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commit-graph: persist existence of changed-paths

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The changed-path Bloom filters were released in v2.27.0, but have a
significant drawback. A user can opt-in to writing the changed-path
filters using the "--changed-paths" option to "git commit-graph write"
but the next write will drop the filters unless that option is
specified.

This becomes even more important when considering the interaction with
gc.writeCommitGraph (on by default) or fetch.writeCommitGraph (part of
features.experimental). These config options trigger commit-graph writes
that the user did not signal, and hence there is no --changed-paths
option available.

Allow a user that opts-in to the changed-path filters to persist the
property of "my commit-graph has changed-path filters" automatically. A
user can drop filters using the --no-changed-paths option.

In the process, we need to be extremely careful to match the Bloom
filter settings as specified by the commit-graph. This will allow future
versions of Git to customize these settings, and the version with this
change will persist those settings as commit-graphs are rewritten on
top.

Use the trace2 API to signal the settings used during the write, and
check that output in a test after manually adjusting the correct bytes
in the commit-graph file.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Derrick Stolee
2020-07-01 13:27:24 +00:00
committed by Junio C Hamano
parent 949197420e
commit 0087a87ba8
5 changed files with 67 additions and 6 deletions
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5
Documentation/git-commit-graph.txt
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@ -60,7 +60,10 @@ existing commit-graph file.
With the `--changed-paths` option, compute and write information about the
paths changed between a commit and it's first parent. This operation can
take a while on large repositories. It provides significant performance gains
for getting history of a directory or a file with `git log -- <path>`.
for getting history of a directory or a file with `git log -- <path>`. If
this option is given, future commit-graph writes will automatically assume
that this option was intended. Use `--no-changed-paths` to stop storing this
data.
+
With the `--split` option, write the commit-graph as a chain of multiple
commit-graph files stored in `<dir>/info/commit-graphs`. The new commits