Sporadic reproducible GetListing, Download and other failures: IOException on control connection

This has been reported quite a few times.

What does the failure look like?

Typically, it manifests as an IOException. Depending on the API call being performed, this exception can cause

• a GetListing result to be empty while the exeption is ignored
• other commands to fail with an IOException.

If you look closely at the IOException, you might see: an attempt was made to access a socket in a way forbidden by its access permissions

When does it happen?

It happens on an control connection (only if encrypted) when you have issued many API calls (that execute commands and get the response).

Reported by (ignore the incorrect title):

@omanThYp https://github.com/robinrodricks/FluentFTP/issues/104#issuecomment-632437591 @oleksabor https://github.com/robinrodricks/FluentFTP/issues/104#issuecomment-953899294

#752 could such a case, but sadly there is no log

See also #723 : one of the “strange” ways this problem can show up.

A newer issue of this kind: #1130

[robinrodricks]

Made a small change to only honor SslSessionLength if the FTP connection is using FTPS (IsEncrypted).

If that was not your intention, I can change the behavior, let me know.

[FanDjango]

This is now released as V42.1.0