set realtime priority for stream threads in alsa and wasapi

implements #939. As someone who doesn't know a whole lot about
cpal's inner workings it looks to me like these are the only
threads that the library spawns itself, leading me to think these
are the only threads that require manual priority changes. However,
`audio_thread_priority` claims to also support MacOS, so maybe
someone could look into whether that's applicable here?

Cargo.toml

@@ -39,12 +39,14 @@ windows = { version = "0.54.0", features = [
     "Win32_Media_Multimedia",
     "Win32_UI_Shell_PropertiesSystem"
 ]}
+audio_thread_priority = "0.33.0"
 asio-sys = { version = "0.2", path = "asio-sys", optional = true }
 num-traits = { version = "0.2.6", optional = true }
 
 [target.'cfg(any(target_os = "linux", target_os = "dragonfly", target_os = "freebsd", target_os = "netbsd"))'.dependencies]
 alsa = "0.9"
 libc = "0.2"
+audio_thread_priority = "0.33.0"
 jack = { version = "0.13.0", optional = true }
 
 [target.'cfg(any(target_os = "macos", target_os = "ios"))'.dependencies]

src/host/alsa/mod.rs

@@ -10,6 +10,7 @@ use crate::{
     SupportedBufferSize, SupportedStreamConfig, SupportedStreamConfigRange,
     SupportedStreamConfigsError,
 };
+use audio_thread_priority::promote_current_thread_to_real_time;
 use std::cell::Cell;
 use std::cmp;
 use std::convert::TryInto;
@@ -589,6 +590,17 @@ fn input_stream_worker(
     error_callback: &mut (dyn FnMut(StreamError) + Send + 'static),
     timeout: Option<Duration>,
 ) {
+    let buffer_size = if let BufferSize::Fixed(buffer_size) = stream.conf.buffer_size {
+        buffer_size
+    } else {
+        // if the buffer size isn't fixed, let audio_thread_priority choose a sensible default value
+        0
+    };
+
+    if let Err(err) = promote_current_thread_to_real_time(buffer_size, stream.conf.sample_rate.0) {
+        eprintln!("Failed to promote audio thread to real-time priority: {err}");
+    }
+
     let mut ctxt = StreamWorkerContext::new(&timeout);
     loop {
         let flow =
@@ -640,6 +652,17 @@ fn output_stream_worker(
     error_callback: &mut (dyn FnMut(StreamError) + Send + 'static),
     timeout: Option<Duration>,
 ) {
+    let buffer_size = if let BufferSize::Fixed(buffer_size) = stream.conf.buffer_size {
+        buffer_size
+    } else {
+        // if the buffer size isn't fixed, let audio_thread_priority choose a sensible default value
+        0
+    };
+
+    if let Err(err) = promote_current_thread_to_real_time(buffer_size, stream.conf.sample_rate.0) {
+        eprintln!("Failed to promote audio thread to real-time priority: {err}");
+    }
+
     let mut ctxt = StreamWorkerContext::new(&timeout);
     loop {
         let flow =

src/host/wasapi/stream.rs

@@ -4,6 +4,7 @@ use crate::{
     BackendSpecificError, Data, InputCallbackInfo, OutputCallbackInfo, PauseStreamError,
     PlayStreamError, SampleFormat, StreamError,
 };
+use audio_thread_priority::promote_current_thread_to_real_time;
 use std::mem;
 use std::ptr;
 use std::sync::mpsc::{channel, Receiver, SendError, Sender};
@@ -269,7 +270,16 @@ fn run_input(
     data_callback: &mut dyn FnMut(&Data, &InputCallbackInfo),
     error_callback: &mut dyn FnMut(StreamError),
 ) {
-    boost_current_thread_priority();
+    let buffer_size = if let BufferSize::Fixed(buffer_size) = run_ctxt.stream.config.buffer_size {
+        buffer_size
+    } else {
+        // if the buffer size isn't fixed, let audio_thread_priority choose a sensible default value
+        0
+    };
+
+    if let Err(err) = promote_current_thread_to_real_time(buffer_size, stream.conf.sample_rate.0) {
+        eprintln!("Failed to promote audio thread to real-time priority: {err}");
+    }
 
     loop {
         match process_commands_and_await_signal(&mut run_ctxt, error_callback) {
@@ -298,7 +308,16 @@ fn run_output(
     data_callback: &mut dyn FnMut(&mut Data, &OutputCallbackInfo),
     error_callback: &mut dyn FnMut(StreamError),
 ) {
-    boost_current_thread_priority();
+    let buffer_size = if let BufferSize::Fixed(buffer_size) = run_ctxt.stream.config.buffer_size {
+        buffer_size
+    } else {
+        // if the buffer size isn't fixed, let audio_thread_priority choose a sensible default value
+        0
+    };
+
+    if let Err(err) = promote_current_thread_to_real_time(buffer_size, stream.conf.sample_rate.0) {
+        eprintln!("Failed to promote audio thread to real-time priority: {err}");
+    }
 
     loop {
         match process_commands_and_await_signal(&mut run_ctxt, error_callback) {
@@ -322,17 +341,6 @@ fn run_output(
     }
 }
 
-fn boost_current_thread_priority() {
-    unsafe {
-        let thread_id = Threading::GetCurrentThreadId();
-
-        let _ = Threading::SetThreadPriority(
-            HANDLE(thread_id as isize),
-            Threading::THREAD_PRIORITY_TIME_CRITICAL,
-        );
-    }
-}
-
 enum ControlFlow {
     Break,
     Continue,