Flatten AggregateException with single inner exception

Author: SergeiPavlov

Orm/Xtensive.Orm/Orm/Domain.cs

@@ -58,7 +58,7 @@ public sealed class Domain : IDisposable, IAsyncDisposable, IHasExtensions, ISes
     public event EventHandler Disposing;
 
     /// <summary>
-    /// Gets the <see cref="Domain"/> of the current <see cref="Session"/>. 
+    /// Gets the <see cref="Domain"/> of the current <see cref="Session"/>.
     /// </summary>
     /// <seealso cref="Session.Current"/>
     /// <seealso cref="Demand"/>
@@ -70,7 +70,7 @@ public static Domain Current {
     }
 
     /// <summary>
-    /// Gets the <see cref="Domain"/> of the current <see cref="Session"/>, or throws <see cref="InvalidOperationException"/>, 
+    /// Gets the <see cref="Domain"/> of the current <see cref="Session"/>, or throws <see cref="InvalidOperationException"/>,
     /// if active <see cref="Session"/> is not found.
     /// </summary>
     /// <returns>Current domain.</returns>
@@ -80,7 +80,7 @@ public static Domain Demand()
     {
       return Session.Demand().Domain;
     }
-    
+
     /// <summary>
     /// Gets the domain configuration.
     /// </summary>
@@ -356,8 +356,11 @@ internal async Task<Session> OpenSessionInternalAsync(SessionConfiguration confi
                   session.AttachToScope(sessionScope);
                 }
               }
-              else if (t.Exception != null) {
-                exceptionDispatchInfo = ExceptionDispatchInfo.Capture(t.Exception);
+              else if (t.Exception is Exception ex) {
+                if (ex is System.AggregateException aggregateException && aggregateException.InnerExceptions.Count == 1) {
+                  ex = aggregateException.InnerExceptions[0];
+                }
+                exceptionDispatchInfo = ExceptionDispatchInfo.Capture(ex);
               }
             }, TaskContinuationOptions.ExecuteSynchronously)
             .ConfigureAwait(false);
@@ -380,7 +383,7 @@ internal async Task<Session> OpenSessionInternalAsync(SessionConfiguration confi
 
     /// <inheritdoc/>
     public IExtensionCollection Extensions { get; private set; }
-    
+
     #endregion
 
     /// <summary>