Exchanging code for access token fails when using Sign in with Google in Dropbox - dropbox

Question

We have an application that uses Dropbox API. When the user goes through the Dropbox OAuth 2 flow and signs-in using their email address and password, all works fine and we get the access_token. However, when the user uses the "Sign in with Google" flow in the Dropbox authorization dialog, we get back code which we then try to exchange for access token but the request fails with {"error_description": "code doesn't exist or has expired", "error": "invalid_grant"}.
Here's the steps we use:
1.
var dbx = new Dropbox.Dropbox({ clientId: clientId });
var authUrl = dbx.getAuthenticationUrl('https://www.dropbox.com/1/oauth2/redirect_receiver');
This gives us url https://www.dropbox.com/oauth2/authorize?response_type=token&client_id=...&redirect_uri=https://www.dropbox.com/1/oauth2/redirect_receiver.
2.
Open authUrl in a popup.
3.
User uses "Sign in with Google"
4.
We get a redirect to the URL below that contains the code:
https://www.dropbox.com/google/authcallback?state=...&code=...&scope=...
Now trying to exchange the code for access token with POST to https://api.dropboxapi.com/oauth2/token gives us:
{"error_description": "code doesn't exist or has expired", "error": "invalid_grant"}

Answer 1

The problem here is that, given the use of the Google Sign In flow, there are actually two OAuth authorization flow instances occurring; the Google Sign In flow is nested inside the Dropbox app authorization flow. Your app doesn't actually need to know about this though.
That https://www.dropbox.com/google/authcallback URL is Dropbox's redirect URL for the Google Sign In flow, and accordingly the code given there is for the Google OAuth flow, not the Dropbox OAuth flow. Attempting to use it for the Dropbox OAuth 2 flow will accordingly fail as you've seen (since it actually came from Google, not Dropbox).
You should have your app wait until your own redirect URL (in your shared code, https://www.dropbox.com/1/oauth2/redirect_receiver) is accessed, and only then take the code from there and exchange it for a Dropbox access token.