SocialPub/PrivaPub/Infrastructure/Http/EdgeHints.cs

using PrivaPub.Infrastructure.Geo;

using System.Collections.Concurrent;

namespace PrivaPub.Infrastructure.Http
{
	public interface IEdgeHints
	{
		void Remember(string host, CdnProvider cdn);
		CdnProvider Of(string host);
	}

	// The CDN each remote host's responses came through, as their headers tell it (CdnCatalog fingerprints), for the
	// weekly description of the server. Like ConnectedAddresses, memory only: what a host answers is never stored per
	// request.
	public class EdgeHints : IEdgeHints
	{
		const int MaxHosts = 50_000;

		readonly ConcurrentDictionary<string, CdnProvider> _hints = new(StringComparer.OrdinalIgnoreCase);

		public void Remember(string host, CdnProvider cdn)
		{
			if (string.IsNullOrEmpty(host))
				return;
			if (cdn == default)
			{
				_hints.TryRemove(host, out _);
				return;
			}
			if (_hints.Count >= MaxHosts)
				_hints.Clear();
			_hints[host] = cdn;
		}

		public CdnProvider Of(string host) => host != default && _hints.TryGetValue(host, out var cdn) ? cdn : default;
	}

	// On the federation client: reads each response's CDN fingerprint for its host. A response without one forgets the
	// host's hint, so a server that leaves its CDN is seen as itself again.
	public class EdgeHintsHandler : DelegatingHandler
	{
		readonly IEdgeHints _hints;

		public EdgeHintsHandler(IEdgeHints hints) => _hints = hints;

		protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
		{
			var response = await base.SendAsync(request, cancellationToken);
			if (request.RequestUri is { } uri && (int)response.StatusCode is >= 200 and < 500)
				_hints.Remember(uri.Host, CdnCatalog.FromResponse(response));
			return response;
		}
	}
}