There are some use cases (namely using
Pantalaimon with bots)
which could benefit from knowing the device_id associated with a token.
The /account/whoami endpoint receives an additional response field for the device_id
associated with the access token. The field is optional because appservice users may not
have a real device associated with them. Non-appservice users should always have a device
associated with them.
Access tokens are already associated with at most 1 device, and devices are associated with exactly 1 access token. Because of this, we do not need to worry about multiple devices causing problems. For more information, see https://matrix.org/docs/spec/client_server/r0.4.0.html#relationship-between-access-tokens-and-devices
Note: Pantalaimon would likely require a device_id be returned and error requests
otherwise. This should be considered expected behaviour by Pantalaimon in the MSC author's
opinion.
We could introduce a /device/whoami endpoint, however that is a less superior option. Most
calls to /device/whoami would additionally need to call /account/whoami to determine the
user ID of the account. We had might as well bundle the two pieces of information into the
same request.