feat: add atomic ephemeral state (#6)

Closes #6

src/FinalFactory.Rendezvous.Server/Program.cs

@@ -2,6 +2,7 @@ using System.Net;
 using FinalFactory.Rendezvous.Contracts;
 using FinalFactory.Rendezvous.Server.Http;
 using FinalFactory.Rendezvous.Server.Provisioning;
+using FinalFactory.Rendezvous.Server.State;
 using FinalFactory.Rendezvous.Server.Transport;
 using Microsoft.OpenApi;
 
@@ -35,6 +36,13 @@ builder.Services.AddOpenApi("v1", static options =>
 builder.Services.ConfigureHttpJsonOptions(static options =>
     ContractJson.Configure(options.SerializerOptions));
 
+SystemRendezvousClock rendezvousClock = new();
+InMemoryEphemeralRendezvousStore stateStore = new(
+    new EphemeralStoreOptions(),
+    rendezvousClock,
+    rendezvousClock);
+builder.Services.AddSingleton<IEphemeralRendezvousStore>(stateStore);
+
 if (isOpenApiGeneration)
 {
     builder.Services.AddSingleton(new ProvisioningReadiness(false));
@@ -74,6 +82,7 @@ if (!isOpenApiGeneration)
 }
 
 WebApplication app = builder.Build();
+app.Lifetime.ApplicationStopping.Register(() => stateStore.BeginDrain());
 
 app.MapOpenApi();
 app.MapRendezvousContractEndpoints();
@@ -85,8 +94,14 @@ app.MapGet(
     .WithTags("Health");
 app.MapGet(
     "/health/ready",
-    static (UdpMediatorService mediator, ProvisioningReadiness provisioning) =>
-        mediator.LocalEndpoint is null || !provisioning.IsReady
+    static (
+        UdpMediatorService mediator,
+        ProvisioningReadiness provisioning,
+        IEphemeralRendezvousStore state) =>
+        mediator.LocalEndpoint is null
+        || !provisioning.IsReady
+        || !state.IsAvailable
+        || state.IsDraining
         ? Results.StatusCode(StatusCodes.Status503ServiceUnavailable)
         : Results.Ok(new HealthResponse { Status = "ready" }))
     .Produces<HealthResponse>()