title: Facebook OAuth: custom callback_uri parameters
body: <p>I'd like to have a dynamic redirect URL for my Facebook OAuth2 integration.  For example, if my redirect URL is this in my Facebook app:</p>
  
  <pre>http://www.mysite.com/oauth_callback?foo=bar</pre>
  
  <p>I'd like the redirect URL for a specific request be something like this, so that on the server, I have some context about how to process the auth code:</p>
  
  <pre>http://www.mysite.com/oauth_callback?foo=bar&user=6234</pre>
  
  <p>My redirect gets invoked after the authorization dialog is submitted, and I get back an auth code, but when I try to get my access token, I'm getting an OAuthException error back from Facebook.  My request looks like this (line breaks added for clarity):</p>
  
  <pre>
  https://graph.facebook.com/oauth/access_token
      ?client_id=MY_CLIENT_ID
      &redirect_uri=http%3A%2F%2Fwww.mysite.com%2Foauth_callback%3Ffoo%3Dbar%26user%3D6234
      &client_secret=MY_SECRET
      &code=RECEIVED_CODE
  </pre>
  
  <p>All of my parameters are URL-encoded, and the code looks valid, so my only guess is that the problem parameter is my redirect_uri.  I've tried setting <code>redirect_uri</code> to all of the following, to no avail:</p>
  
  <ol>
  <li>The actual URL of the request to my site</li>
  <li>The URL of the request to my site, minus the <code>code</code> parameter</li>
  <li>The URL specified in my Facebook application's configuration</li>
  </ol>
  
  <p>Are custom redirect URI parameters supported?  If so, am I specifying them correctly?  If not, will I be forced to set a cookie, or is there some better pattern for supplying context to my web site?</p>