UnixFS reading (or exporter)

[koivunej]

UnixFS as in UnixFSv1.5 reading per https://github.com/ipfs/specs/blob/master/UNIXFS.md.

• reading single-block files
• reading files with arbitary trees
• reading directories and files under them
• reading symlinks
• http: /cat
• http: /get

[koivunej]

I am still looking into getting the third working, then work on the /cat. There doesn't seem to be much of an issue into making this into a fn reader(block: &'a [u8]) -> SomeReader<'a> but that will leave the block reading into the hands of the caller, similar to how refs is handled at the moment over at http. The "block reading" or "link following" has some things to handle as well, such as size validation. I am expecting this to be wrangleable into something like:

pub async fn cat_unixfs(&self, path: impl Into<IpfsPath>, slice: Option<Range<u64>>) -> 
  Result<impl TryStream<Ok = SomeBytes, Error = UnixFsError>, ipfs::Error>;

cat operation is more simple than the get as it can only cat files and maybe symlinks if I understand correctly.

Hopefully this will allow supporting many different blockstore/persistence appoaches.

I haven't still done but the preliminary work on tar support at /get, and I am also a bit unsure how to represent a async fn get_unixfs(&self, path: impl Into<IpfsPath>) -> Result<impl TryStream<Ok = ???, Error = _>, _> as the TryStream::Ok must be able to contain multiple further Streams.

[koivunej]

Related js-ipfs PR: ipfs/js-ipfs#3093