DM room is encrypted on one side but not the other

[kegsay]

Steps to reproduce

• Sign up for a brand new matrix.org account
• Start a DM with me. (v6)
• The DM is not encrypted for you but it is for me.

I dug into this and the reason why is because the initiator does not have the entire room state: it lacks the m.room.encryption event, whereas I have:

On the other side, they ONLY had m.room.create and m.room.member. Looking at the logs for this, it looks like the code which processes /sync fell over which has resulted in partial state:

2021-11-11T15:59:02.408Z I No usable key backup: not enabling key backup
2021-11-11T15:59:02.424Z I LL: got 1 members from server for room !GtAZwcgIRgGMvUCNyF:matrix.org
2021-11-11T15:59:02.425Z I LL: RoomState about to set 1 OOB members ...
2021-11-11T15:59:02.425Z I LL: RoomState put in finished state ...
2021-11-11T15:59:02.425Z I LL: telling store to write 0 members for room !GtAZwcgIRgGMvUCNyF:matrix.org
2021-11-11T15:59:02.438Z I Event $f1lzUDKNqNHHBNWeGZCYmupQzXc4ppsycYSNkrKUNU0 already in timeline !GtAZwcgIRgGMvUCNyF:matrix.org:2021-11-11T15:59:02.128Z
2021-11-11T15:59:02.759Z I Saving device tracking data m2462457537~2.2462457541~3.2462457541_757284961_7414302_1102398485_1047423627_2750527_358471574_2268960259_210115
2021-11-11T15:59:05.426Z E Caught /sync error TypeError: Cannot read property 'getContent' of undefined
    at Y.<anonymous> (vector://vector/webapp/bundles/d443354bdb439e133de8/vendors~init.js:2:157412)
    at Y.s.emit (vector://vector/webapp/bundles/d443354bdb439e133de8/vendors~init.js:2:11111)
    at vector://vector/webapp/bundles/d443354bdb439e133de8/vendors~init.js:2:3166333
    at Array.forEach (<anonymous>)
    at y.processSyncResponse (vector://vector/webapp/bundles/d443354bdb439e133de8/vendors~init.js:2:3166188)
    at y.doSync (vector://vector/webapp/bundles/d443354bdb439e133de8/vendors~init.js:2:3163984)

Leaving the DM room and making a new DM room fixed this.

The initiator has:

Element 1.9.4
Olm 3.2.3
OS: Windows

Outcome

This should work fine and be encrypted.

Operating system

Windows

Application version

Element 1.9.4 Olm 3.2.3

How did you install the app?

From element.io

Homeserver

matrix.org

Will you send logs?

Yes

[dbkr]

Decoder ring says the top stack frame is {"source":"webpack:///node_modules/matrix-react-sdk/src/stores/SpaceStore.ts","line":709,"column":42,"name":"getContent"}

[t3chguy]

That looks to be assuming that accountData changes always have a lastEvent, will fix, looks to be exploding the sync loop, wonder how it took this long to spot