fix(sandbox): Relax upload permissions for aio sandbox sync (#1409)

* Relax upload permissions for aio sandbox sync

* Harden upload permission sync checks

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/tests/test_uploads_router.py

@@ -1,4 +1,5 @@
 import asyncio
+import stat
 from io import BytesIO
 from pathlib import Path
 from unittest.mock import AsyncMock, MagicMock, patch
@@ -69,6 +70,85 @@ def test_upload_files_syncs_non_local_sandbox_and_marks_markdown_file(tmp_path):
     sandbox.update_file.assert_any_call("/mnt/user-data/uploads/report.md", b"converted")
 
 
+def test_upload_files_makes_non_local_files_sandbox_writable(tmp_path):
+    thread_uploads_dir = tmp_path / "uploads"
+    thread_uploads_dir.mkdir(parents=True)
+
+    provider = MagicMock()
+    provider.acquire.return_value = "aio-1"
+    sandbox = MagicMock()
+    provider.get.return_value = sandbox
+
+    async def fake_convert(file_path: Path) -> Path:
+        md_path = file_path.with_suffix(".md")
+        md_path.write_text("converted", encoding="utf-8")
+        return md_path
+
+    with (
+        patch.object(uploads, "get_uploads_dir", return_value=thread_uploads_dir),
+        patch.object(uploads, "ensure_uploads_dir", return_value=thread_uploads_dir),
+        patch.object(uploads, "get_sandbox_provider", return_value=provider),
+        patch.object(uploads, "convert_file_to_markdown", AsyncMock(side_effect=fake_convert)),
+        patch.object(uploads, "_make_file_sandbox_writable") as make_writable,
+    ):
+        file = UploadFile(filename="report.pdf", file=BytesIO(b"pdf-bytes"))
+        result = asyncio.run(uploads.upload_files("thread-aio", files=[file]))
+
+    assert result.success is True
+    make_writable.assert_any_call(thread_uploads_dir / "report.pdf")
+    make_writable.assert_any_call(thread_uploads_dir / "report.md")
+
+
+def test_upload_files_does_not_adjust_permissions_for_local_sandbox(tmp_path):
+    thread_uploads_dir = tmp_path / "uploads"
+    thread_uploads_dir.mkdir(parents=True)
+
+    provider = MagicMock()
+    provider.acquire.return_value = "local"
+    sandbox = MagicMock()
+    provider.get.return_value = sandbox
+
+    with (
+        patch.object(uploads, "get_uploads_dir", return_value=thread_uploads_dir),
+        patch.object(uploads, "ensure_uploads_dir", return_value=thread_uploads_dir),
+        patch.object(uploads, "get_sandbox_provider", return_value=provider),
+        patch.object(uploads, "_make_file_sandbox_writable") as make_writable,
+    ):
+        file = UploadFile(filename="notes.txt", file=BytesIO(b"hello uploads"))
+        result = asyncio.run(uploads.upload_files("thread-local", files=[file]))
+
+    assert result.success is True
+    make_writable.assert_not_called()
+
+
+def test_make_file_sandbox_writable_adds_write_bits_for_regular_files(tmp_path):
+    file_path = tmp_path / "report.pdf"
+    file_path.write_bytes(b"pdf-bytes")
+    os_chmod_mode = stat.S_IRUSR | stat.S_IRGRP | stat.S_IROTH
+    file_path.chmod(os_chmod_mode)
+
+    uploads._make_file_sandbox_writable(file_path)
+
+    updated_mode = stat.S_IMODE(file_path.stat().st_mode)
+    assert updated_mode & stat.S_IWUSR
+    assert updated_mode & stat.S_IWGRP
+    assert updated_mode & stat.S_IWOTH
+
+
+def test_make_file_sandbox_writable_skips_symlinks(tmp_path):
+    file_path = tmp_path / "target-link.txt"
+    file_path.write_text("hello", encoding="utf-8")
+    symlink_stat = MagicMock(st_mode=stat.S_IFLNK)
+
+    with (
+        patch.object(uploads.os, "lstat", return_value=symlink_stat),
+        patch.object(uploads.os, "chmod") as chmod,
+    ):
+        uploads._make_file_sandbox_writable(file_path)
+
+    chmod.assert_not_called()
+
+
 def test_upload_files_rejects_dotdot_and_dot_filenames(tmp_path):
     thread_uploads_dir = tmp_path / "uploads"
     thread_uploads_dir.mkdir(parents=True)