Don't include redundant prev_state in new events

[DMRobertson]

These need to remain on any existing events so that hashes and signatures etc remain valid. But there is no reason for them hang around. As Rich wrote on MSC2176:

The following event keys are to be removed from the list of those to be
preserved by a redaction:

• membership
• prev_state

(Note this refers to the event-level membership property, rather than the
richvdh marked this conversation as resolved.
similarly-named sub-property under the content key.)

Rationale: neither of the above properties have defined meanings any more in the Matrix
protocol, so there is no reason for them to be special-cased in this way.

See matrix-org/matrix-spec#1091 and matrix-org/matrix-spec-proposals#2176

I think we can remove prev_state from here

synapse/synapse/events/builder.py

Lines 155 to 165 in e0bb268

	event_dict: Dict[str, Any] = {
	"auth_events": auth_events,
	"prev_events": prev_events,
	"type": self.type,
	"room_id": self.room_id,
	"sender": self.sender,
	"content": self.content,
	"unsigned": self.unsigned,
	"depth": depth,
	"prev_state": [],
	}

and here

synapse/synapse/federation/federation_client.py

Lines 800 to 804 in bf0c3ca

	# The protoevent received over the JSON wire may not have all
	# the required fields. Lets just gloss over that because
	# there's some we never care about
	if "prev_state" not in pdu_dict:
	pdu_dict["prev_state"] = []

[richvdh]

related: #4787

[DMRobertson]

Thanks Rich. (I did have a quick search this time, but I think prev_state shows up in a lot of error messages and event dumps.)