commit-graph: add new config for changed-paths & recommend it in scalar

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The changed-path Bloom filters feature has proven stable and reliable
over several years of use, delivering significant performance
improvement for file history computation in large monorepos. Currently
a user can opt-in to writing the changed-path Bloom filters using the
"--changed-paths" option to "git commit-graph write". The filters will
be persisted until the user drops the filters using the
"--no-changed-paths" option. For this functionality, refer to 0087a87
(commit-graph: persist existence of changed-paths, 2020-07-01).

Large monorepos using Git's background maintenance to build and update
commit-graph files could use an easy switch to enable this feature
without a foreground computation. In this commit, we're proposing a new
config option "commitGraph.changedPaths":

* "true" value will write Bloom filters, like "--changed-paths";
* "false" or "unset", existing Bloom filters will persist while new
Bloom filters will only be written when explicitly requested via
"--changed-paths". Note "false" can disable a previous "true" config
value but doesn't imply "--no-changed-paths".

This config will always respect the precedence of command line option
"--changed-paths" and "--no-changed-paths".

We also set this new config as optional recommended config in scalar to
turn on this feature for large repos.

Helped-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Emily Yang <emilyyang.git@gmail.com>

Author: emilyyang-ms

Documentation/config/commitgraph.adoc

@@ -8,6 +8,14 @@ commitGraph.maxNewFilters::
 	Specifies the default value for the `--max-new-filters` option of `git
 	commit-graph write` (c.f., linkgit:git-commit-graph[1]).
 
+commitGraph.changedPaths::
+	If true, then `git commit-graph write` will compute and write
+	changed-path Bloom filters by default, equivalent to passing
+	`--changed-paths`. If false or unset, existing Bloom filters will persist,
+	while new ones will only be written when explicitly requested via
+	`--changed-paths`. Command-line option `--changed-paths` always takes
+	precedence over this configuration. Defaults to unset.
+
 commitGraph.readChangedPaths::
 	Deprecated. Equivalent to commitGraph.changedPathsVersion=-1 if true, and
 	commitGraph.changedPathsVersion=0 if false. (If commitGraph.changedPathVersion

builtin/commit-graph.c

@@ -210,6 +210,8 @@ static int git_commit_graph_write_config(const char *var, const char *value,
 {
 	if (!strcmp(var, "commitgraph.maxnewfilters"))
 		write_opts.max_new_filters = git_config_int(var, value, ctx->kvi);
+	else if (!strcmp(var, "commitgraph.changedpaths"))
+		opts.enable_changed_paths = git_config_bool(var, value) ? 1 : -1;
 	/*
 	 * No need to fall-back to 'git_default_config', since this was already
 	 * called in 'cmd_commit_graph()'.

scalar.c

@@ -166,6 +166,7 @@ static int set_recommended_config(int reconfigure)
 #endif
 		/* Optional */
 		{ "status.aheadBehind", "false" },
+		{ "commitGraph.changedPaths", "true" },
 		{ "commitGraph.generationVersion", "1" },
 		{ "core.autoCRLF", "false" },
 		{ "core.safeCRLF", "false" },

t/t5318-commit-graph.sh

@@ -946,4 +946,48 @@ test_expect_success 'stale commit cannot be parsed when traversing graph' '
 	)
 '
 
+test_expect_success 'config commitGraph.changedPaths acts like --changed-paths' '
+	git init config-changed-paths &&
+	(
+		cd config-changed-paths &&
+
+		# commitGraph.changedPaths is not set and it should not write Bloom filters
+		test_commit first &&
+		GIT_PROGRESS_DELAY=0 git commit-graph write --reachable --progress 2>error &&
+		test_grep ! "Bloom filters" error &&
+
+		# Set commitGraph.changedPaths to true and it should write Bloom filters
+		test_commit second &&
+		git config commitGraph.changedPaths true &&
+		GIT_PROGRESS_DELAY=0 git commit-graph write --reachable --progress 2>error &&
+		test_grep "Bloom filters" error &&
+
+		# Add one more config commitGraph.changedPaths as false to disable the previous true config value
+		# It should still write Bloom filters due to existing filters
+		test_commit third &&
+		git config --add commitGraph.changedPaths false &&
+		GIT_PROGRESS_DELAY=0 git commit-graph write --reachable --progress 2>error &&
+		test_grep "Bloom filters" error &&
+
+		# commitGraph.changedPaths is still false and command line options should take precedence
+		test_commit fourth &&
+		GIT_PROGRESS_DELAY=0 git commit-graph write --no-changed-paths --reachable --progress 2>error &&
+		test_grep ! "Bloom filters" error &&
+		GIT_PROGRESS_DELAY=0 git commit-graph write --reachable --progress 2>error &&
+		test_grep ! "Bloom filters" error &&
+
+		# commitGraph.changedPaths is all cleared and then set to false again, command line options should take precedence
+		test_commit fifth &&
+		git config --unset-all commitGraph.changedPaths &&
+		git config commitGraph.changedPaths false &&
+		GIT_PROGRESS_DELAY=0 git commit-graph write --changed-paths --reachable --progress 2>error &&
+		test_grep "Bloom filters" error &&
+
+		# commitGraph.changedPaths is still false and it should write Bloom filters due to existing filters
+		test_commit sixth &&
+		GIT_PROGRESS_DELAY=0 git commit-graph write --reachable --progress 2>error &&
+		test_grep "Bloom filters" error
+	)
+'
+
 test_done