Handler returned immediately when email shape was invalid or user was
unknown; on a found user it generated an Argon2 token, INSERT'd into
app_user_password_resets, built a link, and called email.send (tens of
ms + DB write + SMTP RTT). The wall-clock delta was enormous and
externally observable.
Add a dummy generate_session_token() call on both no-match branches so
the observable cost matches the matching branch up to the Argon2 hash.
Doesn't equalise the DB INSERT + SMTP send (those would require either
real side effects or a deterministic dev-mode null sink), but it
neutralises the highest-bit-of-info side channel (handler-immediate-
return vs handler-token-mint).
Combined with F-S-003 (find_by_email now requires a principal),
enumeration via this surface from an anonymous public route is closed.
AUDIT.md anchor: F-S-004.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
9247680ab6