relation_id() fails with multiple relations to the same service

[Stuart Bishop]

hookenv.relation_id() has the option to specify (relation_name + service_or_unit). This is not a key (the only key for relations is the relation id). If the unit has been related multiple times to the remote service, then relation_id() may return the incorrect relation_id().

eg.

juju deploy local:a
juju deploy local:b
juju add-relation a:rela b:relx
juju add-relation a:relb b:relx

If a unit in service b called relation_id('relx', 'a') it would get one of the two relation ids randomly.

[Stuart Bishop (stub)]

The only fixes are deprecating and removing the options, or raising an exception and have the charm fail explicitly in this situation (out of its control - it is triggered by how the remote service is implemented).