Avoid race condition. Fixes #1263

Author: bgrainger

src/MySqlConnector/Core/ConnectionPool.cs

@@ -246,22 +246,35 @@ public void Dispose()
 	/// </summary>
 	private async Task RecoverLeakedSessionsAsync(IOBehavior ioBehavior)
 	{
-		var recoveredSessions = new List<ServerSession>();
+		var recoveredSessions = new List<(ServerSession Session, MySqlConnection Connection)>();
 		lock (m_leasedSessions)
 		{
 			m_lastRecoveryTime = unchecked((uint) Environment.TickCount);
 			foreach (var session in m_leasedSessions.Values)
 			{
 				if (!session.OwningConnection!.TryGetTarget(out var _))
-					recoveredSessions.Add(session);
+				{
+					// create a dummy MySqlConnection so that any thread running RecoverLeakedSessionsAsync doesn't process this one
+					var connection = new MySqlConnection();
+					session.OwningConnection = new(connection);
+					recoveredSessions.Add((session, connection));
+				}
 			}
 		}
 		if (recoveredSessions.Count == 0)
 			Log.Trace("Pool{0} recovered no sessions", m_logArguments);
 		else
 			Log.Warn("Pool{0}: RecoveredSessionCount={1}", m_logArguments[0], recoveredSessions.Count);
-		foreach (var session in recoveredSessions)
+
+		foreach (var (session, connection) in recoveredSessions)
+		{
+			// bypass MySqlConnection.Dispose(Async), because it's a dummy MySqlConnection that's not set up
+			// properly, and simply return the session to the pool directly
 			await session.ReturnToPoolAsync(ioBehavior, null).ConfigureAwait(false);
+
+			// be explicit about keeping the associated MySqlConnection alive until the session has been returned
+			GC.KeepAlive(connection);
+		}
 	}
 
 	private async Task CleanPoolAsync(IOBehavior ioBehavior, Func<ServerSession, bool> shouldCleanFn, bool respectMinPoolSize, CancellationToken cancellationToken)