config: create core.multiPackIndex setting

The core.multiPackIndex config setting controls the multi-pack-
index (MIDX) feature. If false, the setting will disable all reads
from the multi-pack-index file.

Read this config setting in the new prepare_multi_pack_index_one()
which is called during prepare_packed_git(). This check is run once
per repository.

Add comparison commands in t5319-multi-pack-index.sh to check
typical Git behavior remains the same as the config setting is turned
on and off. This currently includes 'git rev-list' and 'git log'
commands to trigger several object database reads. Currently, these
would only catch an error in the prepare_multi_pack_index_one(), but
with later commits will catch errors in object lookups, abbreviations,
and approximate object counts.

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

Documentation/config.txt

@@ -908,6 +908,11 @@ core.commitGraph::
 	Enable git commit graph feature. Allows reading from the
 	commit-graph file.
 
+core.multiPackIndex::
+	Use the multi-pack-index file to track multiple packfiles using a
+	single index. See link:technical/multi-pack-index.html[the
+	multi-pack-index design document].
+
 core.sparseCheckout::
 	Enable "sparse checkout" feature. See section "Sparse checkout" in
 	linkgit:git-read-tree[1] for more information.

midx.c

@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "config.h"
 #include "csum-file.h"
 #include "dir.h"
 #include "lockfile.h"
@@ -177,6 +178,30 @@ struct multi_pack_index *load_multi_pack_index(const char *object_dir)
 	return NULL;
 }
 
+int prepare_multi_pack_index_one(struct repository *r, const char *object_dir)
+{
+	struct multi_pack_index *m = r->objects->multi_pack_index;
+	struct multi_pack_index *m_search;
+	int config_value;
+
+	if (repo_config_get_bool(r, "core.multipackindex", &config_value) ||
+	    !config_value)
+		return 0;
+
+	for (m_search = m; m_search; m_search = m_search->next)
+		if (!strcmp(object_dir, m_search->object_dir))
+			return 1;
+
+	r->objects->multi_pack_index = load_multi_pack_index(object_dir);
+
+	if (r->objects->multi_pack_index) {
+		r->objects->multi_pack_index->next = m;
+		return 1;
+	}
+
+	return 0;
+}
+
 static size_t write_midx_header(struct hashfile *f,
 				unsigned char num_chunks,
 				uint32_t num_packs)

midx.h

@@ -1,7 +1,11 @@
 #ifndef __MIDX_H__
 #define __MIDX_H__
 
+#include "repository.h"
+
 struct multi_pack_index {
+	struct multi_pack_index *next;
+
 	int fd;
 
 	const unsigned char *data;
@@ -25,6 +29,7 @@ struct multi_pack_index {
 };
 
 struct multi_pack_index *load_multi_pack_index(const char *object_dir);
+int prepare_multi_pack_index_one(struct repository *r, const char *object_dir);
 
 int write_midx_file(const char *object_dir);
 

object-store.h

@@ -105,6 +105,13 @@ struct raw_object_store {
 	 */
 	struct oidmap *replace_map;
 
+	/*
+	 * private data
+	 *
+	 * should only be accessed directly by packfile.c and midx.c
+	 */
+	struct multi_pack_index *multi_pack_index;
+
 	/*
 	 * private data
 	 *

packfile.c

@@ -15,6 +15,7 @@
 #include "tree-walk.h"
 #include "tree.h"
 #include "object-store.h"
+#include "midx.h"
 
 char *odb_pack_name(struct strbuf *buf,
 		    const unsigned char *sha1,
@@ -935,10 +936,13 @@ static void prepare_packed_git(struct repository *r)
 
