In 80673b4 (https://go.dev/cl/157820) I added a never-shrinking
package-global cache to remember which auto-detected auth style (HTTP
headers vs POST) was supported by a certain OAuth2 server, keyed by
its URL.

Unfortunately, some multi-tenant SaaS OIDC servers behave poorly and
have one global OpenID configuration document for all of their
customers which says ("we support all auth styles! you pick!") but
then give each customer control of which style they specifically
accept. This is bogus behavior on their part, but the oauth2 package's
global caching per URL isn't helping. (It's also bad to have a
package-global cache that can never be GC'ed)

So, this change moves the cache to hang off the oauth *Configs
instead. Unfortunately, it does so with some backwards compatiblity
compromises (an atomic.Value hack), lest people are using old versions
of Go still or copying a Config by value, both of which this package
previously accidentally supported, even though they weren't tested.

This change also means that anybody that's repeatedly making ephemeral
oauth.Configs without an explicit auth style will be losing &
reinitializing their cache on any auth style failures + fallbacks to
the other style. I think that should be pretty rare. People seem to
make an oauth2.Config once earlier and stash it away somewhere (often
deep in a token fetcher or HTTP client/transport).

Change-Id: I91f107368ab3c3d77bc425eeef65372a589feb7b
Signed-off-by: Brad Fitzpatrick <[email protected]>
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/515675
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Roland Shoemaker <[email protected]>
Reviewed-by: Adrian Dewhurst <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Update golang.org/x dependencies to their latest tagged versions.

Change-Id: I2fb95ca59417e20377bc315094221fa7165128c8
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/525675
Reviewed-by: Heschi Kreinick <[email protected]>
Run-TryBot: Gopher Robot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Gopher Robot <[email protected]>