fix(latency): threadpool renders + RENDER_LOCK + client pre-render cache

app/core/vst_engine.py

@@ -3,8 +3,21 @@ import os
 import json
 import numpy as np
 import functools
+import threading
 from ctypes import c_char_p
 
+# pedalboard VST3 + pyfluidsynth KHÔNG thread-safe: tuần tự hóa mọi native render
+RENDER_LOCK = threading.RLock()
+
+
+def _render_locked(fn):
+    """Chạy hàm native (VST3/pyfluidsynth) dưới RENDER_LOCK — không thread-safe."""
+    @functools.wraps(fn)
+    def wrapper(*args, **kwargs):
+        with RENDER_LOCK:
+            return fn(*args, **kwargs)
+    return wrapper
+
 
 def _ensure_fluidsynth_dll_on_path():
     """Frozen (PyInstaller): pyfluidsynth tim dll qua ctypes.util.find_library
@@ -25,6 +38,7 @@ def midi_note_to_freq(note_number: int) -> float:
     return 440.0 * (2.0 ** ((note_number - 69) / 12.0))
 
 
+@_render_locked
 def render_soundfont_midi_to_audio(midi_events: list, sf_path: str, bank: int = 0,
                                    program: int = 0, sr: int = 44100, bpm: float = 120.0) -> np.ndarray:
     """Native FluidSynth (pyfluidsynth) — render MIDI events bằng soundfont THẬT.
@@ -92,6 +106,7 @@ def render_soundfont_midi_to_audio(midi_events: list, sf_path: str, bank: int =
         except Exception:
             pass
 
+@_render_locked
 def render_midi_events_to_audio(midi_events: list, sr: int = 44100, bpm: float = 120.0, instrument: str = 'synth') -> np.ndarray:
     beat_duration_sec = 60.0 / max(30.0, bpm)
     max_duration_sec = 2.0
@@ -336,6 +351,7 @@ class PluginManager:
                     }
         return list(sf_map.values())
 
+    @_render_locked
     def load_vst(self, plugin_name: str, preset_data: dict = None):
         if not HAS_PEDALBOARD:
             return None