Signed-off-by: Derrick Stolee <dsto...@microsoft.com>
---
 Documentation/technical/pack-format.txt |  5 +++
 builtin/midx.c                          |  7 ++++
 midx.c                                  | 56 +++++++++++++++++++++++--
 object-store.h                          |  2 +
 t/t5319-midx.sh                         | 11 +++--
 5 files changed, 75 insertions(+), 6 deletions(-)

diff --git a/Documentation/technical/pack-format.txt 
b/Documentation/technical/pack-format.txt
index 2b37be7b33..29bf87283a 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -296,6 +296,11 @@ CHUNK LOOKUP:
 
 CHUNK DATA:
 
+       Packfile Name Lookup (ID: {'P', 'L', 'O', 'O'}) (P * 4 bytes)
+           P * 4 bytes storing the offset in the packfile name chunk for
+           the null-terminated string containing the filename for the
+           ith packfile.
+
        Packfile Names (ID: {'P', 'N', 'A', 'M'})
            Stores the packfile names as concatenated, null-terminated strings.
            Packfiles must be listed in lexicographic order for fast lookups by
diff --git a/builtin/midx.c b/builtin/midx.c
index fe56560853..3a261e9bbf 100644
--- a/builtin/midx.c
+++ b/builtin/midx.c
@@ -16,6 +16,7 @@ static struct opts_midx {
 
 static int read_midx_file(const char *object_dir)
 {
+       uint32_t i;
        struct midxed_git *m = load_midxed_git(object_dir);
 
        if (!m)
@@ -30,11 +31,17 @@ static int read_midx_file(const char *object_dir)
 
        printf("chunks:");
 
+       if (m->chunk_pack_lookup)
+               printf(" pack_lookup");
        if (m->chunk_pack_names)
                printf(" pack_names");
 
        printf("\n");
 
+       printf("packs:\n");
+       for (i = 0; i < m->num_packs; i++)
+               printf("%s\n", m->pack_names[i]);
+
        printf("object_dir: %s\n", m->object_dir);
 
        return 0;
diff --git a/midx.c b/midx.c
index d4f4a01a51..923acda72e 100644
--- a/midx.c
+++ b/midx.c
@@ -13,8 +13,9 @@
 #define MIDX_HASH_LEN 20
 #define MIDX_MIN_SIZE (MIDX_HEADER_SIZE + MIDX_HASH_LEN)
 
-#define MIDX_MAX_CHUNKS 1
+#define MIDX_MAX_CHUNKS 2
 #define MIDX_CHUNK_ALIGNMENT 4
+#define MIDX_CHUNKID_PACKLOOKUP 0x504c4f4f /* "PLOO" */
 #define MIDX_CHUNKID_PACKNAMES 0x504e414d /* "PNAM" */
 #define MIDX_CHUNKLOOKUP_WIDTH (sizeof(uint32_t) + sizeof(uint64_t))
 
@@ -85,6 +86,10 @@ struct midxed_git *load_midxed_git(const char *object_dir)
                uint64_t chunk_offset = get_be64(m->data + 16 + 
MIDX_CHUNKLOOKUP_WIDTH * i);
 
                switch (chunk_id) {
+                       case MIDX_CHUNKID_PACKLOOKUP:
+                               m->chunk_pack_lookup = (uint32_t *)(m->data + 
chunk_offset);
+                               break;
+
                        case MIDX_CHUNKID_PACKNAMES:
                                m->chunk_pack_names = m->data + chunk_offset;
                                break;
@@ -102,9 +107,32 @@ struct midxed_git *load_midxed_git(const char *object_dir)
                }
        }
 
+       if (!m->chunk_pack_lookup)
+               die("MIDX missing required pack lookup chunk");
        if (!m->chunk_pack_names)
                die("MIDX missing required pack-name chunk");
 
