address review: widened integer rate-ratio check and a distinct rate-limit error

crates/engine/src/clip_group_file.rs

@@ -103,7 +103,7 @@ fn decode_group_audio(
                 let bytes = crate::io::read_file(&full)?;
                 let (_, buf) = soundcraft_audio_io::decode(&bytes, Some("wav")).map_err(|err| EngineError::Io(format!("{rel}: {err}")))?;
                 let buf = crate::io::at_rate(buf, rate, crate::io::MAX_IMPORT_SAMPLES)
-                    .ok_or_else(|| EngineError::Io(format!("{rel}: too large to import")))?;
+                    .map_err(|r| EngineError::Io(format!("{rel}: {}", r.message("too large to import"))))?;
                 decoded.push((rel.clone(), buf));
                 index.insert(key, decoded.len() - 1);
                 decoded.len() - 1

crates/engine/src/cmd/sampler.rs

@@ -75,7 +75,7 @@ pub(crate) fn attach(e: &mut Engine, t: TrackId, name: &str, bytes: &[u8], path:
         return Err(EngineError::Io(format!("{name}: no audio")));
     }
     let buf = crate::io::at_rate(buf, e.session().sample_rate.hz(), MAX_SAMPLE_VALUES)
-        .ok_or_else(|| EngineError::Io(format!("{name}: too long for a sample (ten minutes of 192 kHz stereo at most)")))?;
+        .map_err(|r| EngineError::Io(format!("{name}: {}", r.message("too long for a sample (ten minutes of 192 kHz stereo at most)"))))?;
     let channels = buf.num_channels();
     let s = e.session_mut();
     let src = crate::io::add_source(s, name, buf, path, format.format);

crates/engine/src/io/import.rs

@@ -7,18 +7,48 @@ use super::{
 /// Largest import we accept (samples per channel × channels), ~4 GiB of f32.
 pub(crate) const MAX_IMPORT_SAMPLES: usize = 1 << 30;
 
-/// `buf` converted to `rate`, unless it would then hold more than `limit` samples: a file with a
-/// tiny sample rate grows by orders of magnitude.
-pub(crate) fn at_rate(mut buf: AudioBuffer, rate: u32, limit: usize) -> Option<AudioBuffer> {
+/// The most a file's audio may grow when converted to the session rate. Sessions run at 8 kHz to
+/// 768 kHz (96×); past this a file is lying about its rate, and converting a few kilobytes would
+/// otherwise allocate and filter hundreds of megabytes.
+const MAX_RATE_EXPANSION: u32 = 128;
+
+/// Why [`at_rate`] refused a conversion.
+#[derive(Debug, PartialEq, Eq)]
+pub(crate) enum RateError {
+    /// The converted audio would hold more samples than the limit.
+    TooLarge,
+    /// The file's rate is so far below the session's that converting would grow it past [`MAX_RATE_EXPANSION`].
+    Ratio { from: u32, to: u32 },
+}
+
+impl RateError {
+    /// The refusal as a sentence: `size` says what a size refusal means to the caller.
+    pub(crate) fn message(&self, size: &str) -> String {
+        match self {
+            RateError::TooLarge => size.to_string(),
+            RateError::Ratio { from, to } => {
+                format!("its {from} Hz sample rate is too low to convert to the session's {to} Hz (at most {MAX_RATE_EXPANSION}-fold)")
+            }
+        }
+    }
+}
+
+/// `buf` converted to `rate`, unless it would then hold more than `limit` samples, or grow more
+/// than [`MAX_RATE_EXPANSION`]-fold: a file with a tiny sample rate grows by orders of magnitude.
+pub(crate) fn at_rate(mut buf: AudioBuffer, rate: u32, limit: usize) -> std::result::Result<AudioBuffer, RateError> {
     let from = buf.sample_rate.max(1);
+    // widened integers: dividing first would truncate 128.86-fold down to 128
+    if u64::from(rate) > u64::from(from) * u64::from(MAX_RATE_EXPANSION) {
+        return Err(RateError::Ratio { from, to: rate });
+    }
     if buf.frames() as f64 * buf.num_channels() as f64 * f64::from(rate) / f64::from(from) > limit as f64 {
-        return None;
+        return Err(RateError::TooLarge);
     }
     if from != rate {
         buf.channels = soundcraft_dsp::offline::resample(&buf.channels, from, rate);
         buf.sample_rate = rate;
     }
-    Some(buf)
+    Ok(buf)
 }
 
