document token ownership risks for Config.Client and ReuseTokenSource

[mattbnz]

When used with tokens issued by a server supporting refresh token rotation it is unsafe to continuing using the token provided to ReuseTokenSource (including via the Client method of Config) outside of the returned TokenSource and/or Client as it leads to a race condition when the first renewal happens:

• If ReuseTokenSource renews its token first, the original token's RefreshToken is now invalid (revoked) and any use/renewal attempt will fail.
• If the original token renews its token first, the ReuseTokenSource holds the invliad RefreshToken and will fail on the next usage attempt.

#84 has extensive discussion of related risks and complications when trying to cache or store the RefreshToken, but the generic risk of race conditions exists regardless of whether any caching or storage is being attempted and API users must be warned of this possibility.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gopherbot]

This PR (HEAD: a03b4ae) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/oauth2/+/531535.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to register for Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/531535.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Gopher Robot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
A maintainer will review your change and provide feedback. See
https://go.dev/doc/contribute#review for more info and tips to get your
patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/531535.
After addressing review feedback, remember to publish your drafts!