Fix: hand off process killing to blocking thread, await all children.

This should make dropping `ChildContainer`s and their parent `Input`s safer in async contexts.

It seems like SIGINT is insufficient to make wait terminate, but SIGKILL suffices. This introduced a new problem, namely that we have to remember and wait on *every* pid we create. This should, hopefully, put the issue of zombie processes to bed for good.

Cargo.toml

@@ -52,10 +52,6 @@ version = "0.10"
 [dependencies.futures]
 version = "0.3"
 
-[dependencies.nix]
-version = "0.19"
-optional = true
-
 [dependencies.parking_lot]
 optional = true
 version = "0.11"
@@ -137,7 +133,6 @@ driver = [
     "byteorder",
     "discortp",
     "flume",
-    "nix",
     "parking_lot",
     "rand",
     "serenity-voice-model",

src/input/child.rs

@@ -1,55 +1,79 @@
 use super::*;
 use std::{
     io::{BufReader, Read},
+    mem,
     process::Child,
 };
+use tokio::runtime::Handle;
 use tracing::debug;
 
-#[cfg(unix)]
-use nix::{
-    sys::signal::{self, Signal},
-    unistd::Pid,
-};
-
 /// Handle for a child process which ensures that any subprocesses are properly closed
 /// on drop.
+///
+/// # Warning
+/// To allow proper cleanup of child processes, if you create a process chain you must
+/// make sure to use `From<Vec<Child>>`. Here, the *last* process in the `Vec` will be
+/// used as the audio byte source.
 #[derive(Debug)]
-pub struct ChildContainer(Child);
+pub struct ChildContainer(Vec<Child>);
 
-pub(crate) fn child_to_reader<T>(child: Child) -> Reader {
+pub(crate) fn children_to_reader<T>(children: Vec<Child>) -> Reader {
     Reader::Pipe(BufReader::with_capacity(
         STEREO_FRAME_SIZE * mem::size_of::<T>() * CHILD_BUFFER_LEN,
-        ChildContainer(child),
+        ChildContainer(children),
     ))
 }
 
 impl From<Child> for Reader {
     fn from(container: Child) -> Self {
-        child_to_reader::<f32>(container)
+        children_to_reader::<f32>(vec![container])
+    }
+}
+
+impl From<Vec<Child>> for Reader {
+    fn from(container: Vec<Child>) -> Self {
+        children_to_reader::<f32>(container)
     }
 }
 
 impl Read for ChildContainer {
     fn read(&mut self, buffer: &mut [u8]) -> IoResult<usize> {
-        self.0.stdout.as_mut().unwrap().read(buffer)
+        match self.0.last_mut() {
+            Some(ref mut child) => child.stdout.as_mut().unwrap().read(buffer),
+            None => Ok(0),
+        }
     }
 }
 
 impl Drop for ChildContainer {
     fn drop(&mut self) {
-        #[cfg(not(unix))]
-        let attempt = self.0.kill();
+        let children = mem::take(&mut self.0);
 
-        #[cfg(unix)]
-        let attempt = {
-            let pid = Pid::from_raw(self.0.id() as i32);
-            let _ = signal::kill(pid, Signal::SIGINT);
-
-            self.0.wait()
-        };
-
-        if let Err(e) = attempt {
-            debug!("Error awaiting child process: {:?}", e);
+        if let Ok(handle) = Handle::try_current() {
+            handle.spawn_blocking(move || {
+                cleanup_child_processes(children);
+            });
+        } else {
+            cleanup_child_processes(children);
         }
     }
 }
+
+fn cleanup_child_processes(mut children: Vec<Child>) {
+    let attempt = if let Some(child) = children.last_mut() {
+        child.kill()
+    } else {
+        return;
+    };
+
+    let attempt = attempt.and_then(|_| {
+        children
+            .iter_mut()
+            .rev()
+            .try_for_each(|child| child.wait().map(|_| ()))
+    });
+
+    if let Err(e) = attempt {
+        debug!("Error awaiting child process: {:?}", e);
+    }
+}

src/input/ffmpeg_src.rs

@@ -1,5 +1,5 @@
 use super::{
-    child_to_reader,
+    children_to_reader,
     error::{Error, Result},
     Codec,
     Container,
@@ -115,7 +115,7 @@ pub(crate) async fn _ffmpeg_optioned(
 
     Ok(Input::new(
         is_stereo,
-        child_to_reader::<f32>(command),
+        children_to_reader::<f32>(vec![command]),
         Codec::FloatPcm,
         Container::Raw,
         Some(metadata),

src/input/ytdl_src.rs

@@ -1,5 +1,5 @@
 use super::{
-    child_to_reader,
+    children_to_reader,
     error::{Error, Result},
     Codec,
     Container,
@@ -92,12 +92,14 @@ pub(crate) async fn _ytdl(uri: &str, pre_args: &[&str]) -> Result<Input> {
 
     youtube_dl.stderr = Some(returned_stderr);
 
+    let taken_stdout = youtube_dl.stdout.take().ok_or(Error::Stdout)?;
+
     let ffmpeg = Command::new("ffmpeg")
         .args(pre_args)
         .arg("-i")
         .arg("-")
         .args(&ffmpeg_args)
-        .stdin(youtube_dl.stdout.ok_or(Error::Stdout)?)
+        .stdin(taken_stdout)
         .stderr(Stdio::null())
         .stdout(Stdio::piped())
         .spawn()?;
@@ -108,7 +110,7 @@ pub(crate) async fn _ytdl(uri: &str, pre_args: &[&str]) -> Result<Input> {
 
     Ok(Input::new(
         true,
-        child_to_reader::<f32>(ffmpeg),
+        children_to_reader::<f32>(vec![youtube_dl, ffmpeg]),
         Codec::FloatPcm,
         Container::Raw,
         Some(metadata),