GoogleCredential OAuth access token invalid_grant error - google-oauth

Question

I'm trying to acquire a 2 legged oauth2 access token for google's storage api through a service account.
The relevant documentation is contained here:
https://developers.google.com/identity/protocols/OAuth2ServiceAccount
I first tried attempted to acquire the token through their HTTP/REST api. I eventually gave up being unable to get past a 400 response with the following details:
{ "error" : "invalid_grant" }
I then turned to the recommended method: using the java Google api client library (I'm using scala).
val credentialBuilder = new GoogleCredential.Builder()
.setTransport(GoogleNetHttpTransport.newTrustedTransport())
.setJsonFactory(JacksonFactory.getDefaultInstance())
.setServiceAccountId(Configuration.GoogleAPI.ServiceAccount.email)
.setServiceAccountPrivateKeyFromP12File(new File(Configuration.GoogleAPI.ServiceAccount.pkcs12))
.setServiceAccountScopes(Collections.singleton(StorageScopes.DEVSTORAGE_FULL_CONTROL))
def updateToken = {
val credential = credentialBuilder.build()
credential.refreshToken()
// ... do stuff here with token
}
Running the above, I get a TokenResponseException, with a payload identical to the error I was getting directly accessing the REST api. 400 Bad Request, invalid_grant error.
Doing some research (on both stackoverflow and elsewhere), I find that the most common reasons for this error message are:
1) The local clock is not synced with Google's servers. I suspected this even back when I was using the REST api because I've encountered this issue for other services before. I'm fairly sure its not a clock issue however, because I have repeatedly synced my clocks with an external ntp server:
sudo ntpdate -s ntp.ubuntu.com
2) The other common cause for this error is setting the service account client ID incorrectly. Most people set it to their service account client id (ending with "apps.googleusercontent.com") instead of their service account client email. Its pretty clear that this isn't the problem, since I'm correctly pointing to the email (the docs instructed this, so I followed the directions), ending with "#developer.gserviceaccount.com".
I'm stuck. I've checked the user documentation, the javadocs, various forums. The error message isn't very helpful (probably designed that way, since it's a security endpoint).
What other possible causes for this error exist? What can I do to get my service account access token?

Answer 1

If you are using OAuth2.0 to access a google analytics account then there could be one more reason of getting invalid_grant. As per developer docs:
If the application attempts to use an invalidated refresh token, an
invalid_grant error response is returned. The limit for each unique
pair of OAuth 2.0 client and Google Analytics account is 25 refresh
tokens (note that this limit is subject to change). If the application
continues to request refresh tokens for the same Client/Account pair,
once the 26th token is issued, the 1st refresh token that was
previously issued will become invalid. The 27th requested refresh
token would invalidate the 2nd previously issued token and so on.