The final pair of chunks for the multi-pack-index file stores the object
offsets. We default to using 32-bit offsets as in the pack-index version
1 format, but if there exists an offset larger than 32-bits, we use a
trick similar to the pack-index version 2 format by storing all offsets
at least 2^31 in a 64-bit table; we use the 32-bit table to point into
that 64-bit table as necessary.

We only store these 64-bit offsets if necessary, so create a test that
manipulates a version 2 pack-index to fake a large offset. This allows
us to test that the large offset table is created, but the data does not
match the actual packfile offsets. The multi-pack-index offset does match
the (corrupted) pack-index offset, so a future feature will compare these
offsets during a 'verify' step.

Signed-off-by: Derrick Stolee <dsto...@microsoft.com>
---
 Documentation/technical/pack-format.txt |  15 +++-
 midx.c                                  | 100 +++++++++++++++++++++++-
 object-store.h                          |   2 +
 t/helper/test-read-midx.c               |   4 +
 t/t5319-multi-pack-index.sh             |  45 ++++++++---
 5 files changed, 151 insertions(+), 15 deletions(-)

diff --git a/Documentation/technical/pack-format.txt 
b/Documentation/technical/pack-format.txt
index 3215f7bfcd..cab5bdd2ff 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -311,7 +311,20 @@ CHUNK DATA:
            The OIDs for all objects in the MIDX are stored in lexicographic
            order in this chunk.
 
-       (This section intentionally left incomplete.)
+       Object Offsets (ID: {'O', 'O', 'F', 'F'})
+           Stores two 4-byte values for every object.
+           1: The pack-int-id for the pack storing this object.
+           2: The offset within the pack.
+               If all offsets are less than 2^31, then the large offset chunk
+               will not exist and offsets are stored as in IDX v1.
+               If there is at least one offset value larger than 2^32-1, then
+               the large offset chunk must exist. If the large offset chunk
+               exists and the 31st bit is on, then removing that bit reveals
+               the row in the large offsets containing the 8-byte offset of
+               this object.
+
+       [Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
+           8-byte offsets into large packfiles.
 
 TRAILER:
 
diff --git a/midx.c b/midx.c
index 0f773e2585..71ca493107 100644
--- a/midx.c
+++ b/midx.c
@@ -14,13 +14,18 @@
 #define MIDX_HASH_LEN 20
 #define MIDX_MIN_SIZE (MIDX_HEADER_SIZE + MIDX_HASH_LEN)
 
-#define MIDX_MAX_CHUNKS 3
+#define MIDX_MAX_CHUNKS 5
 #define MIDX_CHUNK_ALIGNMENT 4
 #define MIDX_CHUNKID_PACKNAMES 0x504e414d /* "PNAM" */
 #define MIDX_CHUNKID_OIDFANOUT 0x4f494446 /* "OIDF" */
 #define MIDX_CHUNKID_OIDLOOKUP 0x4f49444c /* "OIDL" */
+#define MIDX_CHUNKID_OBJECTOFFSETS 0x4f4f4646 /* "OOFF" */
+#define MIDX_CHUNKID_LARGEOFFSETS 0x4c4f4646 /* "LOFF" */
 #define MIDX_CHUNKLOOKUP_WIDTH (sizeof(uint32_t) + sizeof(uint64_t))
 #define MIDX_CHUNK_FANOUT_SIZE (sizeof(uint32_t) * 256)
+#define MIDX_CHUNK_OFFSET_WIDTH (2 * sizeof(uint32_t))
+#define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))
+#define MIDX_LARGE_OFFSET_NEEDED 0x80000000
 
 static char *get_midx_filename(const char *object_dir)
 {
@@ -113,6 +118,14 @@ struct multi_pack_index *load_multi_pack_index(const char 
*object_dir)
                                m->chunk_oid_lookup = m->data + chunk_offset;
                                break;
 
+                       case MIDX_CHUNKID_OBJECTOFFSETS:
+                               m->chunk_object_offsets = m->data + 
chunk_offset;
+                               break;
+
+                       case MIDX_CHUNKID_LARGEOFFSETS:
+                               m->chunk_large_offsets = m->data + chunk_offset;
+                               break;
+
                        case 0:
                                die(_("terminating multi-pack-index chunk id 
appears earlier than expected"));
                                break;
@@ -132,6 +145,8 @@ struct multi_pack_index *load_multi_pack_index(const char 
*object_dir)
                die(_("multi-pack-index missing required OID fanout chunk"));
        if (!m->chunk_oid_lookup)
                die(_("multi-pack-index missing required OID lookup chunk"));
+       if (!m->chunk_object_offsets)
+               die(_("multi-pack-index missing required object offsets 
chunk"));
 
        m->num_objects = ntohl(m->chunk_oid_fanout[255]);
 
@@ -466,6 +481,56 @@ static size_t write_midx_oid_lookup(struct hashfile *f, 
unsigned char hash_len,
        return written;
 }
 
