feature: Enable fetching of OAuth credentials from the GCE metadata service about yup-oauth2

[cristicbz]

Internally we have a hacked together "DefaultApplicationCredentials" thing (as per https://developers.google.com/identity/protocols/application-default-credentials). We'd be happy to open source it, but it needs a bit of cleaning up. Could do this over Christmas if there's interest.

[cristicbz]

This is what I wrote for this use case, seems to work:

header! { (MetadataFlavor, "Metadata-Flavor") => [String] }

struct MetadataServerProvider {
    client: HyperClient,
    token: Token,
}

impl GetToken for MetadataServerProvider {
    fn api_key(&mut self) -> Option<String> {
        None
    }

    fn token<'b, I, T>(&mut self, _scopes: I) -> Result<Token, Box<StdError>>
        where T: AsRef<str> + Ord + 'b,
              I: IntoIterator<Item = &'b T>
    {

        if !self.token.access_token.is_empty() && !self.token.expired() {
            return Ok(self.token.clone());
        }

        let client = HyperClient::new();
        debug!("DefaultApplicationCredentials: checking metadata server...");
        let mut error;
        for _ in 0..3 {
            let response = client.get("http://metadata.google.\
                      internal/computeMetadata/v1/instance/service-accounts/default/token")
                .header(MetadataFlavor("Google".to_owned()))
                .send();
            match response {
                Ok(response) => {
                    self.token = serde_json::from_reader(response)?;
                    return Ok(self.token.clone());
                }
                Err(new_error) => error = new_error,
            }
        }
        Err(error.into())
    }
}
``

[Byron]

Unfortunately I cannot comment on this, as I don't know exactly how this would be useful with oauth just because I am not aware of everything the protocol offers.

[Byron]

@cristicbz I sure find it interesting, even though I wouldn't have a use for it myself. What does @dermesser think about it?

[dermesser]

Hey @cristicbz! Thanks for your proposal, I've been meaning to do it but am not a user of GCE myself (so it's down my priority list). The way you implemented it is definitely similar to how I would've done it. I'd be happy to accept it as addition to this project.

(that is, if @Byron is ok with such vendor-specific code in what is originally his work; although I'm definitely not innocent with regards to that either...)

[Byron]

I am totally fine with that, as I believe we should implement what people need. Given that yup-oauth2 is primarily used by the google-apis-rs project, I think it's alright to cater to google more than to others.
If other usecases show up, we can invest time to generalize once again.

[dermesser]

ping, @cristicbz -- are you still interested in implementing this?

[braincow]

I have created a small bare bones impl of this and would requests comments/contributions on it before creating an pull request: https://github.com/braincow/yup-oauth2/tree/gcp_instance_metadata

[dermesser]

@braincow thank you, this looks interesting! I'd encourage you to directly create a PR anyway, it's easier to see and evaluate the diff that way :)

[braincow]

@dermesser #156 here you go. Its a draft for now since I did not have time currently to properly go through the projects guidelines on opening pull requests. I hope you dont mind my haste :)

[ingwinlu]

This is something I really want to see. I went ahead and rebased @braincow s draft onto the current master and fixed the failing tests in #164 .

[braincow]

nice work @ingwinlu! and thanks for updating/rebasing the code since I have been busy with other things to follow up on this. still would love to see this merged into yup-oauth2

[ingwinlu]

@dermesser i think this and the original pr can be closed now.

[braincow]

I concur. The PR draft and subsequent updated PR by @ingwinlu are now merged so this feature is implemented.