I see the following in /var/log/chrome/chrome on a recent caroline build after a test logs in using autotestPrivate:

[5548:5548:0612/143205.430220:ERROR:object_proxy.cc(616)] Failed to call method: org.chromium.CryptohomeInterface.NeedsDircryptoMigration: object_path= /org/chromium/Cryptohome: org.freedesktop.DBus.GLib.ErrorError: Method invoked for NeedsDircryptoMigration returned FALSE but did not set error
[5548:5548:0612/143205.430265:ERROR:user_selection_screen.cc(310)] Failed to call cryptohome NeedsDircryptoMigration.

Here's the corresponding log message from cryptohome:

2018-06-12T14:32:05.427755-07:00 ERR cryptohomed[1561]: Unknown user.

I think that this is a widespread issue throughout platform2/cryptohome/service.cc; most of its D-Bus method call handlers return FALSE without updating the passed-in GError struct. Updating the GError would presumably result in a useful error message being returned to the caller.

So that's something that should be fixed in cryptohome, but should Chrome also avoid calling this method in the first place when a fake user is logging in for testing? I'm assuming that that's the reason for the "Unknown user" error -- I don't see it when I log in as a real user on the same device.