+static size_t write_midx_object_offsets(struct hashfile *f, int 
large_offset_needed,
+                                       struct pack_midx_entry *objects, 
uint32_t nr_objects)
+{
+       struct pack_midx_entry *list = objects;
+       uint32_t i, nr_large_offset = 0;
+       size_t written = 0;
+
+       for (i = 0; i < nr_objects; i++) {
+               struct pack_midx_entry *obj = list++;
+
+               hashwrite_be32(f, obj->pack_int_id);
+
+               if (large_offset_needed && obj->offset >> 31)
+                       hashwrite_be32(f, MIDX_LARGE_OFFSET_NEEDED | 
nr_large_offset++);
+               else if (!large_offset_needed && obj->offset >> 32)
+                       BUG("object %s requires a large offset (%"PRIx64") but 
the MIDX is not writing large offsets!",
+                           oid_to_hex(&obj->oid),
+                           obj->offset);
+               else
+                       hashwrite_be32(f, (uint32_t)obj->offset);
+
+               written += MIDX_CHUNK_OFFSET_WIDTH;
+       }
+
+       return written;
+}
+
+static size_t write_midx_large_offsets(struct hashfile *f, uint32_t 
nr_large_offset,
+                                      struct pack_midx_entry *objects, 
uint32_t nr_objects)
+{
+       struct pack_midx_entry *list = objects;
+       size_t written = 0;
+
+       while (nr_large_offset) {
+               struct pack_midx_entry *obj = list++;
+               uint64_t offset = obj->offset;
+
+               if (!(offset >> 31))
+                       continue;
+
+               hashwrite_be32(f, offset >> 32);
+               hashwrite_be32(f, offset & 0xffffffffUL);
+               written += 2 * sizeof(uint32_t);
+
+               nr_large_offset--;
+       }
+
+       return written;
+}
+
 int write_midx_file(const char *object_dir)
 {
        unsigned char cur_chunk, num_chunks = 0;
@@ -478,8 +543,9 @@ int write_midx_file(const char *object_dir)
        uint64_t written = 0;
        uint32_t chunk_ids[MIDX_MAX_CHUNKS + 1];
        uint64_t chunk_offsets[MIDX_MAX_CHUNKS + 1];
-       uint32_t nr_entries;
+       uint32_t nr_entries, num_large_offsets = 0;
        struct pack_midx_entry *entries = NULL;
+       int large_offsets_needed = 0;
 
        midx_name = get_midx_filename(object_dir);
        if (safe_create_leading_directories(midx_name)) {
@@ -506,13 +572,19 @@ int write_midx_file(const char *object_dir)
        sort_packs_by_name(packs.names, packs.nr, pack_perm);
 
        entries = get_sorted_entries(packs.list, pack_perm, packs.nr, 
&nr_entries);
+       for (i = 0; i < nr_entries; i++) {
+               if (entries[i].offset > 0x7fffffff)
+                       num_large_offsets++;
+               if (entries[i].offset > 0xffffffff)
+                       large_offsets_needed = 1;
+       }
 
        hold_lock_file_for_update(&lk, midx_name, LOCK_DIE_ON_ERROR);
        f = hashfd(lk.tempfile->fd, lk.tempfile->filename.buf);
        FREE_AND_NULL(midx_name);
 
        cur_chunk = 0;
-       num_chunks = 3;
+       num_chunks = large_offsets_needed ? 5 : 4;
 
        written = write_midx_header(f, num_chunks, packs.nr);
 
@@ -528,9 +600,21 @@ int write_midx_file(const char *object_dir)
        chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + 
MIDX_CHUNK_FANOUT_SIZE;
 
        cur_chunk++;
-       chunk_ids[cur_chunk] = 0;
+       chunk_ids[cur_chunk] = MIDX_CHUNKID_OBJECTOFFSETS;
        chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + nr_entries * 
MIDX_HASH_LEN;
 
+       cur_chunk++;
+       chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + nr_entries * 
MIDX_CHUNK_OFFSET_WIDTH;
+       if (large_offsets_needed) {
+               chunk_ids[cur_chunk] = MIDX_CHUNKID_LARGEOFFSETS;
+
+               cur_chunk++;
+               chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] +
+                                          num_large_offsets * 
MIDX_CHUNK_LARGE_OFFSET_WIDTH;
+       }
+
+       chunk_ids[cur_chunk] = 0;
+
        for (i = 0; i <= num_chunks; i++) {
                if (i && chunk_offsets[i] < chunk_offsets[i - 1])
                        BUG("incorrect chunk offsets: %"PRIu64" before %"PRIu64,
@@ -568,6 +652,14 @@ int write_midx_file(const char *object_dir)
                                written += write_midx_oid_lookup(f, 
MIDX_HASH_LEN, entries, nr_entries);
                                break;
 
