Use tokio to asynchronously read from pipe

Avoids either `spawn_blocking`, or potentially blocking call in an async
function (though it shouldn't block for long).

src/desktop.rs

@@ -335,7 +335,7 @@ where
     // https://systemd.io/DESKTOP_ENVIRONMENTS
     //
     // Similar to what Gnome sets, for now.
-    if let Ok(Some(pid)) = tokio::task::spawn_blocking(|| crate::process::spawn(cmd)).await {
+    if let Some(pid) = crate::process::spawn(cmd).await {
         if let Ok(session) = zbus::Connection::session().await {
             if let Ok(systemd_manager) = SystemdMangerProxy::new(&session).await {
                 let _ = systemd_manager

src/process.rs

@@ -1,9 +1,10 @@
 use std::fs::File;
 use std::io;
 use std::process::{exit, Command, Stdio};
+use tokio::io::AsyncReadExt;
 
 /// Performs a double fork with setsid to spawn and detach a command.
-pub fn spawn(mut command: Command) -> Option<u32> {
+pub async fn spawn(mut command: Command) -> Option<u32> {
     command
         .stdin(Stdio::null())
         .stdout(Stdio::null())
@@ -15,16 +16,11 @@ pub fn spawn(mut command: Command) -> Option<u32> {
 
     match unsafe { libc::fork() } {
         // Parent process
-        child @ 1.. => {
-            let child = rustix::process::Pid::from_raw(child).unwrap();
-            let _res = rustix::process::waitpid(Some(child), rustix::process::WaitOptions::empty());
+        1.. => {
+            drop(write);
             // Read PID from pipe
-            let mut bytes = [0; 4];
-            if rustix::io::read(read, &mut bytes) == Ok(4) {
-                Some(u32::from_ne_bytes(bytes))
-            } else {
-                None
-            }
+            let mut read = tokio::net::unix::pipe::Receiver::from_owned_fd(read).unwrap();
+            read.read_u32().await.ok()
         }
 
         // Child process
@@ -32,7 +28,7 @@ pub fn spawn(mut command: Command) -> Option<u32> {
             let _res = rustix::process::setsid();
             if let Ok(child) = command.spawn() {
                 // Write PID to pipe
-                let _ = rustix::io::write(write, &child.id().to_ne_bytes());
+                let _ = rustix::io::write(write, &child.id().to_be_bytes());
             }
 
             exit(0)