Version of Sitecore's CustomGetStartUrl processor for sites using CallbackAuthority setting

CustomGetStartUrl.cs

using Sitecore.Abstractions;
using Sitecore.Owin.Authentication.IdentityServer.Extensions;
using Sitecore.Owin.Authentication.Pipelines.CookieAuthentication.SignedIn;
using Sitecore.Pipelines.GetStartUrl;
using Sitecore.Sites;
using System;

namespace Identity.Pipelines.CookieAuthentication.SignedIn
{
    public class CustomGetStartUrl : Sitecore.Owin.Authentication.Pipelines.CookieAuthentication.SignedIn.GetStartUrl
    {
        BaseSettings Settings { get; }

        public CustomGetStartUrl(BaseCorePipelineManager corePipelineManager, BaseSiteContextFactory siteContextFactory, BaseLog log, BaseSettings settings)
            : base(corePipelineManager, siteContextFactory, log)
        {
            Settings = settings;
        }

        public override void Process(SignedInArgs args)
        {
            SiteContext site = args.Site;
            if (site.IsBackend)
                site = SiteContextFactory.GetSiteContext("shell");
            
            string callbackAuthority = Settings.IdentityServer().CallbackAuthority;
            Uri baseUrl = args.Context.Request.Uri;
            if(!string.IsNullOrEmpty(callbackAuthority))
                Uri.TryCreate(callbackAuthority, UriKind.Absolute, out baseUrl);

            GetStartUrlArgs newArgs = new GetStartUrlArgs(args.User.InnerUser, site, baseUrl, args.Context.Options.ReturnUrlParameter)
            {
                AppendClientLanguage = true
            };
            GetStartUrlPipeline.Run(CorePipelineManager, newArgs, Log);
            args.StartUrl = newArgs.Result.ToString();
        }
    }
}