- Reduce the duplicate code by merging the flows and
determining the flow type by looking at the provided options.
- Options as a function type allows us to validate an individual
an option in its scope and makes it easier to compose the
built-in options with the third-party ones.
This change is for both App Engine and Managed VMs so that these apps can scale without running into the app_identity_service quota limit due to calling appengine.AccessToken too frequently. An added benefit of caching is that calls to Google APIs will be significantly sped up due to removing the round-trip calls to the api_identity_service.
GAE managed VM runtime contains both appengine and appenginevm
build tags. Currently, appengine.go is being built even if user
wants to use the package on managed VM that errors with no
"appengine" package to import.