make oauth2 token refresh more multi-process-friendly

[walgitrus]

• add optional retrieve_tokens callback to OAuth2 to fetch possibly refreshed tokens from local store
• addresses issue Token Refresh in a Multiple Process Environment #71

[boxcla]

Verified that @walgitrus has signed the CLA. Thanks for the pull request!

[walgitrus]

looks like requirements.txt should fix cryptography at 0.9.2 (or at least below 1.0) in order for pypy to work

[jmoldow]

Thanks for the PR. First look through looks good. We'll fix the cryptography issue (there's a fix in #70, though I might try something else), and then look to merge this.

[jmoldow]

I merged a fix for the PyPy issue. Going to close and re-open this PR to try to get Travis to generate the new test matrix.

[walgitrus]

rebased to pull in the changes from master

[jmoldow]

Since this is similar to store_tokens (and they'll probably be used together), let's put this below store_tokens in the argument list. Same in the docstring.

[walgitrus]

changes made and pushed

[landscape-bot]

Code quality remained the same when pulling 18fc5d5 on walgitrus:enh-oauth2-retrieve-tokens into 63b60a2 on box:master.

[jmoldow]

Let's expand on this slightly.

We should specify that the callback takes no arguments, and that it should either return None (if no credentials are currently stored), or a (access token, refresh token) tuple.

And maybe we should add a brief description of what this feature would be used for. Something like "When using multiple processes or multiple separate OAuth2 objects, provide a retrieve_tokens function so that the same credentials can be reused across all your OAuth2 objects. The first object that needs to authenticate will do so, and store the tokens with the store_tokens callback (or the authentication and storage might happen entirely outside of the SDK). All subsequent objects, rather than re-authenticating, will use the tokens provided by retrieve_tokens. And the same behavior will occur when they all need to do token refresh." Feel free to reword or make that more concise.

[jmoldow]

Hi @walgitrus,

We're going to handle this in PR #81 (see the CooperativelyManagedOAuth2Mixin class). If you want, take a look over there and see if the proposed interface will work for you.