_check_threepid in auth.py incorrect for MSISDN

[jryans]

Looking at this snippet:

synapse/synapse/handlers/auth.py

Lines 446 to 463 in f99a9c9

	if self.hs.config.threepid_behaviour_email == ThreepidBehaviour.REMOTE:
	if medium == "email":
	threepid = yield identity_handler.threepid_from_creds(
	self.hs.config.account_threepid_delegate_email, threepid_creds
	)
	elif medium == "msisdn":
	threepid = yield identity_handler.threepid_from_creds(
	self.hs.config.account_threepid_delegate_msisdn, threepid_creds
	)
	else:
	raise SynapseError(400, "Unrecognized threepid medium: %s" % (medium,))
	elif self.hs.config.threepid_behaviour_email == ThreepidBehaviour.LOCAL:
	row = yield self.store.get_threepid_validation_session(
	medium,
	threepid_creds["client_secret"],
	sid=threepid_creds["sid"],
	validated=True,
	)

...the MSISDN handling should not depend on the threepid_behaviour_email setting. Instead, we should use the MSISDN delegate if it exists or else fail.

Riot Web hits this block when trying to provide a MSISDN during registration.

[jryans]

Fixed by #6104