runtime: tighten queue, generator, and late-write semantics

internal/app/engine.go

@@ -194,7 +194,7 @@ func (e *Engine) SetStreamSource(src *audio.StreamSource) {
 	}
 	resampler := audio.NewStreamResampler(src, compositeRate)
 	e.generator.SetExternalSource(resampler)
-	log.Printf("engine: live audio stream — %d Hz → %.0f Hz (buffer %d frames)",
+	log.Printf("engine: live audio stream wired — initial %d Hz → %.0f Hz composite (buffer %d frames); actual decoded rate auto-corrects on first chunk",
 		src.SampleRate, compositeRate, src.Stats().Capacity)
 }
 

internal/offline/generator.go

@@ -120,8 +120,15 @@ func NewGenerator(cfg cfgpkg.Config) *Generator {
 
 // SetExternalSource sets a live audio source (e.g. StreamResampler) that
 // takes priority over WAV/tone sources. Must be called before the first
-// GenerateFrame() call (i.e. before init).
+// GenerateFrame() call; calling it after init() has no effect because
+// g.source is already wired to the old source.
 func (g *Generator) SetExternalSource(src frameSource) {
+	if g.initialized {
+		// init() already called sourceFor() and wired g.source. Updating
+		// g.externalSource here would have no effect on the live DSP chain.
+		// This is a programming error — log loudly rather than silently break.
+		panic("generator: SetExternalSource called after GenerateFrame; call it before the engine starts")
+	}
 	g.externalSource = src
 }
 
@@ -189,12 +196,14 @@ func (g *Generator) init() {
 	g.mpxNotch19, g.mpxNotch57 = dsp.NewCompositeProtection(g.sampleRate)
 	// BS.412 MPX power limiter (EU/CH requirement for licensed FM)
 	if g.cfg.FM.BS412Enabled {
-		chunkSec := 0.05 // 50ms chunks (matches engine default)
+		// chunkSec is not known at init time (Engine.chunkDuration may differ).
+		// Pass 0 here; GenerateFrame computes the actual chunk duration from
+		// the real sample count and updates BS.412 accordingly.
 		g.bs412 = dsp.NewBS412Limiter(
 			g.cfg.FM.BS412ThresholdDBr,
 			g.cfg.FM.PilotLevel,
 			g.cfg.FM.RDSInjection,
-			chunkSec,
+			0,
 		)
 	}
 	if g.cfg.FM.FMModulationEnabled {
@@ -360,8 +369,14 @@ func (g *Generator) GenerateFrame(duration time.Duration) *output.CompositeFrame
 		}
 	}
 
-	// BS.412: feed this chunk's average audio power for next chunk's gain calculation
+	// BS.412: feed this chunk's actual duration and average audio power for
+	// the next chunk's gain calculation. Using the real sample count avoids
+	// the error that occurred when chunkSec was hardcoded to 0.05 — any
+	// SetChunkDuration() call from the engine would silently miscalibrate
+	// the ITU-R BS.412 power measurement window.
 	if g.bs412 != nil && samples > 0 {
+		chunkSec := float64(samples) / g.sampleRate
+		g.bs412.UpdateChunkDuration(chunkSec)
 		g.bs412.ProcessChunk(bs412PowerAccum / float64(samples))
 	}
 

internal/output/frame_queue.go

@@ -80,22 +80,19 @@ func (q *FrameQueue) Capacity() int {
 }
 
 // FillLevel reports the current occupancy as a fraction of capacity.
+// Uses len(ch) directly for accuracy: updateDepth() is called after the
+// channel operation, so q.depth can lag by one frame transiently.
 func (q *FrameQueue) FillLevel() float64 {
-	q.mu.Lock()
-	depth := q.depth
-	q.mu.Unlock()
 	if q.capacity == 0 {
 		return 0
 	}
-	return float64(depth) / float64(q.capacity)
+	return float64(len(q.ch)) / float64(q.capacity)
 }
 
 // Depth returns the current number of frames in the queue.
+// Uses len(ch) directly for accuracy (see FillLevel).
 func (q *FrameQueue) Depth() int {
-	q.mu.Lock()
-	depth := q.depth
-	q.mu.Unlock()
-	return depth
+	return len(q.ch)
 }
 
 // Stats returns a snapshot of the queue metrics.
@@ -104,7 +101,7 @@ func (q *FrameQueue) Stats() QueueStats {
 	fill := q.fillLevelLocked()
 	stats := QueueStats{
 		Capacity:       q.capacity,
-		Depth:          q.depth,
+		Depth:          len(q.ch),
 		FillLevel:      fill,
 		Health:         queueHealthFromFill(fill),
 		HighWaterMark:  q.highWaterMark,
@@ -128,11 +125,15 @@ func (q *FrameQueue) Push(ctx context.Context, frame *CompositeFrame) error {
 		return ErrFrameQueueClosed
 	}
 
+	// BUG-05 fix: increment depth BEFORE the channel send so that Stats()
+	// never reports fill=0 while a frame is in the channel awaiting receive.
+	// On context cancellation, undo the increment.
+	q.updateDepth(+1)
 	select {
 	case q.ch <- frame:
-		q.updateDepth(+1)
 		return nil
 	case <-ctx.Done():
+		q.updateDepth(-1)
 		q.recordPushTimeout()
 		return ctx.Err()
 	}
@@ -211,7 +212,9 @@ func (q *FrameQueue) fillLevelLocked() float64 {
 	if q.capacity == 0 {
 		return 0
 	}
-	return float64(q.depth) / float64(q.capacity)
+	// Use len(ch) rather than q.depth: depth is updated after the channel
+	// operation, so it can be off by one during the Push/Pop window.
+	return float64(len(q.ch)) / float64(q.capacity)
 }
 
 func (q *FrameQueue) recordPushTimeout() {