Re-use GCE credentials if possible about gcsfuse HOT 5 CLOSED

I believe this is the documentation.

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Even better, this is built in to the public oauth2 package. Unfortunately this doesn't expose any capability to find out what scopes are provisioned for the instance, to catch errors early.

The gsutil source code implies that this is possible, though.

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Filed golang/oauth2#102 about querying a list of scopes.

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Looks like the list of scopes can be obtained from the oauth2 API.

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Okay, so more of this is already built than I thought.

Plan:

• Use google.DefaultTokenSource, which automatically gets you the standard flow for finding Google credentials, including GCE VM ones and those used by the gcloud command-line tool.
• Don't worry about detecting the wrong scope for now. The work on better error output (#60) will make the problem more obvious.
• If we do eventually want to do this, we can query the tokeninfo service to get a space-separated list of scopes.
• Add a GOOGLE_APPLICATION_CREDENTIALS environment variable in gcsfuse_mount_helper/sample.sh.
• Test both gcsfuse and the mount tool, on OS X and Linux.
• Kill the --key_file flag and mount option.
• Update documentation.
• Send an announcement to gcsfuse-users.

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