Limit command cancellation attempts.

In a load-balanced environment, we may not connect to the same server when attempting to cancel an existing command, which means that cancelling will fail. Similarly, in a proxied environment, we may not have the right thread ID to cancel. Since cancellation (which opens another connection to the server) executes synchronously when the CancellationToken is checked, enforce a limit on the number of times it happens so that the canceled reader is not perpetually trying to cancel and failing.

Author: bgrainger

src/MySqlConnector/Core/ServerSession.cs

@@ -77,6 +77,8 @@ public bool TryStartCancel(MySqlCommand command)
 				VerifyState(State.Querying, State.CancelingQuery, State.Failed);
 				if (m_state != State.Querying)
 					return false;
+				if (command.CancelAttemptCount++ >= 10)
+					return false;
 				m_state = State.CancelingQuery;
 			}
 
@@ -119,6 +121,7 @@ public void StartQuerying(MySqlCommand command)
 
 				VerifyState(State.Connected);
 				m_state = State.Querying;
+				command.CancelAttemptCount = 0;
 				m_activeCommandId = command.CommandId;
 			}
 		}

src/MySqlConnector/MySql.Data.MySqlClient/MySqlCommand.cs

@@ -230,6 +230,8 @@ internal IDisposable RegisterCancel(CancellationToken token)
 
 		internal int CommandId { get; }
 
+		internal int CancelAttemptCount { get; set; }
+
 		/// <summary>
 		/// Causes the effective command timeout to be reset back to the value specified by <see cref="CommandTimeout"/>.
 		/// </summary>