Another case of state reset in State Resolution v2

[matrixbot]

This issue has been migrated from #15987.

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Update: See matrix-org/synapse#15987 (comment). This original comment contains grossly incorrect conclusions.

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Description

Synapse is responding over the client–server API with a different set of state events depending on whether the requesting user has registered an account or is a guest.

Steps to reproduce

This bug was discovered in room !PfltsPSvJVImWNRDyj:matrix.org. Here is a series of commands to demonstrate the problem:

# Access token for a registered user account
export USER_TOKEN=''
# Access token for a guest user account
export GUEST_TOKEN=''

export ROOMID='!PfltsPSvJVImWNRDyj:matrix.org'
export HS='https://matrix-client.matrix.org'

export FILTER='[ .[] | walk(if type == "object" then delpaths([["age"],["age_ts"],["replaces_state"],["prev_content"],["prev_sender"],["transaction_id"],["unsigned"],["user_id"]]) else . end | if type=="array" then sort else . end) ] | sort_by(.type)'

curl --no-progress-meter -H 'Accept:application/json' -H "Authorization: Bearer $USER_TOKEN" "$HS/_matrix/client/v3/rooms/$ROOMID/state" | jq -rS "$FILTER" > user_state.json
curl --no-progress-meter -H 'Accept:application/json' -H "Authorization: Bearer $GUEST_TOKEN" "$HS/_matrix/client/v3/rooms/$ROOMID/state" | jq -rS "$FILTER" > guest_state.json

Inspection of the output files user_state.json and guest_state.json reveals wildly different sets of state events, even though both come from the same homeserver. For example:

user_state.json

{
  "content": {
    "join_rule": "public"
  },
  "event_id": "$W8ilz2XuqXYU3LilZkU3BLlbGvQgxGMBn5BY81SmBP0",
  "origin_server_ts": 1687249386275,
  "room_id": "!PfltsPSvJVImWNRDyj:matrix.org",
  "sender": "@wj25czxj47bu6q:matrix.org",
  "state_key": "",
  "type": "m.room.join_rules"
}

guest_state.json

{
  "content": {
    "join_rule": "invite"
  },
  "event_id": "$lZoXigGI2_plf_Hdj47IDmYPeoV00te_jgkVQGToJJo",
  "origin_server_ts": 1686004259002,
  "room_id": "!PfltsPSvJVImWNRDyj:matrix.org",
  "sender": "@moderation-tools:matrix.org",
  "state_key": "",
  "type": "m.room.join_rules"
}

In this case, the m.room.join_rules reported to registered users is the correct one, while the m.room.join_rules reported to guest users is an old event which has been replaced.

This also does not appear to have any relation with m.room.guest_access, which was set to "guest_access": "forbidden" long before either of the m.room.join_rules state events displayed above were sent.

Homeserver

matrix.org

Synapse Version

1.88.0

Installation Method

I don't know

Database

I don't know

Workers

I don't know

Platform

I don't know

Configuration

No response

Relevant log output

No response

Anything else that would be useful to know?

This issue seems to have cropped up very recently, perhaps in Synapse v1.88.0 or v1.88.0rc1, though it is also entirely possible that it just went unnoticed until now.