+                       case MIDX_CHUNKID_OBJECTOFFSETS:
+                               written += write_midx_object_offsets(f, 
large_offsets_needed, entries, nr_entries);
+                               break;
+
+                       case MIDX_CHUNKID_LARGEOFFSETS:
+                               written += write_midx_large_offsets(f, 
num_large_offsets, entries, nr_entries);
+                               break;
+
                        default:
                                BUG("trying to write unknown chunk id %"PRIx32,
                                    chunk_ids[i]);
diff --git a/object-store.h b/object-store.h
index 3357e51100..07bcc80e02 100644
--- a/object-store.h
+++ b/object-store.h
@@ -100,6 +100,8 @@ struct multi_pack_index {
        const unsigned char *chunk_pack_names;
        const uint32_t *chunk_oid_fanout;
        const unsigned char *chunk_oid_lookup;
+       const unsigned char *chunk_object_offsets;
+       const unsigned char *chunk_large_offsets;
 
        const char **pack_names;
        char object_dir[FLEX_ARRAY];
diff --git a/t/helper/test-read-midx.c b/t/helper/test-read-midx.c
index d1bb7290ae..20771d1c1d 100644
--- a/t/helper/test-read-midx.c
+++ b/t/helper/test-read-midx.c
@@ -29,6 +29,10 @@ static int read_midx_file(const char *object_dir)
                printf(" oid_fanout");
        if (m->chunk_oid_lookup)
                printf(" oid_lookup");
+       if (m->chunk_object_offsets)
+               printf(" object_offsets");
+       if (m->chunk_large_offsets)
+               printf(" large_offsets");
 
        printf("\nnum_objects: %d\n", m->num_objects);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index ad0d447522..ccde83bca4 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -6,25 +6,28 @@ test_description='multi-pack-indexes'
 midx_read_expect() {
        NUM_PACKS=$1
        NUM_OBJECTS=$2
+       NUM_CHUNKS=$3
+       OBJECT_DIR=$4
+       EXTRA_CHUNKS="$5"
        cat >expect <<- EOF
-       header: 4d494458 1 3 $NUM_PACKS
-       chunks: pack_names oid_fanout oid_lookup
+       header: 4d494458 1 $NUM_CHUNKS $NUM_PACKS
+       chunks: pack_names oid_fanout oid_lookup object_offsets$EXTRA_CHUNKS
        num_objects: $NUM_OBJECTS
        packs:
        EOF
        if [ $NUM_PACKS -ge 1 ]
        then
-               ls pack/ | grep idx | sort >> expect
+               ls $OBJECT_DIR/pack/ | grep idx | sort >> expect
        fi
-       printf "object_dir: .\n" >>expect &&
-       test-tool read-midx . >actual &&
+       printf "object_dir: $OBJECT_DIR\n" >>expect &&
+       test-tool read-midx $OBJECT_DIR >actual &&
        test_cmp expect actual
 }
 
 test_expect_success 'write midx with no packs' '
        test_when_finished rm pack/multi-pack-index &&
        git multi-pack-index --object-dir=. write &&
-       midx_read_expect 0 0
+       midx_read_expect 0 0 4 .
 '
 
 test_expect_success 'create objects' '
@@ -55,13 +58,13 @@ 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 17
+       midx_read_expect 1 17 4 .
 '
 
 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 17
+       midx_read_expect 1 17 4 .
 '
 
 test_expect_success 'Add more objects' '
@@ -91,7 +94,7 @@ test_expect_success 'Add more objects' '
 test_expect_success 'write midx with two packs' '
        git pack-objects --index-version=1 pack/test-2 <obj-list2 &&
        git multi-pack-index --object-dir=. write &&
-       midx_read_expect 2 33
+       midx_read_expect 2 33 4 .
 '
 
 test_expect_success 'Add more packs' '
@@ -122,7 +125,29 @@ test_expect_success 'Add more packs' '
 
 test_expect_success 'write midx with twelve packs' '
        git multi-pack-index --object-dir=. write &&
-       midx_read_expect 12 73
+       midx_read_expect 12 73 4 .
+'
+
+
+# usage: corrupt_data <file> <pos> [<data>]
+corrupt_data() {
+       file=$1
+       pos=$2
+       data="${3:-\0}"
+       printf "$data" | dd of="$file" bs=1 seek="$pos" conv=notrunc
+}
+
+# Force 64-bit offsets by manipulating the idx file.
+# This makes the IDX file _incorrect_ so be careful to clean up after!
+test_expect_success 'force some 64-bit offsets with pack-objects' '
+       mkdir objects64 &&
+       mkdir objects64/pack &&
+       pack64=$(git pack-objects --index-version=2,0x40 objects64/pack/test-64 
<obj-list) &&
+       idx64=objects64/pack/test-64-$pack64.idx &&
+       chmod u+w $idx64 &&
+       corrupt_data $idx64 2899 "\02" &&
+       midx64=$(git multi-pack-index write --object-dir=objects64) &&
+       midx_read_expect 1 62 5 objects64 " large_offsets"
 '
 
 test_done
-- 
2.18.0.24.g1b579a2ee9

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