fix(alsa): Don't leak alsactl monitor process

[exrok]

This bug was found when alsactl was repeatedly failing due to a missing device causing a number of alsactl processes to be running.

This PR uses tokio's kill_on_drop and an added the process handle to a field in the device. I have not checked the rest of the code base for similar issues but at least after this PR alsa is fixed. :)

https://docs.rs/tokio/latest/tokio/process/struct.Command.html#method.kill_on_drop

[bim9262]

Why not just keep the process in the struct Device and get the stdout in wait_for_update?

Like this

diff --git a/src/blocks/sound/alsa.rs b/src/blocks/sound/alsa.rs
index efd2f18dc..4fff9e788 100644
--- a/src/blocks/sound/alsa.rs
+++ b/src/blocks/sound/alsa.rs
@@ -1,6 +1,6 @@
 use std::cmp::{max, min};
 use std::process::Stdio;
-use tokio::process::{ChildStdout, Command};
+use tokio::process::Command;
 
 use super::super::prelude::*;
 use super::SoundDevice;
@@ -11,33 +11,23 @@ pub(super) struct Device {
     natural_mapping: bool,
     volume: u32,
     muted: bool,
-    monitor: ChildStdout,
-    #[expect(
-        dead_code,
-        reason = "Only used in drop to trigger kill and reap of child process"
-    )]
     process: tokio::process::Child,
 }
 
 impl Device {
     pub(super) fn new(name: String, device: String, natural_mapping: bool) -> Result<Self> {
-        let mut process = Command::new("alsactl")
-            .arg("monitor")
-            .kill_on_drop(true)
-            .stdout(Stdio::piped())
-            .spawn()
-            .error("Failed to start alsactl monitor")?;
         Ok(Device {
             name,
             device,
             natural_mapping,
             volume: 0,
             muted: false,
-            monitor: process
-                .stdout
-                .take()
-                .error("Failed to pipe alsactl monitor output")?,
-            process,
+            process: Command::new("alsactl")
+                .arg("monitor")
+                .kill_on_drop(true)
+                .stdout(Stdio::piped())
+                .spawn()
+                .error("Failed to start alsactl monitor")?,
         })
     }
 }
@@ -147,7 +137,10 @@ impl SoundDevice for Device {
 
     async fn wait_for_update(&mut self) -> Result<()> {
         let mut buf = [0u8; 1024];
-        self.monitor
+        self.process
+            .stdout
+            .as_mut()
+            .error("Failed to get stdout of alsactl monitor")?
             .read(&mut buf)
             .await
             .error("Failed to read stdbuf output")?;