[PATCH v2 2/8] builtin/multi-pack-index.c: support `--stdin-packs` mode

[Taylor Blau <me@ttaylorr.com>]

To power a new `--write-midx` mode, `git repack` will want to write a
multi-pack index containing a certain set of packs in the repository.

This new option will be used by `git repack` to write a MIDX which
contains only the packs which will survive after the repack (that is, it
will exclude any packs which are about to be deleted).

This patch effectively exposes the function implemented in the previous
commit via the `git multi-pack-index` builtin. An alternative approach
would have been to call that function from the `git repack` builtin
directly, but this introduces awkward problems around closing and
reopening the object store, so the MIDX will be written out-of-process.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/git-multi-pack-index.txt |  4 ++++
 builtin/multi-pack-index.c             | 27 ++++++++++++++++++++++++++
 t/t5319-multi-pack-index.sh            | 15 ++++++++++++++
 3 files changed, 46 insertions(+)

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index a9df3dbd32..009c989ef8 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -45,6 +45,10 @@ write::
 
 	--[no-]bitmap::
 		Control whether or not a multi-pack bitmap is written.
+
+	--stdin-packs::
+		Write a multi-pack index containing only the set of
+		line-delimited pack index basenames provided over stdin.
 --
 
 verify::
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 73c0113b48..047647b5f2 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -47,6 +47,7 @@ static struct opts_multi_pack_index {
 	const char *preferred_pack;
 	unsigned long batch_size;
 	unsigned flags;
+	int stdin_packs;
 } opts;
 
 static struct option common_opts[] = {
@@ -61,6 +62,16 @@ static struct option *add_common_options(struct option *prev)
 	return parse_options_concat(common_opts, prev);
 }
 
+static void read_packs_from_stdin(struct string_list *to)
+{
+	struct strbuf buf = STRBUF_INIT;
+	while (strbuf_getline(&buf, stdin) != EOF)
+		string_list_append(to, buf.buf);
+	string_list_sort(to);
+
+	strbuf_release(&buf);
+}
+
 static int cmd_multi_pack_index_write(int argc, const char **argv)
 {
 	struct option *options;
@@ -70,6 +81,8 @@ static int cmd_multi_pack_index_write(int argc, const char **argv)
 			   N_("pack for reuse when computing a multi-pack bitmap")),
 		OPT_BIT(0, "bitmap", &opts.flags, N_("write multi-pack bitmap"),
 			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
+		OPT_BOOL(0, "stdin-packs", &opts.stdin_packs,
+			 N_("write multi-pack index containing only given indexes")),
 		OPT_END(),
 	};
 
@@ -86,6 +99,20 @@ static int cmd_multi_pack_index_write(int argc, const char **argv)
 
 	FREE_AND_NULL(options);
 
+	if (opts.stdin_packs) {
+		struct string_list packs = STRING_LIST_INIT_DUP;
+		int ret;
+
+		read_packs_from_stdin(&packs);
+
+		ret = write_midx_file_only(opts.object_dir, &packs,
+					   opts.preferred_pack, opts.flags);
+
+		string_list_clear(&packs, 0);
+
+		return ret;
+
+	}
 	return write_midx_file(opts.object_dir, opts.preferred_pack,
 			       opts.flags);
 }
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index bb04f0f23b..385f0a3efd 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -168,6 +168,21 @@ test_expect_success 'write midx with two packs' '
 
 compare_results_with_midx "two packs"
 
+test_expect_success 'write midx with --stdin-packs' '
+	rm -fr $objdir/pack/multi-pack-index &&
+
+	idx="$(find $objdir/pack -name "test-2-*.idx")" &&
+	basename "$idx" >in &&
+
+	git multi-pack-index write --stdin-packs <in &&
+
+	test-tool read-midx $objdir | grep "\.idx$" >packs &&
+
+	test_cmp packs in
+'
+
+compare_results_with_midx "mixed mode (one pack + extra)"
+
 test_expect_success 'write progress off for redirected stderr' '
 	git multi-pack-index --object-dir=$objdir write 2>err &&
 	test_line_count = 0 err
-- 
2.33.0.96.g73915697e6