LibGit2: fix pass-through for SSH

[StefanKarpinski]

According to the libgit2 documentation, if the callback returns any postive value, it will behave as if the callback were unset (i.e. pass-through), but that's not true. The value that needs to be returned is actually -30 (GIT_PASSTHROUGH).

[Keno]

Hmm?
https://github.com/libgit2/libgit2/blob/b8cdc9c9c59c61fc699550af5245c57744c1bcbd/src/transports/ssh.c#L602-L607

[StefanKarpinski]

Yes, but:

• https://github.com/libgit2/libgit2/blob/20450cbe7d161abbb3eb/src/transports/httpclient.c#L753-L756
• https://github.com/libgit2/libgit2/blob/20450cbe7d161abbb3eb/src/transports/winhttp.c#L313-L314

So while returning a negative positive value to indicate pass-through does work for SSH, it does not work for HTTPS. The previous version of this code was just rejecting invalid certificates, which works for HTTPS but does not seem to work for SSH, which seems to need to try something else after this callback in order to work. The GIT_PASSTHROUGH constant (-30) actually works as a pass-through value for all protocols.

[Keno]

So while returning a negative value to indicate pass-through does work for SSH

You mean a positive value?

[StefanKarpinski]

Yes, that's what I meant.

[Keno]

So the design of NetworkOptions.verify_host is to allow additional hosts, but never be able to reject anything the system allows?

[StefanKarpinski]

Yes, it is a mechanism for allowing unverified access to hosts that would otherwise fail verification. Why would we want a mechanism for rejecting verified hosts?