fix(rb-09): decoder propagates read and decode errors

decode_audio_file's packet loop was `Err(_) => break`, so any non-EOF
read error during playback dropped out silently with whatever samples
had accumulated. Per-packet decode errors were tallied and skipped,
contributing to the same outcome. A corrupt or truncated input
therefore came back as `Ok(partial_samples)` — no way for callers to
distinguish a clean decode from a compromised one.

Every SymphoniaError other than the explicit EOF
(`IoError(UnexpectedEof)`) now maps to `AudioDecodeFailed`. Decoder
errors bubble via `?` rather than being counted. `ResetRequired`
promotes to an error rather than a silent break.

Extracted an internal `decode_media_stream(mss, hint)` so tests can
inject a custom `MediaSource`. Added `FlakyCursor` — a seekable cursor
that returns a synthetic I/O error after N bytes — and a regression
test that confirms mid-stream read failure surfaces as `Err` instead of
returning partial audio. Happy-path and missing-file tests added for
coverage.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/audio/src/decode.rs

@@ -3,6 +3,7 @@ use std::path::Path;
 
 use symphonia::core::audio::SampleBuffer;
 use symphonia::core::codecs::DecoderOptions;
+use symphonia::core::errors::Error as SymphoniaError;
 use symphonia::core::formats::FormatOptions;
 use symphonia::core::io::MediaSourceStream;
 use symphonia::core::meta::MetadataOptions;
@@ -13,6 +14,12 @@ use kon_core::types::AudioSamples;
 
 /// Decode an audio file to mono f32 PCM samples.
 /// Supports all formats symphonia handles: mp3, aac, flac, wav, ogg, etc.
+///
+/// Any read- or decode-side error is propagated as `KonError::AudioDecodeFailed`.
+/// A previous implementation `break`ed out of the packet loop on any read
+/// error and skipped per-packet decode errors, so a truncated or corrupt
+/// input silently returned `Ok` with whatever had decoded before the
+/// failure — flagged by the 2026-04-22 review (RB-09).
 pub fn decode_audio_file(path: &Path) -> Result<AudioSamples> {
     let file = File::open(path)
         .map_err(|e| KonError::AudioDecodeFailed(format!("Cannot open file: {e}")))?;
@@ -23,9 +30,16 @@ pub fn decode_audio_file(path: &Path) -> Result<AudioSamples> {
         hint.with_extension(ext);
     }
 
+    decode_media_stream(mss, &hint)
+}
+
+/// Decode from an already-constructed `MediaSourceStream`. Split out so
+/// tests can inject a custom `MediaSource` (for example, one that
+/// returns a mid-stream I/O error) to verify error propagation.
+fn decode_media_stream(mss: MediaSourceStream, hint: &Hint) -> Result<AudioSamples> {
     let probed = symphonia::default::get_probe()
         .format(
-            &hint,
+            hint,
             mss,
             &FormatOptions::default(),
             &MetadataOptions::default(),
@@ -53,31 +67,35 @@ pub fn decode_audio_file(path: &Path) -> Result<AudioSamples> {
         .map_err(|e| KonError::AudioDecodeFailed(format!("Codec error: {e}")))?;
 
     let mut samples: Vec<f32> = Vec::new();
-    let mut decode_errors = 0u32;
 
     loop {
         let packet = match format.next_packet() {
             Ok(p) => p,
-            Err(symphonia::core::errors::Error::IoError(ref e))
+            Err(SymphoniaError::IoError(ref e))
                 if e.kind() == std::io::ErrorKind::UnexpectedEof =>
             {
+                // Normal end of stream — symphonia signals EOF via UnexpectedEof.
                 break;
             }
-            Err(symphonia::core::errors::Error::ResetRequired) => break,
-            Err(_) => break,
+            Err(SymphoniaError::ResetRequired) => {
+                return Err(KonError::AudioDecodeFailed(
+                    "decoder reset required mid-stream — input contains a discontinuity".into(),
+                ));
+            }
+            Err(e) => {
+                return Err(KonError::AudioDecodeFailed(format!(
+                    "packet read failed: {e}"
+                )));
+            }
         };
 
         if packet.track_id() != track_id {
             continue;
         }
 
-        let decoded = match decoder.decode(&packet) {
-            Ok(d) => d,
-            Err(_) => {
-                decode_errors += 1;
-                continue;
-            }
-        };
+        let decoded = decoder
+            .decode(&packet)
+            .map_err(|e| KonError::AudioDecodeFailed(format!("packet decode failed: {e}")))?;
 
         let spec = *decoded.spec();
         let channels = spec.channels.count();
@@ -96,13 +114,118 @@ pub fn decode_audio_file(path: &Path) -> Result<AudioSamples> {
     }
 
     if samples.is_empty() {
-        if decode_errors > 0 {
-            return Err(KonError::AudioDecodeFailed(format!(
-                "No audio decoded ({decode_errors} packets failed — file may be corrupt)"
-            )));
-        }
         return Err(KonError::AudioDecodeFailed("No audio data decoded".into()));
     }
 
