Re: [Libguestfs] [PATCH nbdkit 2/2] tar: Rewrite the tar plugin (again), this time in C.

On Sun, Jun 28, 2020 at 04:46:59PM +0300, Nir Soffer wrote: Yes the block size is always 512 bytes: https://www.gnu.org/software/tar/manual/html_node/Blocking.html By the way, FreeBSD tar will not work (and presumably other non-GNU tar), so I guess we should probably try to probe tar and come up with a nicer error message: https://www.freebsd.org/cgi/man.cgi?tar(1) Convenience basically. The alternative is to fork and exec the tar command in a subprocess, while reading from a pipe that we have created, which is a pain to write correctly. Yes and no. A few months ago I actually had a proposal to unify all the "file-like" plugins. I believe the latest posted version was this one: https://www.redhat.com/archives/libguestfs/2020-April/msg00083.html but I have some updates in a branch here. Anyway when I was looking at this rewrite I considered resurrecting this work and building on top of it. However it doesn't work directly because: you have to add an offset to each request. Of course we could make a generic file ops which supported offsets (and also a non-FUA extension to deal with the tmpdisk plugin), but it all gets a bit complicated and not as generic as it initially looked. For completeness here are the other ways I investigated: * Make generic read, write, zero, trim, extents operations on local files. The file-like plugins would consume those directly, after doing any adjustments eg for offset. Unfortunately the operations that you have to write are not very "pure" and depend on a bunch of state across calls and you end up not having much common code: https://github.com/libguestfs/nbdkit/blob/f6d4365364f2c90dde0166ae4355f74... * Have the new tar plugin re-exec nbdkit and run the ordinary file plugin + offset filter. After experience with the VDDK plugin which does this through necessity, I don't want to go there right now. See this file to understand the kind of complexity this introduces: https://github.com/libguestfs/nbdkit/blob/master/plugins/vddk/reexec.c * Instead of writing a tar plugin, write a wrapper script which hands off to file plugin + offset filter. From the user's point of view the script would work a lot differently from other plugins. So no good ideas so far. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 100 libraries supported. http://fedoraproject.org/wiki/MinGW