @@ impl State {
                         }
                         Action::Spawn(command) => {
                             if let Some((command, args)) = command.split_first() {
-                                if let Err(err) = Command::new(command).args(args).spawn() {
-                                    warn!("error spawning {command}: {err}");
-                                }
+                                spawn(command, args);
                             }
                         }
                         Action::Screenshot => {

src/main.rs

@@ -25,6 +25,7 @@ use portable_atomic::Ordering;
 use smithay::reexports::calloop::EventLoop;
 use smithay::reexports::wayland_server::Display;
 use tracing_subscriber::EnvFilter;
+use utils::spawn;
 
 #[derive(Parser)]
 #[command(author, version, about, long_about = None)]
@@ -76,9 +77,7 @@ fn main() {
     let mut data = LoopData { display, state };
 
     if let Some((command, args)) = cli.command.split_first() {
-        if let Err(err) = std::process::Command::new(command).args(args).spawn() {
-            warn!("error spawning command: {err:?}");
-        }
+        spawn(command, args);
     }
 
     event_loop

src/utils.rs

@@ -1,6 +1,9 @@
+use std::ffi::OsStr;
 use std::fs::File;
-use std::io::Read;
+use std::io::{self, Read};
+use std::os::unix::process::CommandExt;
 use std::path::PathBuf;
+use std::process::{Command, Stdio};
 use std::time::Duration;
 
 use anyhow::{anyhow, Context};
@@ -107,3 +110,49 @@ pub fn make_screenshot_path() -> anyhow::Result<PathBuf> {
 
     Ok(path)
 }
+
+/// Spawns the command to run independently of the compositor.
+pub fn spawn(command: impl AsRef<OsStr>, args: impl IntoIterator<Item = impl AsRef<OsStr>>) {
+    let _span = tracy_client::span!();
+
+    let command = command.as_ref();
+
+    let mut process = Command::new(command);
+    process
+        .args(args)
+        .stdin(Stdio::null())
+        .stdout(Stdio::null())
+        .stderr(Stdio::null());
+
+    // Double-fork to avoid having to waitpid the child.
+    unsafe {
+        process.pre_exec(|| {
+            match libc::fork() {
+                -1 => return Err(io::Error::last_os_error()),
+                0 => (),
+                _ => libc::_exit(0),
+            }
+
+            Ok(())
+        });
+    }
+
+    let mut child = match process.spawn() {
+        Ok(child) => child,
+        Err(err) => {
+            warn!("error spawning {command:?}: {err:?}");
+            return;
+        }
+    };
+
+    match child.wait() {
+        Ok(status) => {
+            if !status.success() {
+                warn!("child did not exit successfully: {status:?}");
+            }
+        }
+        Err(err) => {
+            warn!("error waiting for child: {err:?}");
+        }
+    }
+}