Add smol implementation for spawn

Although this is used in apps that use `tokio`, if we're going to
support `smol`, this seems suboptimal to require.

This assumes the function will be called by a tokio executor if the
`tokio` feature is used. Otherwise it can be spawned from any executor.
That should be consistent with everything else.

This fails to compile without either the `tokio` or `smol` feature. This
seems reasonable, since `zbus` also fails to compile in that case.

src/process.rs

@@ -1,8 +1,26 @@
+#[cfg(all(feature = "smol", not(feature = "tokio")))]
+use smol::io::AsyncReadExt;
 use std::fs::File;
 use std::io;
+use std::os::fd::OwnedFd;
 use std::process::{exit, Command, Stdio};
+#[cfg(feature = "tokio")]
 use tokio::io::AsyncReadExt;
 
+#[cfg(feature = "tokio")]
+async fn read_from_pipe(read: OwnedFd) -> Option<u32> {
+    let mut read = tokio::net::unix::pipe::Receiver::from_owned_fd(read).unwrap();
+    read.read_u32().await.ok()
+}
+
+#[cfg(all(feature = "smol", not(feature = "tokio")))]
+async fn read_from_pipe(read: OwnedFd) -> Option<u32> {
+    let mut read = smol::Async::new(std::fs::File::from(read)).unwrap();
+    let mut bytes = [0; 4];
+    read.read_exact(&mut bytes).await.ok()?;
+    Some(u32::from_be_bytes(bytes))
+}
+
 /// Performs a double fork with setsid to spawn and detach a command.
 pub async fn spawn(mut command: Command) -> Option<u32> {
     command
@@ -17,10 +35,9 @@ pub async fn spawn(mut command: Command) -> Option<u32> {
     match unsafe { libc::fork() } {
         // Parent process
         1.. => {
+            // Drop copy of write end, then read PID from pipe
             drop(write);
-            // Read PID from pipe
-            let mut read = tokio::net::unix::pipe::Receiver::from_owned_fd(read).unwrap();
-            read.read_u32().await.ok()
+            read_from_pipe(read).await
         }
 
         // Child process