fix(downloader): enforce file size integrity checks

tests/services/test_downloader.py

@@ -0,0 +1,125 @@
+import asyncio
+from datetime import datetime
+from pathlib import Path
+
+import pytest
+
+from py.services.downloader import Downloader
+
+
+class FakeStream:
+    def __init__(self, chunks):
+        self._chunks = list(chunks)
+
+    async def iter_chunked(self, _chunk_size):
+        for chunk in self._chunks:
+            await asyncio.sleep(0)
+            yield chunk
+
+
+class FakeResponse:
+    def __init__(self, status, headers, chunks):
+        self.status = status
+        self.headers = headers
+        self.content = FakeStream(chunks)
+
+    async def __aenter__(self):
+        return self
+
+    async def __aexit__(self, exc_type, exc, tb):
+        return False
+
+
+class FakeSession:
+    def __init__(self, responses):
+        self._responses = list(responses)
+        self._get_calls = 0
+
+    def get(self, url, headers=None, allow_redirects=True, proxy=None):  # noqa: D401 - signature mirrors aiohttp
+        del url, headers, allow_redirects, proxy
+        response_factory = self._responses[self._get_calls]
+        self._get_calls += 1
+        return response_factory()
+
+    async def close(self):
+        return None
+
+
+def _build_downloader(responses, *, max_retries=0):
+    downloader = Downloader()
+    downloader.max_retries = max_retries
+    downloader.base_delay = 0
+    fake_session = FakeSession(responses)
+    downloader._session = fake_session
+    downloader._session_created_at = datetime.now()
+    downloader._proxy_url = None
+    return downloader
+
+
+@pytest.mark.asyncio
+async def test_download_file_fails_when_size_mismatch(tmp_path):
+    target_path = tmp_path / "model" / "file.bin"
+    target_path.parent.mkdir()
+
+    responses = [
+        lambda: FakeResponse(
+            status=200,
+            headers={"content-length": "10"},
+            chunks=[b"abc"],
+        )
+    ]
+
+    downloader = _build_downloader(responses)
+
+    success, message = await downloader.download_file("https://example.com/file", str(target_path))
+
+    assert success is False
+    assert "mismatch" in message.lower()
+    assert not target_path.exists()
+    assert not Path(str(target_path) + ".part").exists()
+
+
+@pytest.mark.asyncio
+async def test_download_file_fails_when_zero_bytes(tmp_path):
+    target_path = tmp_path / "model" / "file.bin"
+    target_path.parent.mkdir()
+
+    responses = [
+        lambda: FakeResponse(
+            status=200,
+            headers={"content-length": "0"},
+            chunks=[],
+        )
+    ]
+
+    downloader = _build_downloader(responses)
+
+    success, message = await downloader.download_file("https://example.com/file", str(target_path))
+
+    assert success is False
+    assert "empty" in message.lower()
+    assert not target_path.exists()
+    assert not Path(str(target_path) + ".part").exists()
+
+
+@pytest.mark.asyncio
+async def test_download_file_succeeds_when_sizes_match(tmp_path):
+    target_path = tmp_path / "model" / "file.bin"
+    target_path.parent.mkdir()
+
+    payload = b"abcdef"
+    responses = [
+        lambda: FakeResponse(
+            status=200,
+            headers={"content-length": str(len(payload))},
+            chunks=[payload],
+        )
+    ]
+
+    downloader = _build_downloader(responses)
+
+    success, result_path = await downloader.download_file("https://example.com/file", str(target_path))
+
+    assert success is True
+    assert Path(result_path).read_bytes() == payload
+    assert not Path(str(target_path) + ".part").exists()