Add Session DbCommandCanceled event (#128)

Author: SergeiPavlov

Orm/Xtensive.Orm.Tests/Storage/SessionEventsTest.cs

@@ -55,8 +55,8 @@ public class EventInfo : SessionBound, IDisposable
     public EntityFieldValueEventArgs EntityFieldValueSettingArgs;
     public EntityFieldValueSetEventArgs EntityFieldValueSetArgs;
 
-    public QueryEventArgs QueryExecuting;
-    public QueryEventArgs QueryExecuted;
+    public QueryEventArgs? QueryExecuting;
+    public QueryEventArgs? QueryExecuted;
 
     public DbCommandEventArgs? DbCommandExecuting;
     public DbCommandEventArgs? DbCommandExecuted;

Orm/Xtensive.Orm/Orm/Providers/StorageDriver.Operations.cs

@@ -469,6 +469,7 @@ private async Task<TResult> ExecuteCommandAsync<TResult>(Session session,
         result = await action(command, commandBehavior, cancellationToken).ConfigureAwait(false);
       }
       catch (OperationCanceledException) {
+        session?.Events.NotifyDbCommandCanceled(command);
         throw;
       }
       catch (Exception exception) {

Orm/Xtensive.Orm/Orm/QueryEventArgs.cs

@@ -12,12 +12,12 @@ namespace Xtensive.Orm
   /// Event args for <see cref="SessionEventAccessor.QueryExecuting"/>
   /// and <see cref="SessionEventAccessor.QueryExecuted"/>.
   /// </summary>
-  public class QueryEventArgs : EventArgs
+  public readonly struct QueryEventArgs
   {
     /// <summary>
     /// Gets executed expression.
     /// </summary>
-    public Expression Expression { get; set; }
+    public Expression Expression { get; }
 
     /// <summary>
     /// Gets exception, thrown during expression execution. <see langword="null" /> if expression executed successfully.

Orm/Xtensive.Orm/Orm/SessionEventAccessor.cs

@@ -42,6 +42,11 @@ public sealed class SessionEventAccessor
     /// </summary>
     public event EventHandler<DbCommandEventArgs> DbCommandExecuted;
 
+    /// <summary>
+    /// Occurs when <see cref="DbCommand"/> is canceled.
+    /// </summary>
+    public event EventHandler<DbCommandEventArgs> DbCommandCanceled;
+
     /// <summary>
     /// Occurs when LINQ query is about to execute.
     /// </summary>
@@ -253,6 +258,9 @@ internal void NotifyDbCommandExecuted(DbCommand command, Exception exception = n
       DbCommandExecuted?.Invoke(this, new DbCommandEventArgs(command, exception));
     }
 
+    internal void NotifyDbCommandCanceled(DbCommand command) =>
+      DbCommandCanceled?.Invoke(this, new DbCommandEventArgs(command));
+
     internal Expression NotifyQueryExecuting(Expression expression)
     {
       var args = new QueryEventArgs(expression);