add WithBaseDir option to tar context

[joshrwolf]

Adds a WithBaseDir option to the tar context to allow building tarballs with more control over the archives root directory.

This is useful when the unarchive process isn't performed by the consumer, but the consumer still wants to adhere to some required directory structure (such as in kontext, where the archive is used to create a container layer unpacked at /)

This option is disabled by default.

[joshrwolf]

this should be equivalent to header.Name = path when c.BaseDir is the zero value, but please correct me if I'm wrong!

[deitch]

I don't fully understand what this change tries to do. Let's say I have a directory tree like this:

foo
  bar
    baz
      filea
      fileb
      subdir
         sub1
         sub2

And I want everything under /foo/bar/baz/ to be in my tarball. Then I already have that, and all subpaths, e.g. /foo/bar/baz/filea and /foo/bar/baz/subdir will appear just like that in the repo, fileaa and subdir, etc.

Unless you are saying you want them to appear in the tarball as prefix/filea and prefix/fileb etc? Is this about modifying what appears inside the tarball?

[joshrwolf]

Unless you are saying you want them to appear in the tarball as prefix/filea and prefix/fileb etc? Is this about modifying what appears inside the tarball?

This. If there's a better way to achieve this within the confines of FullFS, please let me know!

Also, here is a rough draft of the downstream impact this would have for something like kontext

[deitch]

I get it. I don't see any reason not to support it.

I do wonder if this is the best way to achieve it? Thinking about tar commands, usually, if I, as a tar consumer, want my output to be in a certain directory, I would do:

$ mkdir /target/dir
$ tar -C /target/dir -xvf mytarfile.tar

I only create the prefix if I want every potential consumer to have it (which is a valid use case).