[PATCH v4 0/7] Add delta islands support

This patch series is upstreaming work made by GitHub and available in:

https://github.com/peff/git/commits/jk/delta-islands

The above work has been already described in the following article:

https://githubengineering.com/counting-objects/

The above branch contains only one patch. In this patch series the patch has been split into 4 patches (1/7, 3/7, 4/7 and 5/7) with their own commit message, and on top of that 3 new patches (2/7, 6/7 and 7/7) have been added. The new patches implement things that have been requested on the mailing list.

I kept Peff as the author of the original 4 patches and took the liberty to add his Signed-off-by to them.

As explained in details in the "Counting Objects" article referenced above, the goal of the delta island mechanism is for a hosting provider to make it possible to have the "forks" of a repository share as much storage as possible while preventing object packs to contain deltas between different forks.

If deltas between different forks are not prevented, when users clone or fetch a fork, preparing the pack that should be sent to them can be very costly CPU wise, as objects from a different fork should not be sent, which means that a lot of deltas might need to be computed again (instead of reusing existing deltas).

The following changes have been made since the previous iteration:

* suggested by Ramsay: remove the first patch that made
  get_delta_base() in packfile.c non static as it is not necessary

* suggested by Peff: use ALLOC_GROW() in island_config_callback() in
  delta-islands.c in patch 1/7

* suggested by SZEDER Gabor: use ALLOC_ARRAY() in
  deduplicate_islands() in delta-islands.c in patch 1/7

This patch series is also available on GitHub in:

https://github.com/chriscool/git/commits/delta-islands

The previous versions are available there:

V3: https://github.com/chriscool/git/commits/delta-islands29
V2: https://github.com/chriscool/git/commits/delta-islands19
V1: https://github.com/chriscool/git/commits/delta-islands6

V3: https://public-inbox.org/git/20180809155532.26151-1-chriscool@tuxfamily.org/
V2: https://public-inbox.org/git/20180805172525.15278-1-chriscool@tuxfamily.org/
V1: https://public-inbox.org/git/20180722054836.28935-1-chriscool@tuxfamily.org/

Christian Couder (3):
  pack-objects: refactor code into compute_layer_order()
  pack-objects: move tree_depth into 'struct packing_data'
  pack-objects: move 'layer' into 'struct packing_data'

Jeff King (4):
  Add delta-islands.{c,h}
  pack-objects: add delta-islands support
  repack: add delta-islands support
  t: add t5319-delta-islands.sh

 Documentation/config.txt           |  19 ++
 Documentation/git-pack-objects.txt |  97 ++++++
 Documentation/git-repack.txt       |   5 +
 Makefile                           |   1 +
 builtin/pack-objects.c             | 137 +++++---
 builtin/repack.c                   |   9 +
 delta-islands.c                    | 502 +++++++++++++++++++++++++++++
 delta-islands.h                    |  11 +
 pack-objects.c                     |  12 +
 pack-objects.h                     |  39 +++
 t/t5319-delta-islands.sh           | 143 ++++++++
 11 files changed, 932 insertions(+), 43 deletions(-)
 create mode 100644 delta-islands.c
 create mode 100644 delta-islands.h
 create mode 100755 t/t5319-delta-islands.sh