[PATCH v3 0/8] 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 5 patches (1/8, 2/8, 4/8, 5/8 and 6/8) with their own commit message, and on top of that 3 new patches (3/8, 7/8 and 8/8) have been added. The new patches implement things that were requested after the previous iterations.

I kept Peff as the author of the original 5 patches and took the liberty to add his Signed-off-by to them.

As explained in details in the Counting Object 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 Duy: move the code computing the write order for a
  layer to a new separate function called compute_layer_order() in
  builtin/pack-objects.c in new patch 3/8

  I think that indeed this makes the following patch (4/8) shorter and
  easier to understand as well as the overall result nicer.

* suggested by Duy and Peff: rework the way the 'tree_depth' field is
  moved from 'struct object_entry' to 'struct packing_data' in
  pack-object.h in patch 7/8

* suggested by Duy and Peff: move field 'layer' from
  'struct object_entry' to 'struct packing_data' in pack-object.h in
  new patch 8/8

This patch series is also available on GitHub in:

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

The previous versions are available there:

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

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 (5):
  packfile: make get_delta_base() non static
  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                    | 506 +++++++++++++++++++++++++++++
 delta-islands.h                    |  11 +
 pack-objects.c                     |  12 +
 pack-objects.h                     |  39 +++
 packfile.c                         |  10 +-
 packfile.h                         |   7 +
 t/t5319-delta-islands.sh           | 143 ++++++++
 13 files changed, 948 insertions(+), 48 deletions(-)
 create mode 100644 delta-islands.c
 create mode 100644 delta-islands.h
 create mode 100755 t/t5319-delta-islands.sh