Fix an issue with windows lacking have os.pwrite

app/model_downloader/engine/writer.py

@@ -8,18 +8,25 @@ A single file descriptor is opened for the whole download. Segments write to
 their own offsets with ``os.pwrite`` — which is offset-addressed and atomic
 per call, so concurrent segment writers need no extra locking. Per-chunk
 fsync is avoided; we fsync once at completion.
+
+``os.pwrite`` is unavailable on Windows, so there we fall back to
+``os.lseek`` + ``os.write`` guarded by a per-writer lock (the seek/write pair
+is not atomic, so concurrent segment writers must be serialized).
 """
 
 from __future__ import annotations
 
 import asyncio
 import os
+import threading
 from concurrent.futures import ThreadPoolExecutor
 from typing import Optional
 
 # One shared, bounded pool for all download disk I/O.
 _EXECUTOR = ThreadPoolExecutor(max_workers=8, thread_name_prefix="dl-writer")
 
+_HAS_PWRITE = hasattr(os, "pwrite")
+
 
 class FileWriter:
     """Owns the ``.part`` file descriptor for one download."""
@@ -27,6 +34,8 @@ class FileWriter:
     def __init__(self, path: str) -> None:
         self.path = path
         self._fd: Optional[int] = None
+        # Serializes lseek+write on platforms without os.pwrite (Windows).
+        self._seek_lock = threading.Lock()
 
     def _open(self) -> None:
         os.makedirs(os.path.dirname(self.path), exist_ok=True)
@@ -52,18 +61,27 @@ class FileWriter:
         )
 
     def _pwrite_all(self, data: bytes, offset: int) -> None:
-        """``os.pwrite`` may write fewer bytes than requested (signal
+        """A positioned write may write fewer bytes than requested (signal
         interruption, near-ENOSPC); loop until every byte lands so we never
-        leave a gap while the caller advances by the full chunk length."""
+        leave a gap while the caller advances by the full chunk length.
+
+        Uses ``os.pwrite`` where available (offset-addressed, atomic per call).
+        On Windows it falls back to ``os.lseek`` + ``os.write`` under a lock,
+        since that pair is not atomic across concurrent segment writers."""
         assert self._fd is not None, "writer not opened"
         view = memoryview(data)
         written = 0
         total = len(view)
         while written < total:
-            n = os.pwrite(self._fd, view[written:], offset + written)
+            if _HAS_PWRITE:
+                n = os.pwrite(self._fd, view[written:], offset + written)
+            else:
+                with self._seek_lock:
+                    os.lseek(self._fd, offset + written, os.SEEK_SET)
+                    n = os.write(self._fd, view[written:])
             if n == 0:
                 raise OSError(
-                    f"os.pwrite wrote 0 bytes at offset {offset + written} "
+                    f"positioned write wrote 0 bytes at offset {offset + written} "
                     f"({written}/{total} bytes written)"
                 )
             written += n