[Windows] Fix nerdctl build tar parsing error

[TinaMor]

PR Description

Linked Issue: containerd/nerdctl#3629

This PR separates output streams for the buildctl build command to avoid parsing errors specifically on Windows terminals.

The buildctl build command sends both TTY logs and tarball (binary) data to stdout, but Windows terminals cannot differentiate between these, causing errors like "archive/tar: invalid tar header." To resolve this, the tarball output is directed to a file, isolating it from TTY output. If a destination tar file is not specified, a default file is created, which is then loaded into the containerd image store.

[TBBle]

I need to work through this later, but I'm unclear how the current code works on non-Windows. How do those platforms distinguish a tar file from log output on stdout?

They shouldn't be passing through a termInal on the way, the terminal is where nerdctl's stdio is connected, so I don't understand what's different on Windows.

[apostasie]

Hey @TBBle
I’ll have a look later today.
Curious too.

[TinaMor]

@TBBle, @apostasie

Still investigating the issue. This is a temp fix and the PR is WIP (may and may not be the final solution).
Currrently trying to understand the following:

1. What is the performance impact of this fix if using a larger image eg mcr.microsoft.com/windows/servercore:ltsc2022 vs mcr.microsoft.com/windows/nanoserver:ltsc2022
2. Is it the Windows terminal?
3. Why are the build output and tarball all going to the same location on Windows alone?
4. How does it work for progress=plain?

[apostasie]

Deeply confused about this.

Why are the build output and tarball all going to the same location on Windows alone?

This does not make any sense.
I am squinting at nerdctl source and cannot figure it out.

Now suspecting buildctl might be the problem.

[apostasie]

buildctl "output" goes to stderr.

Is it the case on windows?

foo="$(buildctl --addr=unix:///run/user/501/buildkit/buildkitd.sock build --progress=auto --frontend=dockerfile.v0 --local=context=/tmp/ --output=type=docker --local=dockerfile=/tmp --opt=filename=Dockerfile)"

or

buildctl --addr=unix:///run/user/501/buildkit/buildkitd.sock build --progress=auto --frontend=dockerfile.v0 --local=context=/tmp/ --output=type=docker --local=dockerfile=/tmp --opt=filename=Dockerfile >/dev/null