 	if (r->objects->packed_git_initialized)
 		return;
+	prepare_multi_pack_index_one(r, r->objects->objectdir);
 	prepare_packed_git_one(r, r->objects->objectdir, 1);
 	prepare_alt_odb(r);
-	for (alt = r->objects->alt_odb_list; alt; alt = alt->next)
+	for (alt = r->objects->alt_odb_list; alt; alt = alt->next) {
+		prepare_multi_pack_index_one(r, alt->path);
 		prepare_packed_git_one(r, alt->path, 0);
+	}
 	rearrange_packed_git(r);
 	prepare_packed_git_mru(r);
 	r->objects->packed_git_initialized = 1;

t/t5319-multi-pack-index.sh

@@ -3,6 +3,8 @@
 test_description='multi-pack-indexes'
 . ./test-lib.sh
 
+objdir=.git/objects
+
 midx_read_expect () {
 	NUM_PACKS=$1
 	NUM_OBJECTS=$2
@@ -76,18 +78,35 @@ test_expect_success 'create objects' '
 '
 
 test_expect_success 'write midx with one v1 pack' '
-	pack=$(git pack-objects --index-version=1 pack/test <obj-list) &&
-	test_when_finished rm pack/test-$pack.pack pack/test-$pack.idx pack/multi-pack-index &&
-	git multi-pack-index --object-dir=. write &&
-	midx_read_expect 1 18 4 .
+	pack=$(git pack-objects --index-version=1 $objdir/pack/test <obj-list) &&
+	test_when_finished rm $objdir/pack/test-$pack.pack \
+		$objdir/pack/test-$pack.idx $objdir/pack/multi-pack-index &&
+	git multi-pack-index --object-dir=$objdir write &&
+	midx_read_expect 1 18 4 $objdir
 '
 
+midx_git_two_modes () {
+	git -c core.multiPackIndex=false $1 >expect &&
+	git -c core.multiPackIndex=true $1 >actual &&
+	test_cmp expect actual
+}
+
+compare_results_with_midx () {
+	MSG=$1
+	test_expect_success "check normal git operations: $MSG" '
+		midx_git_two_modes "rev-list --objects --all" &&
+		midx_git_two_modes "log --raw"
+	'
+}
+
 test_expect_success 'write midx with one v2 pack' '
-	git pack-objects --index-version=2,0x40 pack/test <obj-list &&
-	git multi-pack-index --object-dir=. write &&
-	midx_read_expect 1 18 4 .
+	git pack-objects --index-version=2,0x40 $objdir/pack/test <obj-list &&
+	git multi-pack-index --object-dir=$objdir write &&
+	midx_read_expect 1 18 4 $objdir
 '
 
+compare_results_with_midx "one v2 pack"
+
 test_expect_success 'add more objects' '
 	for i in $(test_seq 6 10)
 	do
@@ -97,25 +116,31 @@ test_expect_success 'add more objects' '
 '
 
 test_expect_success 'write midx with two packs' '
-	git pack-objects --index-version=1 pack/test-2 <obj-list &&
-	git multi-pack-index --object-dir=. write &&
-	midx_read_expect 2 34 4 .
+	git pack-objects --index-version=1 $objdir/pack/test-2 <obj-list &&
+	git multi-pack-index --object-dir=$objdir write &&
+	midx_read_expect 2 34 4 $objdir
 '
 
+compare_results_with_midx "two packs"
+
 test_expect_success 'add more packs' '
 	for j in $(test_seq 11 20)
 	do
 		generate_objects $j &&
 		commit_and_list_objects &&
-		git pack-objects --index-version=2 pack/test-pack <obj-list
+		git pack-objects --index-version=2 $objdir/pack/test-pack <obj-list
 	done
 '
 
+compare_results_with_midx "mixed mode (two packs + extra)"
+
 test_expect_success 'write midx with twelve packs' '
-	git multi-pack-index --object-dir=. write &&
-	midx_read_expect 12 74 4 .
+	git multi-pack-index --object-dir=$objdir write &&
+	midx_read_expect 12 74 4 $objdir
 '
 
+compare_results_with_midx "twelve packs"
+
 # usage: corrupt_data <file> <pos> [<data>]
 corrupt_data () {
 	file=$1