adding a long hardlink without _fs APIs

[the8472]

Similar to #108, the internal helper method for appending long links is only accessible through APIs that attempt to access the filesystem.

It would be great if it were possible to pass a header (with symlink or hardlink entry_type) and a name + link_name as Path and have the builder insert the necessary long-name headers. I.e. an equivalent to append_data

Maybe some general intermediate-level API is missing for adding headers and data separately to orthogonalize the construction of complicated headers that must be written in several parts (long name, long links, sparse header) and their associated data chunks. A single function taking several enums to cover all those combinations might work too.

[cgwalters]

I'm hitting this in ostreedev/ostree-rs-ext#162

I'm willing to work on a PR here. But reading the code, one thing that strikes me as somewhat odd is that we have this "base interface" in tar::Header and APIs to "cast" it to e.g. a GnuHeader.

But why do we not directly expose all the API methods like set_link_name() on GnuHeader? In my case, I know I am writing a GNU-formatted tar archive. I always want that behavior.

It seems weird to me how many of the Header methods do things like:

    fn _set_path(&mut self, path: &Path) -> io::Result<()> {
        if let Some(ustar) = self.as_ustar_mut() {
            return ustar.set_path(path);
        }
        ...
    // Or base case here

The inverse problem here is that the append_data() behavior of trying the base case but silently "upgrading" to GNU may not be what some people want - they may want to fail instead of writing a GNU entry. I admit this use case is probably obscure, but to rephrase I think we should be consistent in our interface - the format of an archive as a whole is either GNU or PAX or base, etc.

[cgwalters]

PR in #273

[cgwalters]

(Regarding my confusion around Header above, the reason we need a special case append_data() is internally it's represented as two tar entries, so we need to handle it via Builder)