Incorrect membership state with leave/rejoin cycles and state_after support

[richvdh]

0. Ensure that homeserver supports MSC4222.
1. Alice invites Bob and Charlie to a room. Both join.
2. Charlie switches to another room (this means that Charlie will not immediately load the membership list later)
3. Charlie reloads EW. (Not sure if this is necessary)
4. Alice kicks Bob
5. Alice kicks Charlie
6. Alice invites Charlie
7. Alice invites Bob
8. Bob rejoins
9. Charlie switches to the room, and observes the "you have been invited" screen. At this point, Charlie's client does an out-of-band membership load, but already has a cached non-OOB leave membership for Bob, so ignores the result.
10. Charlie rejoins. Due to lazy-loading, Bob's membership state is omitted from the resultant /sync response

Charlie now incorrectly believes that Bob is not in the room, leading to UTDs

[richvdh]

I don't really know why this is happening, given Synapse is supposed to disable LL when the timeline is limited (cf element-hq/element-web#7211, element-hq/element-web#27285), but empirically Synapse certainly omits Geoff's membership event when /sync is called with a filter that enables lazy-loading and includes it normally.

[t3chguy]

given Synapse is supposed to disable LL when the timeline is limited

Are you saying the sync is in fact limited, if so then this would be a clear Synapse bug of spec non-compliance:

When a sync is limited, the server MUST return membership events for events in the gap (between since and the start of the returned timeline), regardless as to whether or not they are redundant.

Could also be related to #18793

[richvdh]

given Synapse is supposed to disable LL when the timeline is limited

Are you saying the sync is in fact limited,

Yes

`/sync` response

In this test, the users are:

• @a:xps9320.sw1v.org - Alice, inviter
• @g:xps9320.sw1v.org - Bob, user whose membership is not returned correctly
• @t:xps9320.sw1v.org - Charlie, user who receives the incorrect /sync

Note the absence of an m.room.member event for @g:xps9320.sw1v.org from state_after.

