feat: add presence-gated session leases (#7)

Closes #7

src/FinalFactory.Rendezvous.Server/Transport/UdpMediatorService.cs

@@ -1,5 +1,8 @@
 using System.Net;
 using System.Net.Sockets;
+using FinalFactory.Rendezvous.Contracts;
+using FinalFactory.Rendezvous.Server.Sessions;
+using FinalFactory.Rendezvous.Server.State;
 using Microsoft.Extensions.Options;
 
 namespace FinalFactory.Rendezvous.Server.Transport;
@@ -7,10 +10,12 @@ namespace FinalFactory.Rendezvous.Server.Transport;
 /// <summary>
 /// Owns the cancellable UDP socket used by the future NAT mediator.
 /// </summary>
-public sealed partial class UdpMediatorService : BackgroundService
+internal sealed partial class UdpMediatorService : BackgroundService
 {
     private readonly ILogger<UdpMediatorService> _logger;
     private readonly UdpMediatorOptions _options;
+    private readonly IEphemeralRendezvousStore _store;
+    private readonly ISessionCapabilityService _capabilities;
     private UdpClient? _udpClient;
 
     /// <summary>
@@ -18,10 +23,14 @@ public sealed partial class UdpMediatorService : BackgroundService
     /// </summary>
     public UdpMediatorService(
         IOptions<UdpMediatorOptions> options,
-        ILogger<UdpMediatorService> logger)
+        ILogger<UdpMediatorService> logger,
+        IEphemeralRendezvousStore store,
+        ISessionCapabilityService capabilities)
     {
         _options = options.Value;
         _logger = logger;
+        _store = store;
+        _capabilities = capabilities;
     }
 
     /// <summary>
@@ -81,7 +90,10 @@ public sealed partial class UdpMediatorService : BackgroundService
         {
             while (!stoppingToken.IsCancellationRequested)
             {
-                _ = await udpClient.ReceiveAsync(stoppingToken).ConfigureAwait(false);
+                UdpReceiveResult received = await udpClient
+                    .ReceiveAsync(stoppingToken)
+                    .ConfigureAwait(false);
+                ProcessDatagram(received.Buffer, received.RemoteEndPoint, stoppingToken);
                 // Bootstrap deliberately emits no UDP response. Protocol handling lands in #11.
             }
         }
@@ -99,6 +111,53 @@ public sealed partial class UdpMediatorService : BackgroundService
         }
     }
 
+    internal UdpPresenceProcessingResult ProcessDatagram(
+        ReadOnlySpan<byte> encoded,
+        IPEndPoint observedSource,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(observedSource);
+        if (!RendezvousUdpCodec.TryDecode(encoded, out PresenceDatagram? datagram, out _)
+            || datagram is null
+            || !_capabilities.TryFingerprint(datagram.Capability, out SecretFingerprint fingerprint))
+        {
+            return UdpPresenceProcessingResult.Dropped;
+        }
+
+        if (datagram.MessageType != UdpPresenceMessageType.HostPresence)
+        {
+            return UdpPresenceProcessingResult.ClientPresenceDeferred;
+        }
+
+        AddressFamilyKind publicFamily = observedSource.AddressFamily switch
+        {
+            AddressFamily.InterNetwork => AddressFamilyKind.Ipv4,
+            AddressFamily.InterNetworkV6 => AddressFamilyKind.Ipv6,
+            _ => 0,
+        };
+        if (publicFamily == 0)
+        {
+            return UdpPresenceProcessingResult.Dropped;
+        }
+
+        ObservedEndpoint publicEndpoint = new(
+            publicFamily,
+            observedSource.Address.ToString(),
+            observedSource.Port);
+        ObservedEndpoint localEndpoint = new(
+            datagram.AddressFamily,
+            datagram.LocalAddress,
+            datagram.LocalPort);
+        StoreResult<StoredListing> bound = _store.BindHostPresence(new(
+            datagram.MediationHandle,
+            fingerprint,
+            publicEndpoint,
+            localEndpoint), cancellationToken);
+        return bound.Succeeded
+            ? UdpPresenceProcessingResult.HostPresenceAccepted
+            : UdpPresenceProcessingResult.HostPresenceRejected;
+    }
+
     [LoggerMessage(
         EventId = 1,
         Level = LogLevel.Information,
@@ -114,3 +173,11 @@ public sealed partial class UdpMediatorService : BackgroundService
         Message = "UDP mediator stopped")]
     private static partial void LogMediatorStopped(ILogger logger);
 }
+
+internal enum UdpPresenceProcessingResult
+{
+    Dropped = 0,
+    HostPresenceAccepted = 1,
+    HostPresenceRejected = 2,
+    ClientPresenceDeferred = 3,
+}