deduplication: Detect holes and create sparse files.

This reduces disk usage of sparse files that are substituted such as
Guile object files (ELF files).  As of Guile 3.0.9, .go files are sparse
due to ELF sections being aligned on 64 KiB boundaries.

This reduces disk usage reported by “du -sh” by 9% for the ‘guix’
package, by 23% for ‘guile’, and by 35% for ‘guile-git’.

* guix/store/deduplication.scm (hole-size, find-holes): New procedures.
(tee)[seekable?]: New variable.
[read!]: Add case when SEEKABLE? is true.
* tests/store-deduplication.scm (cartesian-product): New procedure.
("copy-file/deduplicate, sparse files (holes: ~a/~a/~a)"): New test set.

Change-Id: Iad2ab7830dcb1220e2026f4a127a6c718afa8964

0 files changed, 0 insertions, 0 deletions