Use the timing-safe compare() helper for the CSRF formkey check

[Nexory]

Summary

FORM.accepts() validates the anti-CSRF form key with a plain list-membership test:

if not (formkey and formkeys and formkey in formkeys):

formkey in formkeys compares the attacker-supplied _formkey against each stored key using CPython's ==, which short-circuits on the first differing character and is not constant-time.

web2py already ships a timing-safe comparison helper for exactly this, gluon.utils.compare (documented "Compares two strings and not vulnerable to timing attacks", backed by hmac.compare_digest), and already uses it for the analogous signature check in the same file:

# gluon/html.py:581
return compare(original_sig, sig)

This applies that same helper to the formkey check, so the anti-CSRF token comparison is timing-safe and consistent with the rest of the module:

if not (formkey and formkeys and any(compare(formkey, k) for k in formkeys)):

compare is already imported at the top of the file, so no new import is needed. Behavior is unchanged: a valid formkey is still accepted and an invalid one is still rejected; only the per-key comparison is now constant-time.

Testing

gluon/tests/test_html.py and gluon/tests/test_form.py pass unchanged (86 passed). I also verified directly that a valid formkey is accepted and a wrong formkey is rejected after the change.

This is the same hardening class as #1321 ("fixed timing attack in gluon.utils.compare"), applied to the formkey comparison site.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.71%. Comparing base (1462a3e) to head (fac9c60).
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2650      +/-   ##
==========================================
+ Coverage   50.70%   50.71%   +0.01%     
==========================================
  Files          42       42              
  Lines       15500    15500              
  Branches     3409     3409              
==========================================
+ Hits         7859     7861       +2     
+ Misses       6599     6598       -1     
+ Partials     1042     1041       -1     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.