RAII cleanup wthin the ASIO backend

- Shares the `Device`'s `driver` and `asio_streams` `Arc`s with the
`Stream`s to ensure they remain valid if the `Host` or `Device` are
dropped early.
- Ensures that a stream's callback is removed upon `Drop`.

asio-sys/src/bindings/mod.rs

@@ -235,6 +235,9 @@ struct BufferSizes {
     grans: c_long,
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct CallbackId(usize);
+
 lazy_static! {
     /// A global way to access all the callbacks.
     ///
@@ -244,7 +247,7 @@ lazy_static! {
     /// Options are used so that when a callback is removed we don't change the Vec indices.
     ///
     /// The indices are how we match a callback with a stream.
-    static ref BUFFER_CALLBACK: Mutex<Vec<Option<BufferCallback>>> = Mutex::new(Vec::new());
+    static ref BUFFER_CALLBACK: Mutex<Vec<(CallbackId, BufferCallback)>> = Mutex::new(Vec::new());
 }
 
 impl Asio {
@@ -589,12 +592,26 @@ impl Driver {
     /// Adds a callback to the list of active callbacks.
     ///
     /// The given function receives the index of the buffer currently ready for processing.
-    pub fn set_callback<F>(&self, callback: F)
+    ///
+    /// Returns an ID uniquely associated with the given callback so that it may be removed later.
+    pub fn add_callback<F>(&self, callback: F) -> CallbackId
     where
         F: 'static + FnMut(i32) + Send,
     {
         let mut bc = BUFFER_CALLBACK.lock().unwrap();
-        bc.push(Some(BufferCallback(Box::new(callback))));
+        let id = bc
+            .last()
+            .map(|&(id, _)| CallbackId(id.0.checked_add(1).expect("stream ID overflowed")))
+            .unwrap_or(CallbackId(0));
+        let cb = BufferCallback(Box::new(callback));
+        bc.push((id, cb));
+        id
+    }
+
+    /// Remove the callback with the given ID.
+    pub fn remove_callback(&self, rem_id: CallbackId) {
+        let mut bc = BUFFER_CALLBACK.lock().unwrap();
+        bc.retain(|&(id, _)| id != rem_id);
     }
 
     /// Consumes and destroys the `Driver`, stopping the streams if they are running and releasing
@@ -863,11 +880,9 @@ extern "C" fn buffer_switch_time_info(
 ) -> *mut ai::ASIOTime {
     // This lock is probably unavoidable, but locks in the audio stream are not great.
     let mut bcs = BUFFER_CALLBACK.lock().unwrap();
-    for mut bc in bcs.iter_mut() {
-        if let Some(ref mut bc) = bc {
+    for &mut (_, ref mut bc) in bcs.iter_mut() {
         bc.run(double_buffer_index);
     }
-    }
     time
 }
 

src/host/asio/stream.rs

@@ -6,6 +6,7 @@ use super::Device;
 use std;
 use std::sync::atomic::{Ordering, AtomicBool};
 use std::sync::Arc;
+use super::parking_lot::Mutex;
 use BackendSpecificError;
 use BuildStreamError;
 use Format;
@@ -41,6 +42,10 @@ struct SilenceAsioBuffer {
 
 pub struct Stream {
     playing: Arc<AtomicBool>,
+    // Ensure the `Driver` does not terminate until the last stream is dropped.
+    driver: Arc<sys::Driver>,
+    asio_streams: Arc<Mutex<sys::AsioStreams>>,
+    callback_id: sys::CallbackId,
 }
 
 impl Stream {
@@ -55,8 +60,6 @@ impl Stream {
     }
 }
 
-// TODO: drop implementation
-
 impl Device {
     pub fn build_input_stream<D, E>(
         &self,
@@ -91,7 +94,7 @@ impl Device {
 
         // Set the input callback.
         // This is most performance critical part of the ASIO bindings.
-        self.driver.set_callback(move |buffer_index| unsafe {
+        let callback_id = self.driver.add_callback(move |buffer_index| unsafe {
             // If not playing return early.
             if !playing.load(Ordering::SeqCst) {
                 return
@@ -217,10 +220,18 @@ impl Device {
             }
         });
 
+        let driver = self.driver.clone();
+        let asio_streams = self.asio_streams.clone();
+
         // Immediately start the device?
         self.driver.start().map_err(build_stream_err)?;
 
-        Ok(Stream { playing: stream_playing })
+        Ok(Stream {
+            playing: stream_playing,
+            driver,
+            asio_streams,
+            callback_id,
+        })
     }
 
     pub fn build_output_stream<D, E>(
@@ -255,7 +266,7 @@ impl Device {
         let playing = Arc::clone(&stream_playing);
         let asio_streams = self.asio_streams.clone();
 
-        self.driver.set_callback(move |buffer_index| unsafe {
+        let callback_id = self.driver.add_callback(move |buffer_index| unsafe {
             // If not playing, return early.
             if !playing.load(Ordering::SeqCst) {
                 return
@@ -421,10 +432,18 @@ impl Device {
             }
         });
 
+        let driver = self.driver.clone();
+        let asio_streams = self.asio_streams.clone();
+
         // Immediately start the device?
         self.driver.start().map_err(build_stream_err)?;
 
-        Ok(Stream { playing: stream_playing })
+        Ok(Stream {
+            playing: stream_playing,
+            driver,
+            asio_streams,
+            callback_id,
+        })
     }
 
     /// Create a new CPAL Input Stream.
@@ -508,6 +527,12 @@ impl Device {
     }
 }
 
+impl Drop for Stream {
+    fn drop(&mut self) {
+        self.driver.remove_callback(self.callback_id);
+    }
+}
+
 impl Silence for i16 {
     const SILENCE: Self = 0;
 }