Fix runaway MLX Qwen audio chunks (#964)

* fix runaway MLX Qwen audio chunks

* test: tighten runaway retry coverage

---------

Co-authored-by: huanghua01 <[email protected]>

backend/tests/test_qwen_runaway_retry.py

@@ -0,0 +1,117 @@
+"""Regression coverage for runaway MLX Qwen TTS output."""
+
+from unittest.mock import patch
+
+import numpy as np
+import pytest
+
+from backend.backends import engine_needs_trim, engine_retries_runaway
+from backend.utils.audio import has_tts_runaway
+from backend.utils.chunked_tts import generate_chunked
+
+SAMPLE_RATE = 1000
+
+
+def test_mlx_qwen_enables_runaway_retry_without_aggressive_trim():
+    with patch("backend.backends.get_backend_type", return_value="mlx"):
+        assert engine_needs_trim("qwen") is False
+        assert engine_retries_runaway("qwen") is True
+
+
+def test_pytorch_qwen_keeps_runaway_retry_disabled():
+    with patch("backend.backends.get_backend_type", return_value="pytorch"):
+        assert engine_needs_trim("qwen") is False
+        assert engine_retries_runaway("qwen") is False
+
+
+def test_detector_flags_long_internal_silence():
+    speech = np.full(2 * SAMPLE_RATE, 0.2, dtype=np.float32)
+    runaway_gap = np.zeros(2500, dtype=np.float32)
+    hallucinated_noise = np.full(2 * SAMPLE_RATE, 0.8, dtype=np.float32)
+    audio = np.concatenate([speech, runaway_gap, hallucinated_noise])
+
+    assert has_tts_runaway(audio, SAMPLE_RATE) is True
+
+
+def test_detector_ignores_normal_internal_pause():
+    speech = np.full(SAMPLE_RATE, 0.2, dtype=np.float32)
+    normal_pause = np.zeros(1200, dtype=np.float32)
+    audio = np.concatenate([speech, normal_pause, speech])
+
+    assert has_tts_runaway(audio, SAMPLE_RATE) is False
+
+
+def test_trailing_silence_is_not_a_runaway():
+    speech = np.full(SAMPLE_RATE, 0.2, dtype=np.float32)
+    trailing_silence = np.zeros(2 * SAMPLE_RATE, dtype=np.float32)
+
+    assert (
+        has_tts_runaway(
+            np.concatenate([speech, trailing_silence]),
+            SAMPLE_RATE,
+        )
+        is False
+    )
+
+
+@pytest.mark.asyncio
+async def test_runaway_chunk_is_retried_as_smaller_chunks():
+    class FakeBackend:
+        def __init__(self):
+            self.calls = []
+
+        async def generate(self, text, *_args):
+            self.calls.append(text)
+            if len(text) > 200:
+                speech = np.full(SAMPLE_RATE, 0.2, dtype=np.float32)
+                silence = np.zeros(2500, dtype=np.float32)
+                noise = np.full(SAMPLE_RATE, 0.8, dtype=np.float32)
+                return np.concatenate([speech, silence, noise]), SAMPLE_RATE
+            return np.full(SAMPLE_RATE, 0.2, dtype=np.float32), SAMPLE_RATE
+
+    backend = FakeBackend()
+    text = f"{'A' * 119}. {'B' * 119}."
+
+    audio, sample_rate = await generate_chunked(
+        backend,
+        text,
+        {},
+        max_chunk_chars=800,
+        crossfade_ms=50,
+        runaway_detector=has_tts_runaway,
+    )
+
+    assert sample_rate == SAMPLE_RATE
+    assert backend.calls == [text, f"{'A' * 119}.", f"{'B' * 119}."]
+    assert len(audio) == 1950
+
+
+@pytest.mark.asyncio
+async def test_persistent_runaway_fails_instead_of_returning_corrupt_audio():
+    class AlwaysRunawayBackend:
+        def __init__(self):
+            self.calls = []
+
+        async def generate(self, text, *_args):
+            self.calls.append(text)
+            speech = np.full(SAMPLE_RATE, 0.2, dtype=np.float32)
+            silence = np.zeros(2500, dtype=np.float32)
+            noise = np.full(SAMPLE_RATE, 0.8, dtype=np.float32)
+            return np.concatenate([speech, silence, noise]), SAMPLE_RATE
+
+    backend = AlwaysRunawayBackend()
+    text = f"{'A' * 119}. {'B' * 119}."
+
+    with pytest.raises(
+        RuntimeError,
+        match="remained unstable after retrying smaller text chunks",
+    ):
+        await generate_chunked(
+            backend,
+            text,
+            {},
+            max_chunk_chars=800,
+            runaway_detector=has_tts_runaway,
+        )
+
+    assert [len(call) for call in backend.calls] == [241, 120, 100]