Cannot invite appservice users which do not (yet) exist

[turt2live]

Appservices typically follow this sort of flow:

1. User invites @_example_user:localhost to a room.
2. Homeserver finds out that @_example_user:localhost doesn't exist, but is in the namespace for an appservice.
3. Homeserver queries the appservice so it can create the user
4. Appservice creates the user
5. Homeserver finishes processing the invite, sending it to the appservice

Recently we added a proof of life check to see if a user exists before proceeding with the invite though (matrix-org/matrix-react-sdk#2317). This fails on Synapse because the user query isn't done, so the request for a profile fails, which means Riot won't try and invite the user, which means the steps above mean nothing because they aren't executed.

TLDR: Synapse needs to hit the appservice's /users endpoint for profile lookups too.

[turt2live]

(reported as the user-facing bug - will open a synapse issue to track the backend issue)

[valentinab25]

We also have this problem. We are using mxisd as our identity server ( as a synapse password provider integrated with our LDAP ). Since 0.17.8 no new users can be invited in any rooms. We can find the user, but on the SEND INVITES button we receive the error:

Failed to invite the following users to the TEST room:
@LDAPUID:OUR_HOMESERVER: User @LDAPUID:OUR_HOMESERVER does not exist

[maxidorius]

This issue is not limited to appservices but to at least the following set of existing tools:

• Password providers
• mxisd
• Profile enhancers or Gateways which may return 404 at their own discretion as allowed by the profile endpoint spec even if a profile exists

A more detailed explanation of why this needs to be rolled back is available on the PR.

[lampholder]

A spec change is being discussed as to the merits of checking /profile and how we might enhance this API (or add a new one) to handle checking whether a user exists. However, we've decided that for now we can power through client side if we:

• expose a warning when you try and invite a mxid we can't confirm exists (but let you continue with the invite if you wish)
• make this warning optional, to be configured on either a per user or per riot deploy basis

[turt2live]

matrix-org/matrix-react-sdk#2434 implements the behaviour @lampholder describes. For those looking to have it always work regardless of account setting, add this to your config.json:

"settingDefaults": {
    "alwaysInviteUnknownUsers": true
}

[turt2live]

Fixed by matrix-org/matrix-react-sdk#2434