[PATCH v4 5/6] pack-objects: create pack.useSparse setting

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <dstolee@microsoft.com>

The '--sparse' flag in 'git pack-objects' changes the algorithm
used to enumerate objects to one that is faster for individual
users pushing new objects that change only a small cone of the
working directory. The sparse algorithm is not recommended for a
server, which likely sends new objects that appear across the
entire working directory.

Create a 'pack.useSparse' setting that enables this new algorithm.
This allows 'git push' to use this algorithm without passing a
'--sparse' flag all the way through four levels of run_command()
calls.

If the '--no-sparse' flag is set, then this config setting is
overridden.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 Documentation/config/pack.txt  |  9 +++++++++
 builtin/pack-objects.c         |  4 ++++
 t/t5322-pack-objects-sparse.sh | 15 +++++++++++++++
 3 files changed, 28 insertions(+)

diff --git a/Documentation/config/pack.txt b/Documentation/config/pack.txt
index edac75c83f..425c73aa52 100644
--- a/Documentation/config/pack.txt
+++ b/Documentation/config/pack.txt
@@ -105,6 +105,15 @@ pack.useBitmaps::
 	true. You should not generally need to turn this off unless
 	you are debugging pack bitmaps.
 
+pack.useSparse::
+	When true, git will default to using the '--sparse' option in
+	'git pack-objects' when the '--revs' option is present. This
+	algorithm only walks trees that appear in paths that introduce new
+	objects. This can have significant performance benefits when
+	computing a pack to send a small change. However, it is possible
+	that extra objects are added to the pack-file if the included
+	commits contain certain types of direct renames.
+
 pack.writeBitmaps (deprecated)::
 	This is a deprecated synonym for `repack.writeBitmaps`.
 
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 7d5b0735e3..124b1bafc4 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -2711,6 +2711,10 @@ static int git_pack_config(const char *k, const char *v, void *cb)
 		use_bitmap_index_default = git_config_bool(k, v);
 		return 0;
 	}
+	if (!strcmp(k, "pack.usesparse")) {
+		sparse = git_config_bool(k, v);
+		return 0;
+	}
 	if (!strcmp(k, "pack.threads")) {
 		delta_search_threads = git_config_int(k, v);
 		if (delta_search_threads < 0)
diff --git a/t/t5322-pack-objects-sparse.sh b/t/t5322-pack-objects-sparse.sh
index 45dba6e014..8f5699bd91 100755
--- a/t/t5322-pack-objects-sparse.sh
+++ b/t/t5322-pack-objects-sparse.sh
@@ -121,4 +121,19 @@ test_expect_success 'sparse pack-objects' '
 	test_cmp expect_sparse_objects.txt sparse_objects.txt
 '
 
+test_expect_success 'pack.useSparse enables algorithm' '
+	git config pack.useSparse true &&
+	git pack-objects --stdout --revs <packinput.txt >sparse.pack &&
+	git index-pack -o sparse.idx sparse.pack &&
+	git show-index <sparse.idx | awk "{print \$2}" >sparse_objects.txt &&
+	test_cmp expect_sparse_objects.txt sparse_objects.txt
+'
+
+test_expect_success 'pack.useSparse overridden' '
+	git pack-objects --stdout --revs --no-sparse <packinput.txt >sparse.pack &&
+	git index-pack -o sparse.idx sparse.pack &&
+	git show-index <sparse.idx | awk "{print \$2}" >sparse_objects.txt &&
+	test_cmp expect_objects.txt sparse_objects.txt
+'
+
 test_done
-- 
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