Implement API to allow replacing root CIDs in a CARv1 or CARv2

[masih]

Implement an API that allows a caller to replace root CIDs in an
existing CAR file, may it be v1 or v2, as long as the resulting
serialized header is of identical size to the existing header.

Assert that the new API works in a variety of CARv1 and CARv2 files
along with failure scenarios.

Fixes #245

[mvdan]

I think having it take a path on disk is fair - we only exposed OpenReadWrite after all, and no variant that attempts to use io interfaces, because we just need too many methods.

And if we want to later on expose a FinalizeWithRoots, we could also do that - and it would internally share the same implementation as ReplaceRootsInFile, perhaps via an unexported API that works on *os.File.

[masih]

And if we want to later on expose a FinalizeWithRoots, we could also do that

That was my thought too. I think we probably need both: Basic APIs to work with CAR files, and equivalent functionality used through blockstore.

I think we have a separate ticket already for blockstore support: #196

[mvdan]

Yep agreed. Was just thinking outloud :)

[willscott]

we could potentially get rid of the comment saying it doesn't shift padding - i'd prefer that to show up in this method once supported, rather than maintaining two different exposed APIs for it.

[mvdan]

i'd prefer that to show up in this method once supported, rather than maintaining two different exposed APIs for it.

My only worry is that it would end up being an API that might be very cheap (just rewrite the first kilobyte of data) versus very expensive (copy potentially gigabytes of data, depending on how large the CAR is).

[masih]

I think what Will is taking about is to use the padding after CARv2 header, before inner CARv1 to grow roots when needed. That should still be cheap for CARv2 files and it just means DataOffset in CARv2 header needs to be updated.

So we could expand the implementation to support replacing larger roots as long as they fit into the padding+current v1 header size

[mvdan]

Ah, that's fair. As long as it's equally cheap :)