As of yesterday, my application is able to authenticate with google (youtube) via oAuth 2.0 the first time no problem but the 2nd time (re-authentication, same app + same user), when I exchange the requestToken for an accessToken I get an error:
error : invalid_grant
I'm using:
grant_type = authorization_code
like they suggest. What used to happen, before 2 days ago, was on re-authentication the web page would say 'you've already authenticated with this application before, would you like to grant it access again?'. What has changed or what could I be doing wrong?
I found a way to fix this (at least for us). When redirecting to the accounts.google.com/o/oauth2/auth url, we had to add both approval_prompt=force and access_type=offline. If one was missing, we got no refresh token.
This url documents all of the parameters, but says nothing about the refresh token: http://code.google.com/apis/accounts/docs/OAuth2WebServer.html
I think I may have found the answer here:
The problem was that that the "code" parameter that you need to send needs to be encoded. It will work when you set the approval_prompt to force because the return code contains URL-safe characters. However, this is not the case for the return code upon a second authorization request.
For me this fixed it, I hope it does for you as well.
来源：https://stackoverflow.com/questions/8433990/when-authenticating-with-oauth-and-youtube-always-get-error-invalid-grant-on