multi-pack-index: add 'write' verb

In anticipation of writing multi-pack-indexes, add a skeleton
'git multi-pack-index write' subcommand and send the options to a
write_midx_file() method. Also create a skeleton test script that
tests the 'write' subcommand.

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

Documentation/git-multi-pack-index.txt

@@ -9,7 +9,7 @@ git-multi-pack-index - Write and verify multi-pack-indexes
 SYNOPSIS
 --------
 [verse]
-'git multi-pack-index' [--object-dir=<dir>]
+'git multi-pack-index' [--object-dir=<dir>] <verb>
 
 DESCRIPTION
 -----------
@@ -23,6 +23,26 @@ OPTIONS
 	`<dir>/packs/multi-pack-index` for the current MIDX file, and
 	`<dir>/packs` for the pack-files to index.
 
+write::
+	When given as the verb, write a new MIDX file to
+	`<dir>/packs/multi-pack-index`.
+
+
+EXAMPLES
+--------
+
+* Write a MIDX file for the packfiles in the current .git folder.
++
+-----------------------------------------------
+$ git multi-pack-index write
+-----------------------------------------------
+
+* Write a MIDX file for the packfiles in an alternate object store.
++
+-----------------------------------------------
+$ git multi-pack-index --object-dir <alt> write
+-----------------------------------------------
+
 
 SEE ALSO
 --------

Makefile

@@ -890,6 +890,7 @@ LIB_OBJS += merge.o
 LIB_OBJS += merge-blobs.o
 LIB_OBJS += merge-recursive.o
 LIB_OBJS += mergesort.o
+LIB_OBJS += midx.o
 LIB_OBJS += name-hash.o
 LIB_OBJS += notes.o
 LIB_OBJS += notes-cache.o

builtin/multi-pack-index.c

@@ -2,9 +2,10 @@
 #include "cache.h"
 #include "config.h"
 #include "parse-options.h"
+#include "midx.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>]"),
+	N_("git multi-pack-index [--object-dir=<dir>] write"),
 	NULL
 };
 
@@ -30,5 +31,17 @@ int cmd_multi_pack_index(int argc, const char **argv,
 	if (!opts.object_dir)
 		opts.object_dir = get_object_directory();
 
-	return 0;
+	if (argc == 0)
+		goto usage;
+
+	if (!strcmp(argv[0], "write")) {
+		if (argc > 1)
+			goto usage;
+
+		return write_midx_file(opts.object_dir);
+	}
+
+usage:
+	usage_with_options(builtin_multi_pack_index_usage,
+			   builtin_multi_pack_index_options);
 }

midx.c

@@ -0,0 +1,7 @@
+#include "cache.h"
+#include "midx.h"
+
+int write_midx_file(const char *object_dir)
+{
+	return 0;
+}

midx.h

@@ -0,0 +1,6 @@
+#ifndef __MIDX_H__
+#define __MIDX_H__
+
+int write_midx_file(const char *object_dir);
+
+#endif

t/t5319-multi-pack-index.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+test_description='multi-pack-indexes'
+. ./test-lib.sh
+
+test_expect_success 'write midx with no packs' '
+	git multi-pack-index --object-dir=. write
+'
+
+test_done