The changed-path Bloom filters work only when we can compute an
explicit Bloom filter key in advance. When a pathspec is given
that allows case-insensitive checks or wildcard matching, we
must disable the Bloom filter performance checks.

By checking the pathspec in prepare_to_use_bloom_filters(), we
avoid setting up the Bloom filter data and thus revert to the
usual logic.

Before this change, the following tests would fail*:

	t6004-rev-list-path-optim.sh (Tests 6-7)
	t6130-pathspec-noglob.sh (Tests 3-6)
	t6131-pathspec-icase.sh (Tests 3-5)

*These tests would fail when using GIT_TEST_COMMIT_GRAPH and
GIT_TEST_COMMIT_GRAPH_BLOOM_FILTERS except that the latter
environment variable was not set up correctly to write the changed-
path Bloom filters in the test suite. That will be fixed in the
next change.

Helped-by: Taylor Blau <[email protected]>
Signed-off-by: Derrick Stolee <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

The GIT_TEST_COMMIT_GRAPH environment variable updates the commit-
graph file whenever "git commit" is run, ensuring that we always
have an updated commit-graph throughout the test suite. The
GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS environment variable was
introduced to write the changed-path Bloom filters whenever "git
commit-graph write" is run. However, the GIT_TEST_COMMIT_GRAPH
trick doesn't launch a separate process and instead writes it
directly.

To expand the number of tests that have commits in the commit-graph
file, add a helper method that computes the commit-graph and place
that helper inside "git commit" and "git merge".

In the helper method, check GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS
to ensure we are writing changed-path Bloom filters whenever
possible.

Signed-off-by: Derrick Stolee <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

The changed-path Bloom filters help reduce the amount of tree
parsing required during history queries. Before calculating a
diff, we can ask the filter if a path changed between a commit
and its first parent. If the filter says "no" then we can move
on without parsing trees. If the filter says "maybe" then we
parse trees to discover if the answer is actually "yes" or "no".

When computing a blame, there is a section in find_origin() that
computes a diff between a commit and one of its parents. When this
is the first parent, we can check the Bloom filters before calling
diff_tree_oid().

In order to make this work with the blame machinery, we need to
initialize a struct bloom_key with the initial path. But also, we
need to add more keys to a list if a rename is detected. We then
check to see if _any_ of these keys answer "maybe" in the diff.

During development, I purposefully left out this "add a new key
when a rename is detected" to see if the test suite would catch
my error. That is how I discovered the issues with
GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS from the previous change.
With that change, we can feel some confidence in the coverage of
this change.

If a user requests copy detection using "git blame -C", then there
are more places where the set of "important" files can expand. I
do not know enough about how this happens in the blame machinery.
Thus, the Bloom filter integration is explicitly disabled in this
mode. A later change could expand the bloom_key data with an
appropriate call (or calls) to add_bloom_key().

If we did not disable this mode, then the following tests would
fail:

	t8003-blame-corner-cases.sh
	t8011-blame-split-file.sh

Generally, this is a performance enhancement and should not
change the behavior of 'git blame' in any way. If a repo has a
commit-graph file with computed changed-path Bloom filters, then
they should notice improved performance for their 'git blame'
commands.

Here are some example timings that I found by blaming some paths
in the Linux kernel repository:

 git blame arch/x86/kernel/topology.c >/dev/null

 Before: 0.83s
  After: 0.24s

 git blame kernel/time/time.c >/dev/null

 Before: 0.72s
  After: 0.24s

 git blame tools/perf/ui/stdio/hist.c >/dev/null

 Before: 0.27s
  After: 0.11s

I specifically looked for "deep" paths that were also edited many
times. As a counterpoint, the MAINTAINERS file was edited many
times but is located in the root tree. This means that the cost of
computing a diff relative to the pathspec is very small. Here are
the timings for that command:

 git blame MAINTAINERS >/dev/null

 Before: 20.1s
  After: 18.0s

These timings are the best of five. The worst-case runs were on the
order of 2.5 minutes for both cases. Note that the MAINTAINERS file
has 18,740 lines across 17,000+ commits. This happens to be one of
the cases where this change provides the least improvement.

The lack of improvement for the MAINTAINERS file and the relatively
modest improvement for the other examples can be easily explained.
The blame machinery needs to compute line-level diffs to determine
which lines were changed by each commit. That makes up a large
proportion of the computation time, and this change does not
attempt to improve on that section of the algorithm. The
MAINTAINERS file is large and changed often, so it takes time to
determine which lines were updated by which commit. In contrast,
the code files are much smaller, and it takes longer to comute
the line-by-line diff for a single patch on the Linux mailing
lists.

Outside of the "-C" integration, I believe there is little more to
gain from the changed-path Bloom filters for 'git blame' after this
patch.

Signed-off-by: Derrick Stolee <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

We don't use the "parent" parameter at all (probably because the bloom
filter for a commit is always defined against a single parent anyway).

Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Signed-off-by: Jeff King <[email protected]>
Reviewed-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

If "test-tool bloom" is not fed a command, or if arguments are missing
for some commands, it will just segfault. Let's check argc and write a
friendlier usage message.

Signed-off-by: Jeff King <[email protected]>
Reviewed-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>