redirect_uri_mismatch with https-only app on AppHarbor - asp.net-mvc

Question

I've deployed MVC 5 site to AppHarbor, got the https-only app working thanks to this gist linked by AppHarbor support.
When I connected to Google OAuth 2 provider locally, everything worked fine - I can log in without problems, but when I try it on AppHarbor, I get error on login "redirect_uri_mismatch". I've got proper key and secret set for web app, the problem is with path, for some reason my page responds with redirect_uri starting with "http://..." instead of "https://..." which I've set in the google project console - (other than that it's the same uri).
I've tried this workaround for URL-based problems, but it doesn't seem to change anything.
As I don't think switching to http for /signin-google would be a good idea - how to fix it?

Answer 1

It's a few months later but I hope this will help someone. I had exactly the same problem with AppHarbor load balancers and loosing the "https" in the redirect_uri string. The simplest solution that I found is to register a few lines of code like a middleware in Startup.Auth.cs before any registration of external login providers:
app.Use(async (context, next) =>
{
if (string.Equals(context.Request.Headers["X-Forwarded-Proto"], "https", StringComparison.InvariantCultureIgnoreCase))
{
context.Request.Scheme = "https";
}
await next.Invoke();
});
This simple middleware checks for AppHarbor header "X-Forwarded-Proto" and if exist just add a correct Scheme property. If you look at the Katana's code correct Scheme property solve the problem:
...
string requestPrefix = Request.Scheme + "://" + Request.Host;
string redirectUri = requestPrefix + Request.PathBase + Options.CallbackPath;
...