     Ok(AudioSamples::new(samples, sample_rate, 1))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::wav::write_wav;
+    use std::io::{Cursor, Read, Seek, SeekFrom};
+    use symphonia::core::io::MediaSource;
+
+    fn temp_path(name: &str) -> std::path::PathBuf {
+        let mut p = std::env::temp_dir();
+        p.push(name);
+        let _ = std::fs::remove_file(&p);
+        p
+    }
+
+    fn valid_wav_bytes(sample_count: usize) -> Vec<u8> {
+        let path = temp_path("kon_decode_tmp_for_bytes.wav");
+        let samples: Vec<f32> = (0..sample_count).map(|i| (i as f32) / 1000.0).collect();
+        let audio = AudioSamples::mono_16khz(samples);
+        write_wav(&path, &audio).unwrap();
+        let bytes = std::fs::read(&path).unwrap();
+        std::fs::remove_file(&path).ok();
+        bytes
+    }
+
+    /// A `MediaSource` that wraps a byte buffer and returns an injected
+    /// I/O error once more than `fail_after_bytes` total bytes have been
+    /// returned successfully. Simulates real-world disk or network read
+    /// failure mid-stream.
+    struct FlakyCursor {
+        inner: Cursor<Vec<u8>>,
+        fail_after_bytes: u64,
+        bytes_read: u64,
+    }
+
+    impl Read for FlakyCursor {
+        fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
+            if self.bytes_read >= self.fail_after_bytes {
+                return Err(std::io::Error::other("injected mid-stream read error"));
+            }
+            let n = self.inner.read(buf)?;
+            self.bytes_read = self.bytes_read.saturating_add(n as u64);
+            Ok(n)
+        }
+    }
+
+    impl Seek for FlakyCursor {
+        fn seek(&mut self, pos: SeekFrom) -> std::io::Result<u64> {
+            self.inner.seek(pos)
+        }
+    }
+
+    impl MediaSource for FlakyCursor {
+        fn is_seekable(&self) -> bool {
+            true
+        }
+        fn byte_len(&self) -> Option<u64> {
+            Some(self.inner.get_ref().len() as u64)
+        }
+    }
+
+    #[test]
+    fn decodes_valid_wav_successfully() {
+        let path = temp_path("kon_decode_valid.wav");
+        let samples: Vec<f32> = (0..4_000).map(|i| (i as f32) / 1000.0).collect();
+        write_wav(&path, &AudioSamples::mono_16khz(samples)).unwrap();
+
+        let loaded = decode_audio_file(&path).expect("valid WAV must decode");
+        assert_eq!(loaded.sample_rate(), 16_000);
+        assert!(!loaded.samples().is_empty());
+
+        std::fs::remove_file(&path).ok();
+    }
+
+    #[test]
+    fn missing_file_surfaces_error() {
+        let path = temp_path("kon_decode_missing.wav");
+        let result = decode_audio_file(&path);
+        assert!(result.is_err(), "missing file must error, got: {result:?}");
+    }
+
+    // RB-09 regression: once probe has succeeded, any mid-stream I/O
+    // error must surface as `Err(AudioDecodeFailed)` rather than being
+    // silently swallowed and returning whatever was decoded so far.
+    //
+    // Pre-fix behaviour: the packet loop had `Err(_) => break`, so an
+    // I/O error during `format.next_packet()` dropped out of the loop
+    // and the function returned `Ok` with partial samples.
+    #[test]
+    fn mid_stream_io_error_propagates_instead_of_returning_partial_audio() {
+        let bytes = valid_wav_bytes(16_000);
+        // Fail after ~1 KiB — probe has seen the RIFF/WAVE header by then,
+        // so probing succeeds. The packet loop hits our injected error
+        // before the stream reaches its natural EOF.
+        let flaky = FlakyCursor {
+            inner: Cursor::new(bytes),
+            fail_after_bytes: 1024,
+            bytes_read: 0,
+        };
+        let mss = MediaSourceStream::new(Box::new(flaky), Default::default());
+        let mut hint = Hint::new();
+        hint.with_extension("wav");
+
+        let result = decode_media_stream(mss, &hint);
+        assert!(
+            result.is_err(),
+            "mid-stream I/O error must surface, got: {result:?}"
+        );
+    }
+}