Google default creds is a combo of ALTS, TLS and OAuth2. The right set of creds will be picked to use based on environment.
This PR contains:
- A new `creds.Bundle` type
- changes to use it in ClientConn and transport
- dial option to set the bundle for a ClientConn
- balancer options and NewSubConnOption to set it for SubConn
- Google default creds implementation by @cesarghali
- grpclb changes to use different creds mode for different servers
- interop client changes for google default creds testing
This PR splits out grpclb from grpc. I have made the PR in several commits so you can see more clearly the steps that happened.
There are a few possibly contentious points that I would like to make clear up front:
* grpclb will no longer autoload as a load balancer. I think this is okay, as service config is not widely (at all?) used, and I believe this is the only way to access it.
* `internal` is used more, as a way of having code shared between packages without exposing types
* ConnectivityStateEvaluator, as used by grpclb, is no longer thread safe. I believe there is an outer mutex that guards access, but I want to point out this subtle change up here.
All but one tests pass with this, due to another cyclic dependency. I can fix this, but it is a little more widely scoped (such as exposing grpc.server and grpc.errorDesc in the internal package). This PR is a nearly-passing sample of that last step to get this working.
PTAL @menghanl @dfawley
The parent ClientConn should re-resolve when grpclb loses connection to the
remote balancer.
When the ClientConn inside grpclb gets a TransientFailure, it calls
lbManualResolver.ResolveNow(), which calls parent ClientConn's ResolveNow, and
eventually results in re-resolve happening in parent ClientConn's resolver (DNS
for example).
This PR adds a method to balancer.ClientConn interface, so balancer can tell
parent ClientConn to re-resolve.
