Fixes Windows symlinks targets

[claudiubelu]

When building with the output type=oci or type=registry, for Windows symlinks, Files/ is prepended to the symlink Target, which can break the symlink entirely if it's an absolute path (C:\path\to\file becomes Files\C:\path\to\file), and for relative paths workarounds were needed (path\to\file would become Files\path\to\file, and it would have worked if that path was valid).

This problem does not occur when the output type is tar.

Removes the Files/ prefix from the Windows symlinks.

Fixes: docker/buildx#373

[tonistiigi]

Probably C:/ needs a special case. Or I wonder if this should be done for symlinks at all. It needs to match the applier behavior and I'm not quite sure what actual wcow builders put in tarball for symlink/hardlink cases.

[claudiubelu]

Probably C:/ needs a special case. Or I wonder if this should be done for symlinks at all. It needs to match the applier behavior and I'm not quite sure what actual wcow builders put in tarball for symlink/hardlink cases.

I've limited this PR to not add the Files/ prefix to symlinks, so the hardlinks behaviour remains unchanged (it wasn't an issue before).

[claudiubelu]

Probably C:/ needs a special case. Or I wonder if this should be done for symlinks at all. It needs to match the applier behavior and I'm not quite sure what actual wcow builders put in tarball for symlink/hardlink cases.

It seems that the buildkit applier uses the containerd archive/tar applier [1], which doesn't trim the Files/ prefix, it just applies the symlink as is [2]. I don't think that we should trim the Files/ prefix in [2], so we can avoid the niche error case in which someone would actually want to symlink to a local folder called Files, something like foo.bar <===> Files/foo.bar. So, it seems it's up to how the images are built in the first place.

I have already mentioned in the PR message that the Files/ prefix is not added when the output type is tar.

[1]

buildkit/util/winlayers/applier.go

Line 84 in 1a7c480

if _, err := archive.Apply(ctx, root, rc2); err != nil {

[2]

buildkit/vendor/github.com/containerd/containerd/archive/tar.go

Line 364 in 1a7c480

if err := os.Symlink(hdr.Linkname, path); err != nil {

[cpuguy83]

What's the status on this? @tonistiigi Was there something you wanted to change here?

[tonistiigi]

@claudiubelu So you think we should also remove the Files/ trim in applier? That sgtm although it would break existing images that were created with the Differ adding the prefix.