Publishing with Lzip

[Pierre Neidhardt <mail@ambrevar.xyz>]

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Hi,

I've just sent a patch of a first draft for the Lzip bindings:

  http://debbugs.gnu.org/cgi/bugreport.cgi?bug=34807

Switching from Gzip (Zlib) to Lzip would save up to some 50% disk usage
for substitutes which would greatly benefit the servers and help
reducing the bandwidth usage.

A few things are missing in the patch and I would probably need some help:

- The %liblz variable in guix/config.scm.in is not set dynamically by
  autoconf.
  This is because lzlib does not have a pkg-config entry.  I don't know
  much about autoconf, so if someone knows how to do this properly...

- I'm still a bit confused about how the API is supposed to work.  There
  is a lz-(de)compress-finish function which is supposed to be called
  according to the examples but the manual does not really says why.  If
  I call it, the tests fail.

- I'm also not 100% I'm doing the right thing with the encoder/decoder:
  should we write everything first, then read as much as we can?  Or
  chain write-read calls like in bbexample.c?

- I'm not 100% sure either that the terminating chunk will always be
  compressed / decompressed.  (It works in the test though.)  I don't
  really understand how Lzlib handles that part.

- How can I map between C enums and Guile with dynamic FFI?  This would
  be useful to have improve error messages.

- lzlib.scm is not used for publishing in the patch.  Will do that
  later.  What are the strategies for transitioning from .gz to .lz?  I
  suggest the following:

  - On publishing, replace .gz with .lz compression.
  - When extracting, check the type and call the appropriate format decompressor.

Feedback welcome!

--
Pierre Neidhardt
https://ambrevar.xyz/

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