WIP - Add ALSA timestamp implementation.

I think this is mostly complete, however there are two major TODOs:

- There seems to be a bug where timestamps are not reported when the
  target device is `pulse`. This causes a `panic!` as the trigger
  timestamp should always precede the timestamp when retrieved while the
  stream is running.
- We need to specify the timestamp type as MONOTONIC_RAW, however the
  `alsa` and `alsa-sys` crates do not yet expose the necessary
  `set_tstamp_type` function.

src/host/alsa/mod.rs

@@ -8,6 +8,7 @@ use crate::{
     PlayStreamError, SampleFormat, SampleRate, StreamConfig, StreamError, SupportedStreamConfig,
     SupportedStreamConfigRange, SupportedStreamConfigsError,
 };
+use std::convert::TryInto;
 use std::sync::Arc;
 use std::thread::{self, JoinHandle};
 use std::vec::IntoIter as VecIntoIter;
@@ -208,7 +209,7 @@ impl Device {
             channel: handle,
             sample_format,
             num_descriptors,
-            num_channels: conf.channels as u16,
+            conf: conf.clone(),
             buffer_len,
             period_len,
             can_pause,
@@ -421,8 +422,8 @@ struct StreamInner {
     // Format of the samples.
     sample_format: SampleFormat,
 
-    // Number of channels, ie. number of samples per frame.
+    // The configuration used to open this stream.
-    num_channels: u16,
+    conf: StreamConfig,
 
     // Number of samples that can fit in the buffer.
     buffer_len: usize,
@@ -479,7 +480,8 @@ fn input_stream_worker(
             PollDescriptorsFlow::Continue => continue,
             PollDescriptorsFlow::Return => return,
             PollDescriptorsFlow::Ready {
-                available_frames: _,
+                avail_frames: _,
+                delay_frames,
                 stream_type,
             } => {
                 assert_eq!(
@@ -487,10 +489,8 @@ fn input_stream_worker(
                     StreamType::Input,
                     "expected input stream, but polling descriptors indicated output",
                 );
-                report_error(
+                let res = process_input(stream, &mut ctxt.buffer, delay_frames, data_callback);
-                    process_input(stream, &mut ctxt.buffer, data_callback),
+                report_error(res, error_callback);
-                    error_callback,
-                );
             }
         }
     }
@@ -514,7 +514,8 @@ fn output_stream_worker(
             PollDescriptorsFlow::Continue => continue,
             PollDescriptorsFlow::Return => return,
             PollDescriptorsFlow::Ready {
-                available_frames,
+                avail_frames,
+                delay_frames,
                 stream_type,
             } => {
                 assert_eq!(
@@ -522,13 +523,15 @@ fn output_stream_worker(
                     StreamType::Output,
                     "expected output stream, but polling descriptors indicated input",
                 );
-                process_output(
+                let res = process_output(
                     stream,
                     &mut ctxt.buffer,
-                    available_frames,
+                    avail_frames,
+                    delay_frames,
                     data_callback,
                     error_callback,
                 );
+                report_error(res, error_callback);
             }
         }
     }
@@ -555,7 +558,8 @@ enum PollDescriptorsFlow {
     Return,
     Ready {
         stream_type: StreamType,
-        available_frames: usize,
+        avail_frames: usize,
+        delay_frames: usize,
     },
 }
 
@@ -614,7 +618,8 @@ fn poll_descriptors_and_prepare_buffer(
         }
     };
     // Get the number of available samples for reading/writing.
-    let available_samples = get_available_samples(stream)?;
+    let (avail_frames, delay_frames) = get_avail_delay(stream)?;
+    let available_samples = avail_frames * stream.conf.channels as usize;
 
     // Only go on if there is at least `stream.period_len` samples.
     if available_samples < stream.period_len {
@@ -624,11 +629,11 @@ fn poll_descriptors_and_prepare_buffer(
     // Prepare the data buffer.
     let buffer_size = stream.sample_format.sample_size() * available_samples;
     buffer.resize(buffer_size, 0u8);
-    let available_frames = available_samples / stream.num_channels as usize;
 
     Ok(PollDescriptorsFlow::Ready {
         stream_type,
-        available_frames,
+        avail_frames,
+        delay_frames,
     })
 }
 
@@ -636,6 +641,7 @@ fn poll_descriptors_and_prepare_buffer(
 fn process_input(
     stream: &StreamInner,
     buffer: &mut [u8],
+    delay_frames: usize,
     data_callback: &mut (dyn FnMut(&Data, &InputCallbackInfo) + Send + 'static),
 ) -> Result<(), BackendSpecificError> {
     stream.channel.io().readi(buffer)?;
@@ -643,7 +649,13 @@ fn process_input(
     let data = buffer.as_mut_ptr() as *mut ();
     let len = buffer.len() / sample_format.sample_size();
     let data = unsafe { Data::from_parts(data, len, sample_format) };
-    let info = crate::InputCallbackInfo {};
+    let callback = stream_timestamp(stream)?;
+    let delay_duration = frames_to_duration(delay_frames, stream.conf.sample_rate);
+    let capture = callback
+        .sub(delay_duration)
+        .expect("`capture` occurs before origin of alsa `StreamInstant`");
+    let timestamp = crate::InputStreamTimestamp { callback, capture };
+    let info = crate::InputCallbackInfo { timestamp };
     data_callback(&data, &info);
 
