Extract analysis mapping to pure function

This commit extracts the logic for mapping audio analysis results into
the `RecordingMeta` fields into a new, pure function
`analysis_to_meta_fields`. This improves testability by separating the
core mapping logic from logging and other side effects.

The previous implementation directly handled logging and conditional
logic within the `run` function. The new function encapsulates these
mapping rules, including handling finite values, silent audio, and error
propagation. The caller, `run`, is now responsible for logging any
analysis failures before calling the helper. Unit tests have been added
to verify the behavior of `analysis_to_meta_fields` in various
scenarios.

server/src/transcribe/worker.rs

@@ -118,38 +118,19 @@ pub async fn run(
             }
         };
 
-        // Translate the analysis outcome into the two `RecordingMeta`
-        // fields. Silent files (volumedetect → -inf dB) keep a valid
-        // duration but skip loudness — `serde_json` refuses non-finite
-        // floats, and persisting `+Inf` gain would loop the worker
-        // forever (write fails → no sidecar → idempotency gate stays
-        // open → retry hits the same trap).
-        let (duration_seconds, loudness) = match analysis_result {
-            Ok(a) if a.mean_db.is_finite() && a.max_db.is_finite() => {
-                let gain_db = loudness::compute_gain_db(a.mean_db, a.max_db);
-                (
-                    Some(a.duration_seconds),
-                    Some(Loudness {
-                        mean_db: a.mean_db,
-                        max_db: a.max_db,
-                        gain_db,
-                    }),
-                )
-            }
-            Ok(a) => {
-                // Silent (or zero-sample) audio: duration is still
-                // valid, but there is no level to amplify against.
-                (Some(a.duration_seconds), None)
-            }
-            Err(e) => {
-                warn!(
-                    audio = %audio_path.display(),
-                    error = %e,
-                    "audio analysis failed — meta will record null duration and loudness"
-                );
-                (None, None)
-            }
-        };
+        // Log analysis failures here (so the helper stays pure and
+        // unit-testable) before mapping the result onto the meta
+        // fields. The mapping rules — finiteness gate, silent-file
+        // handling, error propagation — live in
+        // `analysis_to_meta_fields` with their own tests.
+        if let Err(ref e) = analysis_result {
+            warn!(
+                audio = %audio_path.display(),
+                error = %e,
+                "audio analysis failed — meta will record null duration and loudness"
+            );
+        }
+        let (duration_seconds, loudness) = analysis_to_meta_fields(analysis_result);
 
         // Normalize terminology against the curated vocab before
         // persisting. Downstream consumers (oneliner, analyze) read
@@ -222,6 +203,41 @@ pub async fn run(
     warn!("Transcription worker stopped (channel closed)");
 }
 
+/// Map an [`ffmpeg::analyze_audio`] outcome onto the two
+/// [`RecordingMeta`] fields it can populate.
+///
+/// Pure: no logging, no I/O, no Tokio. The caller is expected to log
+/// failures (so the test fixture for this fn does not have to fight
+/// the global tracing subscriber).
+///
+/// - `Ok(a)` with both `mean_db` and `max_db` finite → both fields
+///   populated, `gain_db` from [`loudness::compute_gain_db`].
+/// - `Ok(a)` with either non-finite (silent / zero-sample input) →
+///   duration kept, loudness dropped: `serde_json` refuses non-finite
+///   floats and persisting `+Inf` gain would loop the worker.
+/// - `Err(_)` → both fields `None`. ffmpeg failed; the recording will
+///   show up in the UI without duration or replay-gain, but the
+///   transcript still gets persisted.
+fn analysis_to_meta_fields(
+    analysis_result: Result<ffmpeg::AudioAnalysis, ffmpeg::FfmpegError>,
+) -> (Option<u32>, Option<Loudness>) {
+    match analysis_result {
+        Ok(a) if a.mean_db.is_finite() && a.max_db.is_finite() => {
+            let gain_db = loudness::compute_gain_db(a.mean_db, a.max_db);
+            (
+                Some(a.duration_seconds),
+                Some(Loudness {
+                    mean_db: a.mean_db,
+                    max_db: a.max_db,
+                    gain_db,
+                }),
+            )
+        }
+        Ok(a) => (Some(a.duration_seconds), None),
+        Err(_) => (None, None),
+    }
+}
+
 /// Atomically rename a failed recording from `<ts>.m4a` to `<ts>.m4a.failed`
 /// so the recovery scan no longer picks it up. The UI still surfaces these
 /// files (with a "failed" flag) so the user can listen to the audio and
@@ -592,4 +608,70 @@ mod tests {
         let dir = tempdir().unwrap();
         assert!(!has_pending_recordings(dir.path()).await);
     }
+
+    /// Successful analysis with finite mean and max — both meta fields
+    /// are populated and `gain_db` matches the loudness formula
+    /// (mean=-22, max=-15 → target-limited at +6 dB; same case as
+    /// `loudness::tests::target_limits_gain`).
+    #[test]
+    fn analysis_to_meta_fields_finite_populates_both() {
+        let analysis = ffmpeg::AudioAnalysis {
+            duration_seconds: 30,
+            mean_db: -22.0,
+            max_db: -15.0,
+        };
+        let (duration, loudness) = analysis_to_meta_fields(Ok(analysis));
+        assert_eq!(duration, Some(30));
+        assert_eq!(
+            loudness,
+            Some(Loudness {
+                mean_db: -22.0,
+                max_db: -15.0,
+                gain_db: 6.0,
+            })
+        );
+    }
+
+    /// Silent file: volumedetect returns `-inf dB` for both. Duration
+    /// is still valid (the decode pass measured it), but loudness must
+    /// be `None` — `serde_json` would refuse to serialize an
+    /// `Infinity` and the worker would loop forever on retry.
+    #[test]
+    fn analysis_to_meta_fields_silent_drops_loudness() {
+        let analysis = ffmpeg::AudioAnalysis {
+            duration_seconds: 5,
+            mean_db: f64::NEG_INFINITY,
+            max_db: f64::NEG_INFINITY,
+        };
+        let (duration, loudness) = analysis_to_meta_fields(Ok(analysis));
+        assert_eq!(duration, Some(5));
+        assert_eq!(loudness, None);
+    }
+
+    /// Mixed case: only one of the two values is non-finite. The gate
+    /// is `&&` so a single non-finite value still drops loudness — we
+    /// never feed a partially-defined measurement into the gain
+    /// formula.
+    #[test]
+    fn analysis_to_meta_fields_partial_inf_drops_loudness() {
+        let analysis = ffmpeg::AudioAnalysis {
+            duration_seconds: 10,
+            mean_db: -27.3,
+            max_db: f64::NEG_INFINITY,
+        };
+        let (duration, loudness) = analysis_to_meta_fields(Ok(analysis));
+        assert_eq!(duration, Some(10));
+        assert_eq!(loudness, None);
+    }
+
+    /// ffmpeg failed: both fields are `None`, the transcript still
+    /// gets persisted by the caller. The worker logs the error before
+    /// calling this helper, so the helper itself stays log-free.
+    #[test]
+    fn analysis_to_meta_fields_err_yields_none_none() {
+        let err = ffmpeg::FfmpegError::Parse("synthetic test error".into());
+        let (duration, loudness) = analysis_to_meta_fields(Err(err));
+        assert_eq!(duration, None);
+        assert_eq!(loudness, None);
+    }
 }