If the changed-path Bloom filters are relatively stable, then I propose
trying to build upon them as a way to discover any deficiencies. Also, it's
good to use them when we can.

The goal I set out to achieve was to use Bloom filters as much as possible
in git blame. I think I have achieved most of that, except I did not
integrate it with the -C mode. In that case, the blob-diff computation takes
a majority of the computation time, so short-circuiting the tree diff using
Bloom filters. Also, it's just really complicated. If someone else thinks
there is an easy win there, then please go ahead and give it a try with the
extra logic presented here in PATCH 3.

While I was playing around with Bloom filters and git blame, I purposefully
got it working with some scenarios but not all. Then I tried to trigger a
failing build in the blame tests using GIT_TEST_COMMIT_GRAPH=1 and
GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS=1. But the tests all succeeded!

Examining the data, I see that the commit-graph didn't have the Bloom filter
chunks at all. This is because we are not setting the flag to write them in
the right spot. The GIT_TEST_COMMIT_GRAPH=1 variable triggers a commit-graph
write during git commit, so we need to update the code there instead of just
inspecting the variable in git commit-graph write. (This is PATCH 2.)

By updating this variable, I saw some test failures in other tests regarding
non-exact pathspecs. I fixed these in PATCH 1 so we keep clean builds.

I based this change on [1] but it would apply cleanly (and logically) on
gs/commit-graph-path-filter

Updates in v2:

 * Added PATCH 3 to write commit-graph files during 'git merge' if
   GIT_TEST_COMMIT_GRAPH is enabled.

 * Updated PATCH 1 with the simplification recommended by Taylor.

 * Fixed the lower-case "bloom" in the commit message.

Thanks, -Stolee

[1]
https://lore.kernel.org/git/pull.601.v2.git.1586437211842.gitgitgadget@gmail.com/

Derrick Stolee (3):
  revision: complicated pathspecs disable filters
  tests: write commit-graph with Bloom filters
  blame: use changed-path Bloom filters

 blame.c          | 139 ++++++++++++++++++++++++++++++++++++++++++++---
 blame.h          |   6 ++
 builtin/blame.c  |  10 ++++
 builtin/commit.c |   4 +-
 builtin/merge.c  |   7 ++-
 commit-graph.c   |  14 +++++
 commit-graph.h   |   9 +++
 revision.c       |  19 ++++++-
 8 files changed, 193 insertions(+), 15 deletions(-)

base-commit: f4df00a0dd448edce0e854a97f63598fefe27d27

Submitted-As: https://lore.kernel.org/git/pull.609.v3.git.1587068044.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.609.git.1586566981.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.609.v2.git.1586789126.gitgitgadget@gmail.com