[RFC PATCH 00/13] Consolidate reachability logic

This RFC is a bit unpolished because I was mostly seeing where the idea could go. I wanted to achieve the following:

1. Consolidate several different commit walks into one file
2. Reduce duplicate reachability logic
3. Increase testability (correctness and performance)
4. Improve performance of reachability queries

My approach is mostly in three parts:

  I. Move code to a new commit-reach.c file.
 II. Add a 'test-tool reach' command to test these methods directly.
III. Modify the logic by improving performance and calling methods with
     similar logic but different prototypes.

The 'test-tool reach' command is helpful to make sure I don't break anything as I change the logic, but also so I can test methods that are normally only exposed by other more complicated commands. For instance, ref_newer() is part of 'git push -f' and ok_to_give_up() is buried deep within fetch negotiation. Both of these methods have some problematic performacne issues that are corrected by this series. As I discovered them, it was clear that it would be better to consolidate walk logic instead of discovering a new walk in another file hidden somewhere.

For the ok_to_give_up() method, I refactored the method so I could pull the logic out of the depths of fetch negotiation. In the commit "commit-reach: make can_all_from_reach... linear" I discuss how the existing algorithm is quadratic and how we can make it linear. Also, we can use heuristic knowledge about the shape of the commit graph and the usual haves/wants to get some extra performance bonus. (The heuristic is to do a DFS with first-parents first, and stop on first found result. We expect haves/wants to include ref tips, which typically have their previous values in their first-parent history.)

The "test-reach" commit in particular is not split well or described well for mailing-list review. I figured I would send the RFC instead of tweaking it carefully, because I will need to re-do most of these changes after more of the object-store series is merged. I don't plan to send a v1 patch until the lookup_commit() and parse_commit() code is stable again.

Thanks, -Stolee

Derrick Stolee (13):
  commit-reach: move walk methods from commit.c
  commit-reach: move ref_newer from remote.c
  commit-reach: move commit_contains from ref-filter
  upload-pack: make reachable() more generic
  upload-pack: refactor ok_to_give_up()
  commit-reach: move can_all_from_reach_with_flag()
  test-reach
  test-reach: test reduce_heads()
  commit-reach: test can_all_from_reach
  commit-reach: test is_descendant_of
  commit-reach: make can_all_from_reach... linear
  commit-reach: use is_descendant_of for ref_newer
  commit-reach: use can_all_from_reach

 Makefile              |   2 +
 builtin/remote.c      |   1 +
 commit-reach.c        | 656 ++++++++++++++++++++++++++++++++++++++++++
 commit-reach.h        |  37 +++
 commit.c              | 365 +----------------------
 commit.h              |   6 +-
 fast-import.c         |   1 +
 fetch-pack.c          |   3 +-
 http-push.c           |   1 +
 ref-filter.c          | 147 +---------
 remote.c              |  48 +---
 remote.h              |   1 -
 sha1-name.c           |   3 +-
 t/helper/test-reach.c | 128 +++++++++
 t/helper/test-tool.c  |   1 +
 t/helper/test-tool.h  |   1 +
 t/t6600-test-reach.sh | 177 ++++++++++++
 upload-pack.c         |  58 +---
 walker.c              |   3 +-
 19 files changed, 1035 insertions(+), 604 deletions(-)
 create mode 100644 commit-reach.c
 create mode 100644 commit-reach.h
 create mode 100644 t/helper/test-reach.c
 create mode 100755 t/t6600-test-reach.sh

base-commit: d4f65b8d141e041eb5e558cd9e763873e29863b9