repo-settings: create feature.manyFiles setting

The feature.manyFiles setting is suitable for repos with many
files in the working directory. By setting index.version=4 and
core.untrackedCache=true, commands such as 'git status' should
improve.

While adding this setting, modify the index version precedence
tests to check how this setting overrides the default for
index.version is unset.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -345,6 +345,8 @@ include::config/difftool.txt[]
 
 include::config/fastimport.txt[]
 
+include::config/feature.txt[]
+
 include::config/fetch.txt[]
 
 include::config/format.txt[]

Documentation/config/core.txt

@@ -86,7 +86,9 @@ core.untrackedCache::
 	it will automatically be removed, if set to `false`. Before
 	setting it to `true`, you should check that mtime is working
 	properly on your system.
-	See linkgit:git-update-index[1]. `keep` by default.
+	See linkgit:git-update-index[1]. `keep` by default, unless
+	`feature.manyFiles` is enabled which sets this setting to
+	`true` by default.
 
 core.checkStat::
 	When missing or is set to `default`, many fields in the stat

Documentation/config/feature.txt

@@ -0,0 +1,15 @@
+feature.*::
+	The config settings that start with `feature.` modify the defaults of
+	a group of other config settings. These groups are created by the Git
+	developer community as recommended defaults and are subject to change.
+	In particular, new config options may be added with different defaults.
+
+feature.manyFiles::
+	Enable config options that optimize for repos with many files in the
+	working directory. With many files, commands such as `git status` and
+	`git checkout` may be slow and these new defaults improve performance:
++
+* `index.version=4` enables path-prefix compression in the index.
++
+* `core.untrackedCache=true` enables the untracked cache. This setting assumes
+that mtime is working on your machine.

Documentation/config/index.txt

@@ -24,3 +24,4 @@ index.threads::
 index.version::
 	Specify the version with which new index files should be
 	initialized.  This does not affect existing repositories.
+	If `feature.manyFiles` is enabled, then the default is 4.

repo-settings.c

@@ -36,9 +36,12 @@ void prepare_repo_settings(struct repository *r)
 		free(strval);
 	}
 
-
 	if (!repo_config_get_bool(r, "pack.usesparse", &value))
 		r->settings.pack_use_sparse = value;
+	if (!repo_config_get_bool(r, "feature.manyfiles", &value) && value) {
+		UPDATE_DEFAULT_BOOL(r->settings.index_version, 4);
+		UPDATE_DEFAULT_BOOL(r->settings.core_untracked_cache, UNTRACKED_CACHE_WRITE);
+	}
 
 	/* Hack for test programs like test-dump-untracked-cache */
 	if (ignore_untracked_cache_config)

t/t1600-index.sh

@@ -59,17 +59,38 @@ test_expect_success 'out of bounds index.version issues warning' '
 	)
 '
 
-test_expect_success 'GIT_INDEX_VERSION takes precedence over config' '
+test_index_version () {
+	INDEX_VERSION_CONFIG=$1 &&
+	FEATURE_MANY_FILES=$2 &&
+	ENV_VAR_VERSION=$3
+	EXPECTED_OUTPUT_VERSION=$4 &&
 	(
 		rm -f .git/index &&
-		GIT_INDEX_VERSION=4 &&
-		export GIT_INDEX_VERSION &&
-		git config --add index.version 2 &&
+		rm -f .git/config &&
+		if test "$INDEX_VERSION_CONFIG" -ne 0
+		then
+			git config --add index.version $INDEX_VERSION_CONFIG
+		fi &&
+		git config --add feature.manyFiles $FEATURE_MANY_FILES
+		if test "$ENV_VAR_VERSION" -ne 0
+		then
+			GIT_INDEX_VERSION=$ENV_VAR_VERSION &&
+			export GIT_INDEX_VERSION
+		else
+			unset GIT_INDEX_VERSION
+		fi &&
 		git add a 2>&1 &&
-		echo 4 >expect &&
+		echo $EXPECTED_OUTPUT_VERSION >expect &&
 		test-tool index-version <.git/index >actual &&
 		test_cmp expect actual
 	)
+}
+
+test_expect_success 'index version config precedence' '
+	test_index_version 2 false 4 4 &&
+	test_index_version 2 true 0 2 &&
+	test_index_version 0 true 0 4 &&
+	test_index_version 0 true 2 2
 '
 
 test_done