Support for Windows OpenSSH agent forwarding

[sschaap]

Buildkit is unable to forward keys from Windows OpenSSH agent. The Microsoft fork of OpenSSH exposes its agent on a named pipe, which cannot be connected to using the net.Dial*() functions that work for UNIX sockets. Trying to pass the named pipe path \\.\pipe\openssh-ssh-agent as SSH socket yields (using 70deac1 on Windows 10 with OpenSSH_for_Windows_8.1p1):

> buildctl build --ssh default=\\.\pipe\openssh-ssh-agent
error: failed to open \\.\pipe\openssh-ssh-agent: open \\.\pipe\openssh-ssh-agent: All pipe instances are busy.

Note that this issue is separate from #914 where the SSH socket seems to be a cygwin or msys socket, rather than a named pipe.

Support for Windows OpenSSH requires:

• Detection of named pipe paths on Windows.
• Use of e.g. winio.DialPipe() to connect to the pipe.
• (optional) Use of named pipe as default SSH agent path if available and not otherwise specified.

This would likely also resolve the downstream docker/buildx#409 issue regarding use of --ssh default on Windows 10.

I have implemented these changes in sschaap/buildkit@d74e356 for personal use. This makes Windows OpenSSH work seamlessly with buildkit --ssh default as well as docker buildx --ssh default (when rebuilt with modified buildkit).

Notably, this also works well with KeeAgent, which implements a compatible named pipe interface.

Is there interest in a PR for these changes? Any specific requirements other than sign-off?

[tonistiigi]

As long as it doesn't break the current cases seems like a fine addition.

[tonistiigi]

#2127