Handle shutdowns better, allow retrying multis

ALttPRandomizer/ShutdownHandler.cs

@@ -0,0 +1,40 @@
+namespace ALttPRandomizer {
+    using System.Collections.Generic;
+    using System.Threading.Tasks;
+    using ALttPRandomizer.Azure;
+    using Microsoft.Extensions.Logging;
+
+    public class ShutdownHandler {
+        public ShutdownHandler(AzureStorage azureStorage, ILogger<ShutdownHandler> logger) {
+            this.AzureStorage = azureStorage;
+            this.Logger = logger;
+        }
+
+        private AzureStorage AzureStorage { get; }
+        private ILogger<ShutdownHandler> Logger { get; }
+
+        private HashSet<string> IdsInProgress { get; } = new();
+
+        public void AddId(string id) {
+            this.IdsInProgress.Add(id);
+        }
+
+        public void RemoveId(string id) {
+            this.IdsInProgress.Remove(id);
+        }
+
+        public async Task HandleShutdown() {
+            this.Logger.LogWarning("Shutdown Received.");
+
+            List<Task> tasks = [];
+
+            foreach (var id in this.IdsInProgress) {
+                tasks.Add(AzureStorage.DeleteFile($"{id}/generating"));
+            }
+
+            this.Logger.LogInformation("Ending {count} in-progress generations.", tasks.Count);
+
+            await Task.WhenAll(tasks);
+        }
+    }
+}