Login API should casefold usernames, matching registration API

[matrixbot]

This issue was originally created by @timmc at https://github.com/matrix-org/dendrite/issues/2059.

Background information

• Dendrite version or git SHA: v0.5.1
• Monolith or Polylith?: monolith
• SQLite3 or Postgres?: Postgres
• Running in Docker?: no
• go version: 1.17.3

Description

User can register with mixed-case names e.g. Test but then cannot log in.

Steps to reproduce

• Register with username Test
• Try to log in with username Test

The registration code case-folds the username to lowercase, so the registered username is actually test. However, the login code doesn't perform this case-folding, so login fails.

References:

• Spec forbids uppercase letters: https://spec.matrix.org/v1.1/appendices/#user-identifiers -- but homeserver is free to downcase
• Previous issue that requested downcasing in login and registration, although only the registration side was actually done: https://github.com/matrix-org/dendrite/issues/325
• ParseUsernameParam may be a good place to make this change, dunno

[matrixbot]

This comment was originally posted by @timmc at https://github.com/matrix-org/dendrite/issues/2059#issuecomment-1041033498.

Looks like this is taken care of by that PR. :100: