Basically a complete re-write/re-design of the forwarding concept introduced in
#4297 (sorry for the rapid churn here). Instead of nonce-services blindly
forwarding nonces around to each other in an attempt to find out who issued the
nonce we add an identifying prefix to each nonce generated by a service. The
WFEs then use this prefix to decide which nonce-service to ask to validate the
nonce.
This requires a slightly more complicated configuration at the WFE/2 end, but
overall I think ends up being a way cleaner, more understandable, easy to
reason about implementation. When configuring the WFE you need to provide two
forms of gRPC config:
* one gRPC config for retrieving nonces, this should be a DNS name that
resolves to all available nonce-services (or at least the ones you want to
retrieve nonces from locally, in a two DC setup you might only configure the
nonce-services that are in the same DC as the WFE instance). This allows
getting a nonce from any of the configured services and is load-balanced
transparently at the gRPC layer.
* a map of nonce prefixes to gRPC configs, this maps each individual
nonce-service to it's prefix and allows the WFE instances to figure out which
nonce-service to ask to validate a nonce it has received (in a two DC setup
you'd want to configure this with all the nonce-services across both DCs so
that you can validate a nonce that was generated by a nonce-service in another
DC).
This balancing is implemented in the integration tests.
Given the current remote nonce code hasn't been deployed anywhere yet this
makes a number of hard breaking changes to both the existing nonce-service
code, and the forwarding code.
Fixes#4303.
af41bea99a