e9707d8f85 exposed OAuth authentication extension point. Using this
extension point plugins can offer OAuth2 authentications.
That is fine for new Gerrit sites, which can restrict the auth scheme
to OAuth2 only.
For the existing sites, that rely on non SSO OpenID auth scheme it
doesn't work to migrate to OAuth2 because of diverse contributors
base that use different OpenID providers. Not all OpenID providers
offer OAuth2 protocol. Particularly, widespread OpenID providers
among open source Gerrit communities are Launchpad/UbuntuOne and
FedoraProject don't offer OAuth2 protocol. To not lock out those
contributors from being able to contribute to open source Gerrit
based projects OpenID must still be supported.
With Google's shut down of their OpenID service in April 2015, big
user base is locked out from contribution to Gerrit based projects
that only support OpenID auth scheme.
The only way to still support OpenID 2.0 providers and new OAuth2
based protocol is native support for hybrid authentication scheme
in Gerrit.
This change extends OpenID auth scheme by making it aware of optional
OAuth plugin-based authentication.
When no oauth-provider plugins are deployed, OpenID auth scheme works
as usual. When OAuth provider plugins are deployed, OAuth2 providers
are offered on the OpenID login form, in addition to hard coded Yahoo!
and Launchpad OpenID providers: [1].
[1] http://imgur.com/IcCrChN
Change-Id: I6d70212f4fea5443a6322c7da683e1e943d058eb
8b5aa48f1d