{
	"next_batch": "s2554_248766_0_1547_5298_2_5443_3849_0_1_1",
	"presence": {
		"events": [
			{
				"type": "m.presence",
				"sender": "@g:xps9320.sw1v.org",
				"content": {
					"presence": "online",
					"last_active_ago": 923,
					"currently_active": true
				}
			},
			{
				"type": "m.presence",
				"sender": "@a:xps9320.sw1v.org",
				"content": {
					"presence": "online",
					"last_active_ago": 2427,
					"currently_active": true
				}
			}
		]
	},
	"device_lists": {
		"changed": [
			"@g:xps9320.sw1v.org",
			"@a:xps9320.sw1v.org",
			"@t:xps9320.sw1v.org"
		]
	},
	"device_one_time_keys_count": {
		"signed_curve25519": 50
	},
	"device_unused_fallback_key_types": [],
	"rooms": {
		"join": {
			"!CuENToYLdemnvoJDDv:xps9320.sw1v.org": {
				"timeline": {
					"events": [
						{
							"type": "m.room.member",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"displayname": "Geoff",
								"membership": "invite"
							},
							"state_key": "@g:xps9320.sw1v.org",
							"origin_server_ts": 1771415095518,
							"unsigned": {
								"membership": "leave",
								"age": 193945
							},
							"event_id": "$57htnjMWpfCyCP4iVRyyIKDW3-poQPmDN3RM11wkFos"
						},
						{
							"type": "m.room.member",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"displayname": "Tim",
								"membership": "invite"
							},
							"state_key": "@t:xps9320.sw1v.org",
							"origin_server_ts": 1771415095687,
							"unsigned": {
								"membership": "invite",
								"age": 193776
							},
							"event_id": "$XtRpYbWOsu-6sOsXTPsgp_xMZokS8UQjUVkq7rI9ls8"
						},
						{
							"type": "m.room.member",
							"sender": "@g:xps9320.sw1v.org",
							"content": {
								"displayname": "Geoff",
								"membership": "join"
							},
							"state_key": "@g:xps9320.sw1v.org",
							"origin_server_ts": 1771415109807,
							"unsigned": {
								"replaces_state": "$57htnjMWpfCyCP4iVRyyIKDW3-poQPmDN3RM11wkFos",
								"prev_content": {
									"displayname": "Geoff",
									"membership": "invite"
								},
								"prev_sender": "@a:xps9320.sw1v.org",
								"membership": "invite",
								"age": 179656
							},
							"event_id": "$p96tWFese4opotjZBODS4fAhOfbVmaqv22VFHYc87Gk"
						},
						{
							"type": "m.room.member",
							"sender": "@t:xps9320.sw1v.org",
							"content": {
								"displayname": "Tim",
								"membership": "join"
							},
							"state_key": "@t:xps9320.sw1v.org",
							"origin_server_ts": 1771415113832,
							"unsigned": {
								"replaces_state": "$XtRpYbWOsu-6sOsXTPsgp_xMZokS8UQjUVkq7rI9ls8",
								"prev_content": {
									"displayname": "Tim",
									"membership": "invite"
								},
								"prev_sender": "@a:xps9320.sw1v.org",
								"membership": "join",
								"age": 175631
							},
							"event_id": "$32j2ZJz9aXF7GFdKS3nBV1C1M-d2Xpnz5Es67RGrI6M"
						},
						{
							"type": "m.room.member",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"membership": "leave"
							},
							"state_key": "@g:xps9320.sw1v.org",
							"origin_server_ts": 1771415158993,
							"unsigned": {
								"replaces_state": "$p96tWFese4opotjZBODS4fAhOfbVmaqv22VFHYc87Gk",
								"prev_content": {
									"displayname": "Geoff",
									"membership": "join"
								},
								"prev_sender": "@g:xps9320.sw1v.org",
								"membership": "join",
								"age": 130470
							},
							"event_id": "$a6iKzIV4BYzF-rNedtkpxWaE_Jl0kgvdwG7w3a36lAg"
						},
						{
							"type": "m.room.member",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"membership": "leave"
							},
							"state_key": "@t:xps9320.sw1v.org",
							"origin_server_ts": 1771415163339,
							"unsigned": {
								"replaces_state": "$32j2ZJz9aXF7GFdKS3nBV1C1M-d2Xpnz5Es67RGrI6M",
								"prev_content": {
									"displayname": "Tim",
									"membership": "join"
								},
								"prev_sender": "@t:xps9320.sw1v.org",
								"membership": "leave",
								"age": 126124
							},
							"event_id": "$Kf0-bnUrgE0r5OORxESDMjORgxlQ2_JAj8j_dBRG9Cg"
						},
						{
							"type": "m.room.member",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"displayname": "Tim",
								"membership": "invite"
							},
							"state_key": "@t:xps9320.sw1v.org",
							"origin_server_ts": 1771415184698,
							"unsigned": {
								"replaces_state": "$Kf0-bnUrgE0r5OORxESDMjORgxlQ2_JAj8j_dBRG9Cg",
								"prev_content": {
									"membership": "leave"
								},
								"prev_sender": "@a:xps9320.sw1v.org",
								"membership": "invite",
								"age": 104765
							},
							"event_id": "$QDsAwXaNQffM8alMAAXPbrlRWtABheCmyS5nPaF5Sco"
						},
						{
							"type": "m.room.member",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"displayname": "Geoff",
								"membership": "invite"
							},
							"state_key": "@g:xps9320.sw1v.org",
							"origin_server_ts": 1771415189208,
							"unsigned": {
								"replaces_state": "$a6iKzIV4BYzF-rNedtkpxWaE_Jl0kgvdwG7w3a36lAg",
								"prev_content": {
									"membership": "leave"
								},
								"prev_sender": "@a:xps9320.sw1v.org",
								"membership": "invite",
								"age": 100255
							},
							"event_id": "$JWqlzV2klNj4WNslCgn7IxEe9WA1BrmBJbTMlBCOjwU"
						},
						{
							"type": "m.room.member",
							"sender": "@g:xps9320.sw1v.org",
							"content": {
								"displayname": "Geoff",
								"membership": "join"
							},
							"state_key": "@g:xps9320.sw1v.org",
							"origin_server_ts": 1771415195923,
							"unsigned": {
								"replaces_state": "$JWqlzV2klNj4WNslCgn7IxEe9WA1BrmBJbTMlBCOjwU",
								"prev_content": {
