feat: implement authenticated NAT mediator (#11)

Closes #11

src/FinalFactory.Rendezvous.Server/State/InMemoryEphemeralRendezvousStore.cs

@@ -4,6 +4,7 @@ namespace FinalFactory.Rendezvous.Server.State;
 
 internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousStore
 {
+    private static readonly TimeSpan UdpMaintenanceInterval = TimeSpan.FromSeconds(1);
     private readonly object _gate = new();
     private readonly EphemeralStoreOptions _options;
     private readonly IMonotonicClock _monotonicClock;
@@ -19,6 +20,8 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
     private readonly Dictionary<string, TimeSpan> _replay = new(StringComparer.Ordinal);
     private readonly Dictionary<string, TimeSpan> _revocations = new(StringComparer.Ordinal);
     private TimeSpan? _drainDeadline;
+    private TimeSpan _nextUdpMaintenance;
+    private long _maintenanceSweepCount;
     private bool _available = true;
 
     public InMemoryEphemeralRendezvousStore(
@@ -38,6 +41,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
     }
 
     public Guid InstanceId { get; }
+    internal long MaintenanceSweepCount => Interlocked.Read(ref _maintenanceSweepCount);
 
     public bool IsAvailable
     {
@@ -270,6 +274,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
 
         if (!_presenceHandles.TryGetValue(command.Handle, out SessionListingId listingId)
             || !_listings.TryGetValue(listingId, out ListingEntry? entry)
+            || entry.LeaseDeadline <= now
             || entry.Definition.HostPresenceFingerprint != command.CapabilityFingerprint)
         {
             return new(StoreResultCode.NotFound);
@@ -285,7 +290,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
             command.LocalEndpoint,
             now + _options.PresenceLifetime);
         return new(StoreResultCode.Success, Snapshot(entry));
-    }, cancellationToken);
+    }, cancellationToken, eagerCleanup: false);
 
     public StoreResult<IReadOnlyList<StoredListing>> BrowseVisibleListings(
         VisibleListingQuery query,
@@ -446,13 +451,18 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
             return new(StoreResultCode.NotFound);
         }
 
+        if (attempt.IntroductionConsumed)
+        {
+            return new(StoreResultCode.Conflict);
+        }
+
         RemoveAttempt(command.AttemptId);
         return new(StoreResultCode.Success, true);
     }, cancellationToken);
 
     public StoreResult<StoredJoinAttempt> BindAttemptEndpoint(
         BindAttemptEndpointCommand command,
-        CancellationToken cancellationToken = default) => Atomic<StoredJoinAttempt>(_ =>
+        CancellationToken cancellationToken = default) => Atomic<StoredJoinAttempt>(now =>
     {
         ArgumentNullException.ThrowIfNull(command);
         if (command.Handle.Value == Guid.Empty
@@ -469,7 +479,13 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
         }
 
         if (!_attemptHandles.TryGetValue(command.Handle, out JoinAttemptId attemptId)
-            || !_attempts.TryGetValue(attemptId, out AttemptEntry? attempt))
+            || !_attempts.TryGetValue(attemptId, out AttemptEntry? attempt)
+            || attempt.Deadline <= now)
+        {
+            return new(StoreResultCode.NotFound);
+        }
+
+        if (!HasFreshHostPresence(attempt, now))
         {
             return new(StoreResultCode.NotFound);
         }
@@ -503,7 +519,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
         }
 
         return new(StoreResultCode.Success, Snapshot(attempt));
-    }, cancellationToken);
+    }, cancellationToken, eagerCleanup: false);
 
     public StoreResult<IntroductionEndpoints> ConsumeIntroduction(
         MediationHandle handle,
@@ -515,7 +531,13 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
         }
 
         if (!_attemptHandles.TryGetValue(handle, out JoinAttemptId attemptId)
-            || !_attempts.TryGetValue(attemptId, out AttemptEntry? attempt))
+            || !_attempts.TryGetValue(attemptId, out AttemptEntry? attempt)
+            || attempt.Deadline <= now)
+        {
+            return new(StoreResultCode.NotFound);
+        }
+
+        if (!HasFreshHostPresence(attempt, now))
         {
             return new(StoreResultCode.NotFound);
         }
@@ -540,7 +562,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
             Snapshot(attempt),
             attempt.HostEndpoint,
             attempt.ClientEndpoint));
-    }, cancellationToken);
+    }, cancellationToken, eagerCleanup: false);
 
     public StoreResult<bool> ConsumeConnectionTicket(
         ConsumeConnectionTicketCommand command,
@@ -687,18 +709,38 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
         }
     }
 
-    private StoreResult<T> Atomic<T>(Func<TimeSpan, StoreResult<T>> operation, CancellationToken cancellationToken)
+    private StoreResult<T> Atomic<T>(
+        Func<TimeSpan, StoreResult<T>> operation,
+        CancellationToken cancellationToken,
+        bool eagerCleanup = true)
     {
         cancellationToken.ThrowIfCancellationRequested();
         lock (_gate)
         {
             cancellationToken.ThrowIfCancellationRequested();
             TimeSpan now = _monotonicClock.Elapsed;
-            Cleanup(now);
+            // Authenticated UDP duplicates need O(1) store work. Their operations
+            // check exact resource deadlines and amortize physical expiry removal.
+            bool drainExpired = _drainDeadline is TimeSpan drainDeadline
+                && now >= drainDeadline;
+            if (drainExpired || eagerCleanup || now >= _nextUdpMaintenance)
+            {
+                Cleanup(now);
+                _nextUdpMaintenance = now + UdpMaintenanceInterval;
+            }
+
             return operation(now);
         }
     }
 
+    private bool HasFreshHostPresence(AttemptEntry attempt, TimeSpan now) =>
+        _listings.TryGetValue(attempt.Command.ListingId, out ListingEntry? listing)
+        && listing.LeaseDeadline > now
+        && _presence.TryGetValue(
+            listing.Definition.HostPresenceHandle,
+            out PresenceEntry? presence)
+        && presence.Deadline > now;
+
     private StoreResult<T>? CheckNewWorkAdmission<T>(string subject)
     {
         if (!_available)
@@ -718,6 +760,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
 
     private void Cleanup(TimeSpan now)
     {
+        _maintenanceSweepCount++;
         if (_drainDeadline is TimeSpan drainDeadline && now >= drainDeadline)
         {
             ClearActiveState();