Three small, independent hardening fixes to source ingestion:
- generate_unique_filename() checked `if not resolved.exists()` then let
a separate write happen later - two concurrent uploads landing on the
same candidate name could both pass the check and clobber each other.
Now atomically claims the name via Path.touch(exist_ok=False) (O_EXCL)
as part of the search loop itself.
- _resolve_source_file() and _is_source_file_available() compared
`resolved_path.startswith(safe_root)` without a trailing separator - a
sibling directory that merely starts with the same string (e.g.
"uploads_evil/") would incorrectly be treated as contained, unlike this
file's other two path checks which already guard with `+ os.sep`. Not
reachable today (source.asset.file_path is only ever set server-side),
but this closes the gap and matches the existing pattern.
- SourceCreate.notebooks/transformations had no length limit; both are
iterated with a per-item DB lookup in create_source(), so an unbounded
array let a single request trigger an unbounded number of sequential DB
round trips. Capped at 50.
tests/test_upload_type_mitigations.py adds no code change - it documents
why the adjacent "no file type allowlist on uploads" finding was
investigated and judged low-risk without one (downloads are already
served as application/octet-stream regardless of actual file type).
