Add a new "tls-mode=preferred" DSN parameter

[dweitzman]

Add a new "tls-mode=preferred" DSN parameter

Description

Separating "preferred" into its own parameter instead of making it a special value in the "tls=" parameter makes it possible to use custom TLS config with this mode. This is useful when clients don't need to authenticate servers using TLS but a server may or may not need to authenticate the client using TLS.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[methane]

It looks very ugly. I dislike it.

I think we should support "tls-mode=false|true|preferred" and deprecate tls=preferred.

[dweitzman]

That makes sense to me. I'll update the diff when I have some time.

[dweitzman]

Updated with tls-mode=preferred. I didn't add tls-mode=required since that would be the same as tls=true. Removing tls=true would be a breaking change that probably won't happen anytime soon, and it seemed best to avoid having two ways to do the same thing.

[methane]

Removing tls=true would be a breaking change that probably won't happen anytime soon, and it seemed best to avoid having two ways to do the same thing.

I don't thinks so. If we add tls-mode, we should support all configuration about tls behavior on it.
On the other hand, it's OK to add only preferred at this pull request.

Options supported by tls-mode must be same to ssl-mode in MySQL. true|false was bad idea.
https://dev.mysql.com/doc/refman/8.0/en/encrypted-connection-options.html#option_general_ssl-mode

[shogo82148]

It is done by #1370