Support GCE_METADATA_HOST

[ijrsvt]

Brings the logic from googleapis/google-auth-library-python#433 to this vendored package.

[dilipped]

Thanks for writing the fix, but I would hold off from merging this PR for a couple of reasons

• Looks like Cloud SDK uses GCE_METADATA_ROOT as well, and so does few other libraries as per the issue you have linked above, I would wait for a consensus to reach before making any changes here.
• This library is now deprecated as mentioned in the README, and hence, only high priority security fixes will be merged in, since there is a work around to use GCE_METADATA_ROOT, we can probably hold this off.

[ijrsvt]

@dilipped Where is it documented that Cloud SDK uses GCE_METADATA_ROOT. The gcloud CLI, Python & Golang libraries use the GCE_METADATA_HOST Env Var.

[dilipped]

Regarding Cloud SDK, I didn't check any documentation, but I've seen it being used in the code (it's not in a public repo so can't share any code). It might be the case that it is used only for some specific case, because looks like google-auth (which is used by Cloud SDK) has started using the new var https://github.com/googleapis/google-auth-library-python/blob/e2b3c98cd8c67b702be1b711c06ee7b9bbedb8ba/google/auth/environment_vars.py#L53.

[ijrsvt]

@dilipped The reason I created this PR was to use the behaviour of the google-auth library. It seems like gsutil is pretty far away from switching to the google-auth library GoogleCloudPlatform/gsutil#565, so backporting this migration to the new env var seems reasonable.

[dilipped]

Fair enough. I think we can merge it, just going to leave a few nit comments.

[ijrsvt]

@dilipped This is ready for review again :)

[dilipped]

Looks good. Were you able to run the tests

[ijrsvt]

@dilipped Yep:

(oauth) ubuntu@ip-172-31-47-243:~/oauth2client$ DJANGO_SETTINGS_MODULE=tests.contrib.django_util.settings pytest -s .
<Output>
=========================================================================================== 519 passed, 72 warnings in 4.20s ============================================================================================

Full output in this gist.

[dilipped]

Thank you!!