 /// A relative `Audio Files/` path no other source in `s` already uses.
@@ -115,7 +145,7 @@ pub fn decode_import(path: &str, rate: u32) -> Result<DecodedImport> {
 fn decode_import_bytes(name: &str, bytes: &[u8], rate: u32) -> Result<(soundcraft_audio_io::AudioInfo, Arc<SourceAudio>)> {
     let ext = Path::new(name).extension().and_then(OsStr::to_str).map(str::to_ascii_lowercase);
     let (info, buf) = soundcraft_audio_io::decode(bytes, ext.as_deref()).map_err(|err| EngineError::Io(format!("{name}: {err}")))?;
-    let buf = at_rate(buf, rate, MAX_IMPORT_SAMPLES).ok_or_else(|| EngineError::Io(format!("{name}: file is too large to import")))?;
+    let buf = at_rate(buf, rate, MAX_IMPORT_SAMPLES).map_err(|e| EngineError::Io(format!("{name}: {}", e.message("file is too large to import"))))?;
     if buf.frames() == 0 {
         return Err(EngineError::Io(format!("{name}: no audio")));
     }
@@ -215,8 +245,22 @@ mod rate_tests {
     #[test]
     fn a_tiny_sample_rate_is_refused_before_it_is_converted() {
         let buf = |rate| AudioBuffer { sample_rate: rate, channels: vec![vec![0.5; 100]; 2] };
-        assert!(at_rate(buf(1), 48_000, 1_000_000).is_none());
+        assert!(at_rate(buf(1), 48_000, 1_000_000).is_err());
         let ok = at_rate(buf(8_000), 48_000, 1_000_000).unwrap();
         assert_eq!((ok.sample_rate, ok.frames()), (48_000, 600));
     }
+
+    #[test]
+    fn an_extreme_rate_expansion_is_refused_whatever_the_sample_budget() {
+        let buf = |rate| AudioBuffer { sample_rate: rate, channels: vec![vec![0.5; 100]; 2] };
+        // 100 frames at 300 Hz would fit the budget as 128 000 samples, but grow 640-fold.
+        assert_eq!(at_rate(buf(300), 192_000, usize::MAX).unwrap_err(), RateError::Ratio { from: 300, to: 192_000 });
+        assert!(at_rate(buf(1_500), 192_000, usize::MAX).is_ok(), "128-fold is the supported extreme");
+        assert!(at_rate(buf(8_000), 768_000, usize::MAX).is_ok(), "the slowest file at the fastest session");
+        // 128.86-fold: integer division would truncate it to 128 and let it through
+        assert_eq!(at_rate(buf(1_490), 192_000, usize::MAX).unwrap_err(), RateError::Ratio { from: 1_490, to: 192_000 });
+        // a short file that is only over the sample budget is a size refusal, not a rate one
+        assert_eq!(at_rate(buf(48_000), 48_000, 10).unwrap_err(), RateError::TooLarge);
+        assert!(RateError::Ratio { from: 300, to: 192_000 }.message("big").contains("300 Hz"));
+    }
 }

crates/engine/src/io/session.rs

@@ -72,7 +72,7 @@ pub(crate) fn open_session_linked(e: &mut Engine, path: &Path, links: &Path) ->
 /// Decode one source's audio file at the session rate.
 pub(super) fn decode_source(bytes: &[u8], ext: Option<&str>, rate: u32) -> std::result::Result<Arc<SourceAudio>, String> {
     let (_, buf) = soundcraft_audio_io::decode(bytes, ext).map_err(|err| err.to_string())?;
-    let buf = at_rate(buf, rate, MAX_IMPORT_SAMPLES).ok_or("too large to load")?;
+    let buf = at_rate(buf, rate, MAX_IMPORT_SAMPLES).map_err(|e| e.message("too large to load"))?;
     Ok(Arc::new(SourceAudio::new(buf)))
 }