Apply strict MSC4311 stripped state validation after 2027-06-01

[MadLittleMods]

MSC4311 says that for "version 12+ rooms, servers SHOULD rather than MAY respond to such requests with 400 M_MISSING_PARAM":

If any of the events are not a PDU, not for the room ID specified, or fail
signature checks, or the m.room.create event is missing, the receiving
server MAY respond to invites with a 400 M_MISSING_PARAM standard Matrix
error (new to the endpoint). For invites to room version 12+ rooms, servers
SHOULD rather than MAY respond to such requests with 400 M_MISSING_PARAM.

-- MSC4311

Currently, we only log in room versions 12+ where this SHOULD be the case already.

After 2027-06-01 (to allow some time for the ecosystem to adapt and support MSC4311), instead of logging, we should reject with 400 M_MISSING_PARAM. Given we have the lee-way to enforce this in all room versions (granted by the MAY langauge in the spec), we might as well. This is marked as a leftover FIXME task in #19723

Definition of done

• Strictly reject events which don't conform to MSC4311 in room versions 12+ (SHOULD)
• Strictly reject events which don't conform to MSC4311 in all room versions (MAY)
  • We could push the date out further for room versions <12 as some homeservers will just never update
• Unskip the TestMSC4311RejectInvalidStrippedStateFederation Complement tests

Dev notes

Synapse PR: #19723

Complement tests: matrix-org/complement#796

[timedoutuk]

Just came to report this as a bug - invites from Synapse are now rejected by Continuwuity as of v26.6.1 (PR1952 specifically) as it now enforces these validation suggestions (on all room verisons). We were initially going to add a compatibility option to log invites that fail additional validation, but it seems Synapse still sends stripped create events in <V12 rooms, which is something we can't work around.

[MadLittleMods]

@timedoutuk This issue is about strictly validating invites but on the Synapse side (like you linked for Continuwuity). The strict validation is happening in the future to give the ecosystem some time to update and adapt to avoid the kind of incompatibility you're seeing.

For compatibility, I think you're actually interested in #19723 which removes the flawed MSC4311 Synapse implementation and actually uses full PDU's in the stripped state.

[timedoutuk]

Ah thanks @MadLittleMods. Had assumed applying the validation would also be part of additionally sending the new format; I'll subscribe to the other issue instead!