[PATCH v2 0/4] Integrate changed-path Bloom filters with 'git blame'

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

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 (4):
  revision: complicated pathspecs disable filters
  commit: write commit-graph with Bloom filters
  commit-graph: write commit-graph in more tests
  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   |   9 +++
 commit-graph.h   |   2 +
 revision.c       |  19 ++++++-
 8 files changed, 181 insertions(+), 15 deletions(-)


base-commit: f4df00a0dd448edce0e854a97f63598fefe27d27
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-609%2Fderrickstolee%2Fbloom-blame-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-609/derrickstolee/bloom-blame-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/609

Range-diff vs v1:

 1:  9cc31c289aa ! 1:  adc03eee4ac revision: complicated pathspecs disable filters
     @@ Commit message
          path Bloom filters in the test suite. That will be fixed in the
          next change.
      
     +    Helped-by: Taylor Blau <me@ttaylorr.com>
          Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
      
       ## revision.c ##
     +@@ revision.c: static void trace2_bloom_filter_statistics_atexit(void)
     + 	jw_release(&jw);
     + }
     + 
     ++static int forbid_bloom_filters(struct pathspec *spec)
     ++{
     ++	if (spec->has_wildcard)
     ++		return 1;
     ++	if (spec->nr > 1)
     ++		return 1;
     ++	if (spec->magic & ~PATHSPEC_LITERAL)
     ++		return 1;
     ++	if (spec->nr && (spec->items[0].magic & ~PATHSPEC_LITERAL))
     ++		return 1;
     ++
     ++	return 0;
     ++}
     ++
     + static void prepare_to_use_bloom_filter(struct rev_info *revs)
     + {
     + 	struct pathspec_item *pi;
      @@ revision.c: static void prepare_to_use_bloom_filter(struct rev_info *revs)
     - 	if (!revs->commits)
     - 	    return;
     + 	int len;
       
     -+	if (revs->prune_data.has_wildcard)
     -+		return;
     -+	if (revs->prune_data.nr > 1)
     -+		return;
     -+	if (revs->prune_data.magic ||
     -+	    (revs->prune_data.nr &&
     -+	     revs->prune_data.items[0].magic))
     + 	if (!revs->commits)
     +-	    return;
      +		return;
      +
     ++	if (forbid_bloom_filters(&revs->prune_data))
     ++		return;
     + 
       	repo_parse_commit(revs->repo, revs->commits->item);
       
     - 	if (!revs->repo->objects->commit_graph)
 2:  bb5ce39d028 < -:  ----------- commit: write commit-graph with bloom filters
 -:  ----------- > 2:  7e8f1aed113 commit: write commit-graph with Bloom filters
 -:  ----------- > 3:  824f8ad067b commit-graph: write commit-graph in more tests
 3:  431fde68031 = 4:  4ae196d6355 blame: use changed-path Bloom filters

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