fix(vosk): refuse to load a model that would exceed available memory (#850)

* fix(vosk): refuse to load a model that would exceed available memory

vosk.Model() allocates the whole model in one spike during loading,
including the RNNLM rescoring component some models ship, with no way
to check the requirement up front or load incrementally. Walk the
model directory, compare its on-disk size against psutil's available
memory with a 1.5x margin, and raise before calling into the native
loader instead of letting the kernel OOM killer take the process down
mid-load.

Fixes #676

* fix(vosk): match memory safety margin to the documented peak

greptile flagged the guard's own comment: loading can spike to ~2x the
model's on-disk size, but the margin only required 1.5x available memory,
so a load between those two multiples could still trigger the OOM this
guard exists to prevent.

---------

Co-authored-by: Jatin K Malik <jatinkrmalik@gmail.com>

src/vocalinux/speech_recognition/recognition_manager.py

@@ -1036,6 +1036,12 @@ def _get_system_model_paths() -> list:
 # Alternative locations for pre-installed models (now dynamic)
 SYSTEM_MODELS_DIRS = _get_system_model_paths()
 
+# vosk.Model()'s peak RSS during loading runs well above the model's on-disk
+# size (issue #676: an RNNLM-bearing model's load-time spike was ~1.5-2x its
+# file size). Require this multiple of available memory before attempting
+# the load.
+_VOSK_MODEL_MEMORY_SAFETY_MARGIN = 2.0
+
 
 def detect_pywhispercpp_gpu_backend() -> str:
     """Detect whether pywhispercpp's native library actually has GPU support."""
@@ -1296,6 +1302,8 @@ class SpeechRecognitionManager:
                 else:
                     logger.info(f"Using existing VOSK model from {self.vosk_model_path}")
 
+            self._check_vosk_model_memory(self.vosk_model_path)
+
             logger.info(f"Loading VOSK model from {self.vosk_model_path}")
             # Ensure previous model/recognizer are released if re-initializing
             self.model = None
@@ -1310,6 +1318,56 @@ class SpeechRecognitionManager:
             self.state = RecognitionState.ERROR
             raise
 
+    @staticmethod
+    def _vosk_model_size_bytes(model_path: str) -> int:
+        """Sum on-disk file sizes under a VOSK model directory.
+
+        Walks the whole directory rather than reading a catalog size, since
+        the RNNLM component (rnnlm/final.raw) that drives the memory spike
+        this guards against is only present for some languages/models and
+        is not reflected in VOSK_MODEL_INFO's nominal download size.
+        """
+        total = 0
+        for root, _dirs, files in os.walk(model_path):
+            for name in files:
+                try:
+                    total += os.path.getsize(os.path.join(root, name))
+                except OSError:
+                    continue
+        return total
+
+    def _check_vosk_model_memory(self, model_path: str) -> None:
+        """Refuse to load a VOSK model that would exhaust available memory.
+
+        vosk.Model() allocates the whole model (acoustic model plus, for
+        models that ship one, the RNNLM rescoring component) in one spike
+        during loading. There is no way to load it incrementally or query
+        vosk for how much it will need up front, so the only guard
+        available here is comparing the model's on-disk footprint against
+        currently available memory before calling into the native loader
+        (issue #676: this spike killed the process via the kernel OOM
+        killer on a 16GB machine, with no exception Vocalinux could catch).
+        """
+        import psutil
+
+        model_bytes = self._vosk_model_size_bytes(model_path)
+        if model_bytes <= 0:
+            return
+
+        available_bytes = psutil.virtual_memory().available
+        required_bytes = model_bytes * _VOSK_MODEL_MEMORY_SAFETY_MARGIN
+
+        if available_bytes < required_bytes:
+            model_mb = model_bytes / (1024 * 1024)
+            available_mb = available_bytes / (1024 * 1024)
+            raise RuntimeError(
+                f"Not enough memory to load the VOSK model at {model_path} "
+                f"(model is ~{model_mb:.0f}MB on disk, {available_mb:.0f}MB "
+                "available). Loading it would likely be killed by the "
+                "system's out-of-memory killer partway through. Close other "
+                "applications or switch to a smaller model size in Settings."
+            )
+
     def _init_whisper(self):
         """Initialize the Whisper speech recognition engine."""
         import warnings