Harden IMAP hook against symlink-followed attachment writes

[Lougarou]

ImapHook._create_file wrote downloaded attachments with a plain
open(file_path, "wb"), which follows a symlink at the destination. The
existing _is_symlink guard in _create_files only inspects the bare
attachment name relative to the current working directory — not the real
joined output path (file_path) that is actually written to.

This hardens _create_file:

• it re-checks os.path.islink on the real joined target and refuses to write
  through a symlink there, matching the existing _is_symlink rejection style;
• it opens via os.open with O_NOFOLLOW (feature-gated with
  getattr(os, "O_NOFOLLOW", 0), so it is a no-op on Windows, which lacks the
  flag) to also close the check-to-open TOCTOU window.

The open flags are O_WRONLY | O_CREAT | O_TRUNC — deliberately without
O_EXCL — so the hook's existing overwrite-on-redownload behaviour is
preserved; the file mode is 0o600. Legitimate downloads (including
redownload/overwrite) behave exactly as before; only writes that would pass
through a symlink are now refused.

test_imap.py is updated: the download_mail_attachments tests now patch
os.open / os.fdopen instead of the builtin open, the symlink test's
islink call-count expectation is relaxed (it is now legitimately consulted in
two places), and two tests are added — one pinning that a symlink at the real
joined target blocks the write, one pinning the O_NOFOLLOW / no-O_EXCL flags.

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Important

🛠️ Maintainer triage note for @Lougarou · by @potiuk · 2026-06-18 13:57 UTC

Paused pending your next update — this PR has been inactive for ~30 days, so it's been moved to draft to keep the review queue clear:

• Rebase on the latest main, address any new failures, and mark it Ready for review when you pick it back up — no rush.
• See the Pull Request quality criteria.

The ball is in your court — you've been assigned to this PR.

_{Automated triage — may be imperfect; a maintainer takes the next look.}

[potiuk]

Note

🗑️ Closing stale draft · by @potiuk · 2026-07-02 18:53 UTC

This draft has been inactive for ~13 days. Closing to keep the queue tidy — no judgment on the change itself. Please reopen (or open a fresh PR) whenever you pick it back up, and a maintainer will be glad to take a look.

_{Automated triage — may be imperfect.}