Remove "DO SLEEP" calls in .NET connector

Summary: Replace "DO SLEEP" calls in .NET connector with SingleStore equivalent "SELECT SLEEP() INTO @dummy".

Test Plan: .circleci: https://app.circleci.com/pipelines/github/memsql/SingleStoreNETConnector/38/workflows/dd7e09af-afb6-495d-befd-c2720cba1311/jobs/42

Reviewers: okramarenko-ua, pmishchenko-ua

Reviewed By: okramarenko-ua, pmishchenko-ua

Subscribers: engineering-list

JIRA Issues: PLAT-5985

Differential Revision: https://grizzly.internal.memcompute.com/D54188

src/SingleStoreConnector/Core/ServerSession.cs

@@ -315,8 +315,8 @@ public void FinishQuerying()
 			// 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
-			Log.Debug("Session{0} sending 'DO SLEEP(0)' command to clear pending cancellation", m_logArguments);
-			var payload = QueryPayload.Create(SupportsQueryAttributes, "DO SLEEP(0);");
+			Log.Debug("Session{0} sending 'SELECT SLEEP(0) INTO @dummy' command to clear pending cancellation", m_logArguments);
+			var payload = QueryPayload.Create(SupportsQueryAttributes, "SELECT SLEEP(0) INTO @dummy;");
 #pragma warning disable CA2012 // Safe because method completes synchronously
 			SendAsync(payload, IOBehavior.Synchronous, CancellationToken.None).GetAwaiter().GetResult();
 			payload = ReceiveReplyAsync(IOBehavior.Synchronous, CancellationToken.None).GetAwaiter().GetResult();

tests/MySqlConnector.Tests/FakeSingleStoreServerConnection.cs

@@ -157,7 +157,7 @@ public async Task RunAsync(TcpClient client, CancellationToken token)
 							m_server.CancelQuery(connectionId);
 							await SendAsync(stream, 1, WriteOk);
 						}
-						else if (query == "DO SLEEP(0);")
+						else if (query == "SELECT SLEEP(0) INTO @dummy;")
 						{
 							var wasSet = CancelQueryEvent.Wait(0, token);
 							await SendAsync(stream, 1, WriteOk);

tests/SideBySide/TransactionScopeTests.cs

@@ -647,11 +647,11 @@ public async Task CommandBehaviorCloseConnection(string connectionString)
 	[MemberData(nameof(ConnectionStrings))]
 	public async Task CancelExecuteNonQueryAsync(string connectionString)
 	{
-		using var connection = new SingleStoreConnection(AppConfig.ConnectionString + ";" + connectionString);
+		using var connection = new SingleStoreConnection(AppConfig.ConnectionString + ";" + connectionString + ";AllowUserVariables=True");
 		using var transactionScope = new TransactionScope(TransactionScopeAsyncFlowOption.Enabled);
 		await connection.OpenAsync();
 
-		using var command = new SingleStoreCommand("DO SLEEP(3);", connection);
+		using var command = new SingleStoreCommand("SELECT SLEEP(3) INTO @dummy", connection);
 		using var tokenSource = new CancellationTokenSource(TimeSpan.FromMilliseconds(100));
 		await command.ExecuteNonQueryAsync(tokenSource.Token);
 	}