									"displayname": "Geoff",
									"membership": "invite"
								},
								"prev_sender": "@a:xps9320.sw1v.org",
								"membership": "invite",
								"age": 93540
							},
							"event_id": "$WfI6u3B48wVao0i0kE2Icfh27dINagRukaL_dABF-ms"
						},
						{
							"type": "m.room.member",
							"sender": "@t:xps9320.sw1v.org",
							"content": {
								"displayname": "Tim",
								"membership": "join"
							},
							"state_key": "@t:xps9320.sw1v.org",
							"origin_server_ts": 1771415289243,
							"unsigned": {
								"replaces_state": "$QDsAwXaNQffM8alMAAXPbrlRWtABheCmyS5nPaF5Sco",
								"prev_content": {
									"displayname": "Tim",
									"membership": "invite"
								},
								"prev_sender": "@a:xps9320.sw1v.org",
								"membership": "join",
								"age": 220
							},
							"event_id": "$ZwukpWI7cOeQ6G8sDEPkoFwocDo54iwhqqmaq4K0tpY"
						}
					],
					"prev_batch": "s2544_248766_0_1547_5298_2_5443_3849_0_1_1",
					"limited": true
				},
				"org.matrix.msc4222.state_after": {
					"events": [
						{
							"type": "m.room.create",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"room_version": "10",
								"creator": "@a:xps9320.sw1v.org"
							},
							"state_key": "",
							"origin_server_ts": 1771414924279,
							"unsigned": {
								"age": 365184
							},
							"event_id": "$RjWK4c-1zf7wMtTuj_n9MG0coGL7-B2Vf10wKwygH_g"
						},
						{
							"type": "m.room.encryption",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"algorithm": "m.megolm.v1.aes-sha2"
							},
							"state_key": "",
							"origin_server_ts": 1771414924427,
							"unsigned": {
								"age": 365036
							},
							"event_id": "$VbbESfFmEdJw_-PxiB64NdMeCRDjpcH78R8Vqu9NQcg"
						},
						{
							"type": "m.room.guest_access",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"guest_access": "can_join"
							},
							"state_key": "",
							"origin_server_ts": 1771414924426,
							"unsigned": {
								"age": 365037
							},
							"event_id": "$nBfv92SWxzH1DGZ9XLBz3KMuuW0xixLyyP3vIo_KN6E"
						},
						{
							"type": "m.room.history_visibility",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"history_visibility": "shared"
							},
							"state_key": "",
							"origin_server_ts": 1771414924425,
							"unsigned": {
								"age": 365038
							},
							"event_id": "$tnpkMbp3D_OYSpUeAJDBcc331W0UjmWJaRMShEt8XDQ"
						},
						{
							"type": "m.room.join_rules",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"join_rule": "invite"
							},
							"state_key": "",
							"origin_server_ts": 1771414924424,
							"unsigned": {
								"age": 365039
							},
							"event_id": "$O5a826jo7L7nX0d0Djejg1Dd66H0cCnXXlx1Y8Cc0ZE"
						},
						{
							"type": "m.room.member",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"displayname": "Alice2",
								"membership": "join"
							},
							"state_key": "@a:xps9320.sw1v.org",
							"origin_server_ts": 1771414924350,
							"unsigned": {
								"age": 365113
							},
							"event_id": "$lraHYK7CDak1WkfPj3VPQmNIw4etXtqGlJDYeZeo7dY"
						},
						{
							"type": "m.room.member",
							"sender": "@t:xps9320.sw1v.org",
							"content": {
								"displayname": "Tim",
								"membership": "join"
							},
							"state_key": "@t:xps9320.sw1v.org",
							"origin_server_ts": 1771415289243,
							"unsigned": {
								"replaces_state": "$QDsAwXaNQffM8alMAAXPbrlRWtABheCmyS5nPaF5Sco",
								"prev_content": {
									"displayname": "Tim",
									"membership": "invite"
								},
								"prev_sender": "@a:xps9320.sw1v.org",
								"age": 220
							},
							"event_id": "$ZwukpWI7cOeQ6G8sDEPkoFwocDo54iwhqqmaq4K0tpY"
						},
						{
							"type": "m.room.name",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"name": "left user test 10"
							},
							"state_key": "",
							"origin_server_ts": 1771414924427,
							"unsigned": {
								"age": 365036
							},
							"event_id": "$qSV5awwhrit9K3Vcof8bjK8fA4RNvbcHySVDMoPJ_dA"
						},
						{
							"type": "m.room.power_levels",
							"sender": "@a:xps9320.sw1v.org",
							"content": {
								"users": {
									"@a:xps9320.sw1v.org": 100
								},
								"users_default": 0,
								"events": {
									"m.room.name": 50,
									"m.room.power_levels": 100,
									"m.room.history_visibility": 100,
									"m.room.canonical_alias": 50,
									"m.room.avatar": 50,
									"m.room.tombstone": 100,
									"m.room.server_acl": 100,
									"m.room.encryption": 100
								},
								"events_default": 0,
								"state_default": 50,
								"ban": 50,
								"kick": 50,
								"redact": 50,
								"invite": 0,
								"historical": 100
							},
							"state_key": "",
							"origin_server_ts": 1771414924408,
							"unsigned": {
								"age": 365055
							},
							"event_id": "$5WuYz6lhK5WM5zkySG6uwyaOy62l0-uz_i3dtXsYCHI"
						}
					]
				},
				"account_data": {
					"events": []
				},
				"ephemeral": {
					"events": []
				},
				"unread_notifications": {
					"notification_count": 0,
					"highlight_count": 0
				},
				"summary": {
					"m.joined_member_count": 3,
					"m.invited_member_count": 0
				}
			}
		}
	}
}

if so then this would be a clear Synapse bug of spec non-compliance:

When a sync is limited, the server MUST return membership events for events in the gap (between since and the start of the returned timeline), regardless as to whether or not they are redundant.

Well, it is included in the timeline, and that bit of spec is frankly unclear about how it relates to state_after, but I think you're right.