fix(alsa): skip drain() on non-Running PCM to prevent USB kernel deadlock

After a write error, try_recover() leaves the PCM in the Prepared state.
Calling drain() on a Prepared-state USB PCM can deadlock the kernel driver,
causing a full system freeze that requires a physical power cycle to recover.

This has been confirmed on Raspberry Pi 3 with the dwc_otg USB controller
and a Focusrite Scarlett 2i4 USB audio interface. Certain tracks trigger
an audio key error which causes an ALSA underrun during stop(); try_recover()
succeeds but leaves the PCM in Prepared state, and the subsequent drain()
call never returns.

Dropping the PCM instead of draining is sufficient to release resources
when the PCM is not actively running.

Author: zeemawn

playback/src/audio_backend/alsa.rs

@@ -4,7 +4,7 @@ use crate::convert::Converter;
 use crate::decoder::AudioPacket;
 use crate::{NUM_CHANNELS, SAMPLE_RATE};
 use alsa::device_name::HintIter;
-use alsa::pcm::{Access, Format, Frames, HwParams, PCM};
+use alsa::pcm::{Access, Format, Frames, HwParams, State, PCM};
 use alsa::{Direction, ValueOr};
 use std::process::exit;
 use thiserror::Error;
@@ -442,7 +442,14 @@ impl Sink for AlsaSink {
 
             let pcm = self.pcm.take().ok_or(AlsaError::NotConnected)?;
 
-            pcm.drain().map_err(AlsaError::DrainFailure)?;
+            // Only drain if the PCM is in Running state. After a write error,
+            // try_recover() leaves the PCM in Prepared state, and calling drain()
+            // on a Prepared-state USB PCM can deadlock the kernel driver
+            // (confirmed on Raspberry Pi 3 with the dwc_otg USB controller).
+            // In that case, dropping the PCM is sufficient to release resources.
+            if pcm.state() == State::Running {
+                pcm.drain().map_err(AlsaError::DrainFailure)?;
+            }
         }
 
         Ok(())