WindowsWriter needs to add `Files` prefix to links about imgutil

[micahyoung]

After some experimenting, we found this is not fixable in imgutil. The only valid type of value for header.LinkName would be a relative path (ex my-link => bin/detect) and those already work without a Files prefix.

Absolute paths also don't work for Linux tar.Writer (they're copied over completely and not rewritten by Docker on extraction) so likewise on Windows, a user would need to set the correct absolute path value for header.Linkname.

Here are some examples from pack. We added some different types of symlinks to a sample buildpack, created a builder image from them, then got this output via docker run on that builder image. Note that both Windows and Linux have identical behavior (they just set the generated symlinks to have the exact values of the original symlinks).

Windows:

12/31/1979  05:00 PM    <DIR>          bin                                
12/31/1979  05:00 PM               162 buildpack.toml                     
12/31/1979  05:00 PM    <SYMLINK>      dirlink-absolute-bin [\Users\micah\workspace\pack\sandbox\hello-windows\bin]                       
12/31/1979  05:00 PM    <SYMLINK>      dirlink-absolute-buildpack.toml [\Users\micah\workspace\pack\sandbox\hello-windows\buildpack.toml] 
12/31/1979  05:00 PM    <SYMLINK>      dirlink-relative-bin [bin]                       
12/31/1979  05:00 PM    <SYMLINK>      dirlink-relative-buildpack.toml [buildpack.toml] 
12/31/1979  05:00 PM    <SYMLINK>      dirlink-relback-bin [..\hello-windows\bin]       
12/31/1979  05:00 PM    <SYMLINK>      dirlink-relback-buildpack.toml [..\hello-windows\buildpack.toml]

Linux:

drwxr-xr-x 2 root root 4096 Jan  1  1980 bin
-rwxr-xr-x 1 root root  162 Jan  1  1980 buildpack.toml
lrwxrwxrwx 1 root root   53 Jan  1  1980 dirlink-absolute-bin -> /Users/micah/workspace/pack/sandbox/hello-windows/bin
lrwxrwxrwx 1 root root   64 Jan  1  1980 dirlink-absolute-buildpack.toml -> /Users/micah/workspace/pack/sandbox/hello-windows/buildpack.toml
lrwxrwxrwx 1 root root    3 Jan  1  1980 dirlink-relative-bin -> bin
lrwxrwxrwx 1 root root   14 Jan  1  1980 dirlink-relative-buildpack.toml -> buildpack.toml
lrwxrwxrwx 1 root root   20 Jan  1  1980 dirlink-relback-bin -> ../hello-windows/bin
lrwxrwxrwx 1 root root   31 Jan  1  1980 dirlink-relback-buildpack.toml -> ../hello-windows/buildpack.toml

[ekcasey]

Absolute paths also don't work for Linux tar.Writer (they're copied over completely and not rewritten by Docker on extraction) so likewise on Windows, a user would need to set the correct absolute path value for header.Linkname.

Can you elaborate on this part a little. If you explicitly set header.Linkname to an absolute path that will exist in the container, docker will not handle it correctly when creating the container?

[micahyoung]

Sorry for no reply. I'm afraid I lost the context on this and I feel like this has been sorted out with the slice-related lifecycle changes, so incoming symlinks are sure to have the correct value on both OSes