[PATCH 0/6] Use generation numbers for --topo-order

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

This patch series performs a decently-sized refactoring of the revision-walk
machinery. Well, "refactoring" is probably the wrong word, as I don't
actually remove the old code. Instead, when we see certain options in the
'rev_info' struct, we redirect the commit-walk logic to a new set of methods
that distribute the workload differently. By using generation numbers in the
commit-graph, we can significantly improve 'git log --graph' commands (and
the underlying 'git rev-list --topo-order').

On the Linux repository, I got the following performance results when
comparing to the previous version with or without a commit-graph:

Test: git rev-list --topo-order -100 HEAD
HEAD~1, no commit-graph: 6.80 s
HEAD~1, w/ commit-graph: 0.77 s
  HEAD, w/ commit-graph: 0.02 s

Test: git rev-list --topo-order -100 HEAD -- tools
HEAD~1, no commit-graph: 9.63 s
HEAD~1, w/ commit-graph: 6.06 s
  HEAD, w/ commit-graph: 0.06 s

If you want to read this series but are unfamiliar with the commit-graph and
generation numbers, then I recommend reading 
Documentation/technical/commit-graph.txt or a blob post [1] I wrote on the
subject. In particular, the three-part walk described in "revision.c:
refactor basic topo-order logic" is present (but underexplained) as an
animated PNG [2].

Since revision.c is an incredibly important (and old) portion of the
codebase -- and because there are so many orthogonal options in 'struct
rev_info' -- I consider this submission to be "RFC quality". That is, I am
not confident that I am not missing anything, or that my solution is the
best it can be. I did merge this branch with ds/commit-graph-with-grafts and
the "DO-NOT-MERGE: write and read commit-graph always" commit that computes
a commit-graph with every 'git commit' command. The test suite passed with
that change, available on GitHub [3]. To ensure that I cover at least the
case I think are interesting, I added tests to t6600-test-reach.sh to verify
the walks report the correct results for the three cases there (no
commit-graph, full commit-graph, and a partial commit-graph so the walk
starts at GENERATION_NUMBER_INFINITY).

One notable case that is not included in this series is the case of a
history comparison such as 'git rev-list --topo-order A..B'. The existing
code in limit_list() has ways to cut the walk short when all pending commits
are UNINTERESTING. Since this code depends on commit_list instead of the
prio_queue we are using here, I chose to leave it untouched for now. We can
revisit it in a separate series later. Since handle_commit() turns on
revs->limited when a commit is UNINTERESTING, we do not hit the new code in
this case. Removing this 'revs->limited = 1;' line yields correct results,
but the performance is worse.

This series is based on ds/reachable.

Thanks, -Stolee

[1] 
https://blogs.msdn.microsoft.com/devops/2018/07/09/supercharging-the-git-commit-graph-iii-generations/
Supercharging the Git Commit Graph III: Generations and Graph Algorithms

[2] 
https://msdnshared.blob.core.windows.net/media/2018/06/commit-graph-topo-order-b-a.png
Animation showing three-part walk

[3] https://github.com/derrickstolee/git/tree/topo-order/testA branch
containing this series along with commits to compute commit-graph in entire
test suite.

Derrick Stolee (6):
  prio-queue: add 'peek' operation
  test-reach: add run_three_modes method
  test-reach: add rev-list tests
  revision.c: begin refactoring --topo-order logic
  commit/revisions: bookkeeping before refactoring
  revision.c: refactor basic topo-order logic

 commit.c                   |  11 +-
 commit.h                   |   8 ++
 object.h                   |   4 +-
 prio-queue.c               |   9 ++
 prio-queue.h               |   6 +
 revision.c                 | 232 ++++++++++++++++++++++++++++++++++++-
 revision.h                 |   6 +
 t/helper/test-prio-queue.c |  10 +-
 t/t6600-test-reach.sh      |  98 +++++++++++++++-
 9 files changed, 361 insertions(+), 23 deletions(-)


base-commit: 6cc017431c1c48f80d1c6512fdcc9866cf4b7f55
Published-As: https://github.com/gitgitgadget/git/releases/tags/pr-25%2Fderrickstolee%2Ftopo-order%2Fprogress-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-25/derrickstolee/topo-order/progress-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/25
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