Perform client side interpolation on macOS for cubeb_stream_get_position

cargo-count.dot

@@ -0,0 +1,102 @@
+digraph dependencies {
+	N0[label="cubeb-coreaudio",shape=diamond,color=green];
+	N1[label="memchr",shape=diamond,color=green];
+	N2[label="atomic",shape=diamond,color=green];
+	N3[label="audio-mixer",shape=diamond,color=green];
+	N4[label="bitflags",shape=diamond,color=green];
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+}

src/backend/mod.rs

@@ -559,6 +559,9 @@ extern "C" fn audiounit_output_callback(
             .store(output_latency_frames, Ordering::SeqCst);
     }
 
+    let now = unsafe { mach_absolute_time() };
+    stm.output_callback_tstamp.store(now, Ordering::SeqCst);
+
     cubeb_logv!(
         "({:p}) output: buffers {}, size {}, channels {}, frames {}.",
         stm as *const AudioUnitStream,
@@ -3153,6 +3156,7 @@ struct AudioUnitStream<'ctx> {
     current_input_latency_frames: AtomicU32,
     total_output_latency_frames: AtomicU32,
     total_input_latency_frames: AtomicU32,
+    output_callback_tstamp: AtomicU64,
     // This is true if a device change callback is currently running.
     switching_device: AtomicBool,
     core_stream_data: CoreStreamData<'ctx>,
@@ -3186,6 +3190,7 @@ impl<'ctx> AudioUnitStream<'ctx> {
             current_input_latency_frames: AtomicU32::new(0),
             total_output_latency_frames: AtomicU32::new(0),
             total_input_latency_frames: AtomicU32::new(0),
+            output_callback_tstamp: AtomicU64::new(0),
             switching_device: AtomicBool::new(false),
             core_stream_data: CoreStreamData::default(),
         }
@@ -3473,7 +3478,15 @@ impl<'ctx> StreamOps for AudioUnitStream<'ctx> {
             let position = if current_output_latency_frames > frames_played {
                 0
             } else {
-                frames_played - current_output_latency_frames
+                // Interpolate here to match other cubeb backends. Only return an interpolated time
+                // if we've played enough frames.
+                const NS2S: u64 = 1_000_000_000;
+                let now = unsafe { mach_absolute_time() };
+                let diff = now - self.output_callback_tstamp.load(Ordering::SeqCst);
+                let frames = (host_time_to_ns(diff)
+                    * self.core_stream_data.output_stream_params.rate() as u64)
+                    / NS2S;
+                (frames_played - current_output_latency_frames) + frames
             };
             return Ok(position);
         }