deal with dualmode sockets

[wfurt]

This got broken by #30036.
We now use DualMode sockets and we can do IPv4 over IPv6 socket. That confuses logic to construct data connection.

With this change I tested both passive and active mode and I can get listing.

[wfurt]

fixes #37175

[stephentoub]

This got broken by #30036.

This is just one piece of code that was apparently dependent on the behavior changed in #30036. Is it not likely that #30036 may have broken other things as well, including in code we don't control?

[davidsh]

Is it not likely that #30036 may have broken other things as well, including in code we don't control?

It is possible. The original change #30036 can be considered a breaking change. I think it's "ok" for things like this to change in a major release of .NET Core, i.e. 3.0. But we should definitely document it in release notes and perhaps add more test coverage to other areas we suspect would be impacted.

@karelz

[wfurt]

I don't know how to assess the risk but it is certainly possible. FTP was particularly vulnerable as it deeply cares about address and manipulates two sockets in coordinated way.
The exposure is only when no AddressFamily is specified. When AddressFamily is used we use one or other socket as specified.

[wfurt]

note that original #29414 request seems reasonable. Better support for IPv6 and/or dual stack seems like a good thing.