TcpClient.Dispose used concurrently with ConnectAsync may not Dispose of the Socket #17611
Description
TcpClient is not documented to have thread-safe members, and in general Dispose implementations should not be used concurrently with other members on the containing types. However, as TcpClient is a thin wrapper for a Socket, code may end up calling Dispose on the TcpClient in order to cancel in-progress async operations, such as ConnectAsync. However, on Unix, ConnectAsync is implemented by creating a new Socket for each address it attempts to connect to, and that Socket isn't stored into the TcpClient until after a connection has been successfully established. As a result, Dispose can't be used to cancel the connect operation, as the Socket won't have been stored yet, and in such cases the Socket won't get disposed of.