features: feature.manyFiles implies fast index writes

The recent addition of the index.skipHash config option allows index
writes to speed up by skipping the hash computation for the trailing
checksum. This is particularly critical for repositories with many files
at HEAD, so add this config option to two cases where users in that
scenario may opt-in to such behavior:

 1. The feature.manyFiles config option enables some options that are
    helpful for repositories with many files at HEAD.

 2. 'scalar register' and 'scalar reconfigure' set config options that
    optimize for large repositories.

In both of these cases, set index.skipHash=true to gain this
speedup. Add tests that demonstrate the proper way that
index.skipHash=true can override feature.manyFiles=true.

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

Documentation/config/feature.txt

@@ -23,6 +23,11 @@ feature.manyFiles::
 	working directory. With many files, commands such as `git status` and
 	`git checkout` may be slow and these new defaults improve performance:
 +
+* `index.skipHash=true` speeds up index writes by not computing a trailing
+  checksum. Note that this will cause Git versions earlier than 2.13.0 to
+  refuse to parse the index and Git versions earlier than 2.40.0 will report
+  a corrupted index during `git fsck`.
++
 * `index.version=4` enables path-prefix compression in the index.
 +
 * `core.untrackedCache=true` enables the untracked cache. This setting assumes

read-cache.c

@@ -2927,7 +2927,8 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 
 	f = hashfd(tempfile->fd, tempfile->filename.buf);
 
-	repo_config_get_bool(r, "index.skiphash", &f->skip_hash);
+	prepare_repo_settings(r);
+	f->skip_hash = r->settings.index_skip_hash;
 
 	for (i = removed = extended = 0; i < entries; i++) {
 		if (cache[i]->ce_flags & CE_REMOVE)

repo-settings.c

@@ -47,6 +47,7 @@ void prepare_repo_settings(struct repository *r)
 	}
 	if (manyfiles) {
 		r->settings.index_version = 4;
+		r->settings.index_skip_hash = 1;
 		r->settings.core_untracked_cache = UNTRACKED_CACHE_WRITE;
 	}
 
@@ -61,6 +62,7 @@ void prepare_repo_settings(struct repository *r)
 	repo_cfg_bool(r, "pack.usesparse", &r->settings.pack_use_sparse, 1);
 	repo_cfg_bool(r, "core.multipackindex", &r->settings.core_multi_pack_index, 1);
 	repo_cfg_bool(r, "index.sparse", &r->settings.sparse_index, 0);
+	repo_cfg_bool(r, "index.skiphash", &r->settings.index_skip_hash, r->settings.index_skip_hash);
 
 	/*
 	 * The GIT_TEST_MULTI_PACK_INDEX variable is special in that

repository.h

@@ -42,6 +42,7 @@ struct repo_settings {
 	struct fsmonitor_settings *fsmonitor; /* lazily loaded */
 
 	int index_version;
+	int index_skip_hash;
 	enum untracked_cache_setting core_untracked_cache;
 
 	int pack_use_sparse;

scalar.c

@@ -143,6 +143,7 @@ static int set_recommended_config(int reconfigure)
 		{ "credential.validate", "false", 1 }, /* GCM4W-only */
 		{ "gc.auto", "0", 1 },
 		{ "gui.GCWarning", "false", 1 },
+		{ "index.skipHash", "false", 1 },
 		{ "index.threads", "true", 1 },
 		{ "index.version", "4", 1 },
 		{ "merge.stat", "false", 1 },

t/t1600-index.sh

@@ -73,6 +73,17 @@ test_expect_success 'index.skipHash config option' '
 	test_cmp expect hash &&
 	git fsck &&
 
+	rm -f .git/index &&
+	git -c feature.manyFiles=true add a &&
+	test_trailing_hash .git/index >hash &&
+	cmp expect hash &&
+
+	rm -f .git/index &&
+	git -c feature.manyFiles=true \
+	    -c index.skipHash=false add a &&
+	test_trailing_hash .git/index >hash &&
+	! cmp expect hash &&
+
 	test_commit start &&
 	git -c protocol.file.allow=always submodule add ./ sub &&
 	git config index.skipHash false &&