Move "KILL QUERY" fix into MySqlSesssion.

Track waiting for a pending cancellation as part of MySqlSession.m_state (instead of a separate property on MySqlCommand) and execute "DO SLEEP(0);" directly on the session.

Author: bgrainger

src/MySqlConnector/MySqlClient/MySqlCommand.cs

@@ -66,7 +66,6 @@ public override void Prepare()
 
 		public override string CommandText { get; set; }
 		public override int CommandTimeout { get; set; }
-		public bool	IsCanceled { get; internal set; }
 
 		public override CommandType CommandType
 		{

src/MySqlConnector/MySqlClient/MySqlDataReader.cs

@@ -288,15 +288,6 @@ private void DoClose()
 				if (!connection.BufferResultSets)
 					connection.Session.FinishQuerying();
 
-				if (Command.IsCanceled)
-				{
-					// KILL QUERY will kill a subsequent query if the command it was intended to cancel has already completed.
-					// In order to handle this case, we issue a dummy query to catch the QueryInterrupted exception.
-					// See https://bugs.mysql.com/bug.php?id=45679
-					var killClearCommand = new MySqlCommand("DO SLEEP(0);", connection);
-					killClearCommand.ExecuteNonQuery();
-				}
-
 				Command.ReaderClosed();
 				if ((m_behavior & CommandBehavior.CloseConnection) != 0)
 				{

src/MySqlConnector/Serialization/MySqlSession.cs

@@ -84,8 +84,6 @@ public void DoCancel(MySqlCommand commandToCancel, MySqlCommand killCommand)
 				// blocking the other thread for an extended duration.
 				killCommand.CommandTimeout = 3;
 				killCommand.ExecuteNonQuery();
-
-				commandToCancel.IsCanceled = true;
 			}
 		}
 
@@ -125,9 +123,30 @@ public void SetActiveReader(MySqlDataReader dataReader)
 
 		public void FinishQuerying()
 		{
+			bool clearConnection = false;
 			lock (m_lock)
 			{
-				VerifyState(State.Querying, State.CancelingQuery);
+				if (m_state == State.CancelingQuery)
+				{
+					m_state = State.ClearingPendingCancellation;
+					clearConnection = true;
+				}
+			}
+
+			if (clearConnection)
+			{
+				// KILL QUERY will kill a subsequent query if the command it was intended to cancel has already completed.
+				// In order to handle this case, we issue a dummy query that will consume the pending cancellation.
+				// See https://bugs.mysql.com/bug.php?id=45679
+				var payload = new PayloadData(new ArraySegment<byte>(PayloadUtilities.CreateEofStringPayload(CommandKind.Query, "DO SLEEP(0);")));
+				SendAsync(payload, IOBehavior.Synchronous, CancellationToken.None).GetAwaiter().GetResult();
+				payload = ReceiveReplyAsync(IOBehavior.Synchronous, CancellationToken.None).GetAwaiter().GetResult();
+				OkPayload.Create(payload);
+			}
+
+			lock (m_lock)
+			{
+				VerifyState(State.Querying, State.ClearingPendingCancellation);
 				m_state = State.Connected;
 				m_activeReader = null;
 				m_activeCommand = null;
@@ -313,7 +332,7 @@ private void VerifyConnected()
 			{
 				if (m_state == State.Closed)
 					throw new ObjectDisposedException(nameof(MySqlSession));
-				if (m_state != State.Connected && m_state != State.Querying && m_state != State.CancelingQuery && m_state != State.Closing)
+				if (m_state != State.Connected && m_state != State.Querying && m_state != State.CancelingQuery && m_state != State.ClearingPendingCancellation && m_state != State.Closing)
 					throw new InvalidOperationException("MySqlSession is not connected.");
 			}
 		}
@@ -669,6 +688,9 @@ private enum State
 			// The session is connected to a server and the active query is being cancelled.
 			CancelingQuery,
 
+			// A cancellation is pending on the server and needs to be cleared.
+			ClearingPendingCancellation,
+
 			// The session is closing.
 			Closing,
 

tests/SideBySide/CancelTests.cs

@@ -251,6 +251,28 @@ value varchar(45)
 
 				await cmd.ExecuteNonQueryAsync().ConfigureAwait(false);
 			}
+
+			using (var cmd = m_database.Connection.CreateCommand())
+			{
+				cmd.CommandText = "select value from cancel_completed_command where id = 1;";
+				var value = (string) await cmd.ExecuteScalarAsync();
+				Assert.Equal("value", value);
+			}
+		}
+
+		[Fact]
+		public void ImplicitCancelWithDapper()
+		{
+			m_database.Connection.Execute(@"drop table if exists cancel_completed_command;
+create table cancel_completed_command(id integer not null primary key, value text null);");
+
+			// a query that returns 0 fields will cause Dapper to cancel the command
+			m_database.Connection.Query<int>("insert into cancel_completed_command(id, value) values (1, null);");
+
+			m_database.Connection.Execute("update cancel_completed_command set value = 'value' where id = 1;");
+
+			var value = m_database.Connection.Query<string>(@"select value from cancel_completed_command where id = 1").FirstOrDefault();
+			Assert.Equal("value", value);
 		}
 
 		[UnbufferedResultSetsFact]