Refactoring: Do not query database when asking for Coinaverage rates, periodically get exchange list

BTCPayServer/Services/SettingsRepository.cs

@@ -8,6 +8,7 @@ using Microsoft.EntityFrameworkCore;
 using BTCPayServer.Models;
 using Microsoft.EntityFrameworkCore.Infrastructure.Internal;
 using Newtonsoft.Json;
+using System.Threading;
 
 namespace BTCPayServer.Services
 {
@@ -51,6 +52,22 @@ namespace BTCPayServer.Services
                     await ctx.SaveChangesAsync();
                 }
             }
+
+            IReadOnlyCollection<TaskCompletionSource<bool>> value;
+            lock (_Subscriptions)
+            {
+                if(_Subscriptions.TryGetValue(typeof(T), out value))
+                {
+                    _Subscriptions.Remove(typeof(T));
+                }
+            }
+            if(value != null)
+            {
+                foreach(var v in value)
+                {
+                    v.TrySetResult(true);
+                }
+            }
         }
 
         private T Deserialize<T>(string value)
@@ -62,5 +79,35 @@ namespace BTCPayServer.Services
         {
             return JsonConvert.SerializeObject(obj);
         }
+
+        MultiValueDictionary<Type, TaskCompletionSource<bool>> _Subscriptions = new MultiValueDictionary<Type, TaskCompletionSource<bool>>();
+        public async Task WaitSettingsChanged<T>(CancellationToken cancellation)
+        {
+            var tcs = new TaskCompletionSource<bool>();
+            using (cancellation.Register(() =>
+             {
+                 try
+                 {
+                     tcs.TrySetCanceled();
+                 }
+                 catch { }
+             }))
+            {
+                lock (_Subscriptions)
+                {
+                    _Subscriptions.Add(typeof(T), tcs);
+                }
+#pragma warning disable CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
+                tcs.Task.ContinueWith(_ =>
+                 {
+                     lock (_Subscriptions)
+                     {
+                         _Subscriptions.Remove(typeof(T), tcs);
+                     }
+                 }, TaskScheduler.Default);
+#pragma warning restore CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
+                await tcs.Task;
+            }
+        }
     }
 }