+       m->pack_names = xcalloc(m->num_packs, sizeof(const char *));
+       for (i = 0; i < m->num_packs; i++) {
+               if (i) {
+                       if (ntohl(m->chunk_pack_lookup[i]) <= 
ntohl(m->chunk_pack_lookup[i - 1])) {
+                               error("MIDX pack lookup value %d before %d",
+                                     ntohl(m->chunk_pack_lookup[i - 1]),
+                                     ntohl(m->chunk_pack_lookup[i]));
+                               goto cleanup_fail;
+                       }
+               }
+
+               m->pack_names[i] = (const char *)(m->chunk_pack_names + 
ntohl(m->chunk_pack_lookup[i]));
+
+               if (i && strcmp(m->pack_names[i], m->pack_names[i - 1]) <= 0) {
+                       error("MIDX pack names out of order: '%s' before '%s'",
+                             m->pack_names[i - 1],
+                             m->pack_names[i]);
+                       goto cleanup_fail;
+               }
+       }
+
        return m;
 
 cleanup_fail:
@@ -162,6 +190,20 @@ static void sort_packs_by_name(char **pack_names, uint32_t 
nr_packs, uint32_t *p
        }
 }
 
+static size_t write_midx_pack_lookup(struct hashfile *f,
+                                    char **pack_names,
+                                    uint32_t nr_packs)
+{
+       uint32_t i, cur_len = 0;
+
+       for (i = 0; i < nr_packs; i++) {
+               hashwrite_be32(f, cur_len);
+               cur_len += strlen(pack_names[i]) + 1;
+       }
+
+       return sizeof(uint32_t) * (size_t)nr_packs;
+}
+
 static size_t write_midx_pack_names(struct hashfile *f,
                                    char **pack_names,
                                    uint32_t num_packs)
@@ -275,13 +317,17 @@ int write_midx_file(const char *object_dir)
        FREE_AND_NULL(midx_name);
 
        cur_chunk = 0;
-       num_chunks = 1;
+       num_chunks = 2;
 
        written = write_midx_header(f, num_chunks, nr_packs);
 
-       chunk_ids[cur_chunk] = MIDX_CHUNKID_PACKNAMES;
+       chunk_ids[cur_chunk] = MIDX_CHUNKID_PACKLOOKUP;
        chunk_offsets[cur_chunk] = written + (num_chunks + 1) * 
MIDX_CHUNKLOOKUP_WIDTH;
 
+       cur_chunk++;
+       chunk_ids[cur_chunk] = MIDX_CHUNKID_PACKNAMES;
+       chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + nr_packs * 
sizeof(uint32_t);
+
        cur_chunk++;
        chunk_ids[cur_chunk] = 0;
        chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + 
pack_name_concat_len;
@@ -311,6 +357,10 @@ int write_midx_file(const char *object_dir)
                            chunk_ids[i]);
 
                switch (chunk_ids[i]) {
+                       case MIDX_CHUNKID_PACKLOOKUP:
+                               written += write_midx_pack_lookup(f, 
pack_names, nr_packs);
+                               break;
+
                        case MIDX_CHUNKID_PACKNAMES:
                                written += write_midx_pack_names(f, pack_names, 
nr_packs);
                                break;
diff --git a/object-store.h b/object-store.h
index 199cf4bd44..1ba50459ca 100644
--- a/object-store.h
+++ b/object-store.h
@@ -100,8 +100,10 @@ struct midxed_git {
        uint32_t num_packs;
        uint32_t num_objects;
 
+       const uint32_t *chunk_pack_lookup;
        const unsigned char *chunk_pack_names;
 
+       const char **pack_names;
        char object_dir[FLEX_ARRAY];
 };
 
diff --git a/t/t5319-midx.sh b/t/t5319-midx.sh
index fdf4f84a90..a31c387c8f 100755
--- a/t/t5319-midx.sh
+++ b/t/t5319-midx.sh
@@ -6,10 +6,15 @@ test_description='multi-pack-indexes'
 midx_read_expect() {
        NUM_PACKS=$1
        cat >expect <<- EOF
-       header: 4d494458 1 1 1 $NUM_PACKS
-       chunks: pack_names
-       object_dir: .
+       header: 4d494458 1 1 2 $NUM_PACKS
+       chunks: pack_lookup pack_names
+       packs:
        EOF
+       if [ $NUM_PACKS -ge 1 ]
+       then
+               ls pack/ | grep idx | sort >> expect
+       fi
+       printf "object_dir: .\n" >>expect &&
        git midx read --object-dir=. >actual &&
        test_cmp expect actual
 }
-- 
2.18.0.rc1

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