     Ok(())
@@ -656,16 +668,23 @@ fn process_output(
     stream: &StreamInner,
     buffer: &mut [u8],
     available_frames: usize,
+    delay_frames: usize,
     data_callback: &mut (dyn FnMut(&mut Data, &OutputCallbackInfo) + Send + 'static),
     error_callback: &mut dyn FnMut(StreamError),
-) {
+) -> Result<(), BackendSpecificError> {
     {
         // We're now sure that we're ready to write data.
         let sample_format = stream.sample_format;
         let data = buffer.as_mut_ptr() as *mut ();
         let len = buffer.len() / sample_format.sample_size();
         let mut data = unsafe { Data::from_parts(data, len, sample_format) };
-        let info = crate::OutputCallbackInfo {};
+        let callback = stream_timestamp(stream)?;
+        let delay_duration = frames_to_duration(delay_frames, stream.conf.sample_rate);
+        let playback = callback
+            .add(delay_duration)
+            .expect("`playback` occurs beyond representation supported by `StreamInstant`");
+        let timestamp = crate::OutputStreamTimestamp { callback, playback };
+        let info = crate::OutputCallbackInfo { timestamp };
         data_callback(&mut data, &info);
     }
     loop {
@@ -693,6 +712,47 @@ fn process_output(
             }
         }
     }
+    Ok(())
+}
+
+// Use the duration since the start of the stream.
+//
+// This ensures positive values that are compatible with our `StreamInstant` representation.
+fn stream_timestamp(stream: &StreamInner) -> Result<crate::StreamInstant, BackendSpecificError> {
+    let status = stream.channel.status()?;
+    let trigger_ts = status.get_trigger_htstamp();
+    // TODO: This is returning `0` on ALSA where default device forwards to pulse.
+    // Possibly related: https://bugs.freedesktop.org/show_bug.cgi?id=88503
+    let ts = status.get_htstamp();
+    let nanos = timespec_diff_nanos(ts, trigger_ts)
+        .try_into()
+        .unwrap_or_else(|_| {
+            panic!(
+                "get_htstamp `{:?}` was earlier than get_trigger_htstamp `{:?}`",
+                ts, trigger_ts
+            );
+        });
+    Ok(crate::StreamInstant::from_nanos(nanos))
+}
+
+// Adapted from `timestamp2ns` here:
+// https://fossies.org/linux/alsa-lib/test/audio_time.c
+fn timespec_to_nanos(ts: libc::timespec) -> i64 {
+    ts.tv_sec as i64 * 1_000_000_000 + ts.tv_nsec
+}
+
+// Adapted from `timediff` here:
+// https://fossies.org/linux/alsa-lib/test/audio_time.c
+fn timespec_diff_nanos(a: libc::timespec, b: libc::timespec) -> i64 {
+    timespec_to_nanos(a) - timespec_to_nanos(b)
+}
+
+// Convert the given duration in frames at the given sample rate to a `std::time::Duration`.
+fn frames_to_duration(frames: usize, rate: crate::SampleRate) -> std::time::Duration {
+    let secsf = frames as f64 / rate.0 as f64;
+    let secs = secsf as u64;
+    let nanos = ((secsf - secs as f64) * 1_000_000_000.0) as u32;
+    std::time::Duration::new(secs, nanos)
 }
 
 impl Stream {
@@ -759,16 +819,16 @@ impl StreamTrait for Stream {
     }
 }
 
-// Determine the number of samples that are available to read/write.
+// Determine the number of frames that are available to read/write along with the latency.
-fn get_available_samples(stream: &StreamInner) -> Result<usize, BackendSpecificError> {
+fn get_avail_delay(stream: &StreamInner) -> Result<(usize, usize), BackendSpecificError> {
-    match stream.channel.avail_update() {
+    match stream.channel.avail_delay() {
         Err(err) if err.errno() == Some(nix::errno::Errno::EPIPE) => {
             // buffer underrun
             // TODO: Notify the user some how.
-            Ok(stream.buffer_len)
+            Ok((stream.buffer_len, 0))
         }
         Err(err) => Err(err.into()),
-        Ok(available) => Ok(available as usize * stream.num_channels as usize),
+        Ok((avail, delay)) => Ok((avail as usize, delay as usize)),
     }
 }
 
@@ -830,6 +890,10 @@ fn set_sw_params_from_format(
         (buffer, period)
     };
 
+    sw_params.set_tstamp_mode(true)?;
+    // TODO: `sw_params.set_tstamp_type(CLOCK_MONOTONIC_RAW)`
+    // Pending addressing https://github.com/diwic/alsa-sys/issues/6
+
     pcm_handle.sw_params(&sw_params)?;
 
     Ok((buffer_len, period_len))