From: Taylor Blau Date: Wed, 14 Jan 2026 19:54:45 +0000 (-0500) Subject: midx: do not require packs to be sorted in lexicographic order X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=8de6741d3a5d50c0ade05661d069077d92f735fc;p=thirdparty%2Fgit.git midx: do not require packs to be sorted in lexicographic order The MIDX file format currently requires that pack files be identified by the lexicographic ordering of their names (that is, a pack having a checksum beginning with "abc" would have a numeric pack_int_id which is smaller than the same value for a pack beginning with "bcd"). As a result, it is impossible to combine adjacent MIDX layers together without permuting bits from bitmaps that are in more recent layer(s). To see why, consider the following example: | packs | preferred pack --------+-------------+--------------- MIDX #0 | { X, Y, Z } | Y MIDX #1 | { A, B, C } | B MIDX #2 | { D, E, F } | D , where MIDX #2's base MIDX is MIDX #1, and so on. Suppose that we want to combine MIDX layers #0 and #1, to create a new layer #0' containing the packs from both layers. With the original three MIDX layers, objects are laid out in the bitmap in the order they appear in their source pack, and the packs themselves are arranged according to the pseudo-pack order. In this case, that ordering is Y, X, Z, B, A, C. But recall that the pseudo-pack ordering is defined by the order that packs appear in the MIDX, with the exception of the preferred pack, which sorts ahead of all other packs regardless of its position within the MIDX. In the above example, that means that pack 'Y' could be placed anywhere (so long as it is designated as preferred), however, all other packs must be placed in the location listed above. Because that ordering isn't sorted lexicographically, it is impossible to compact MIDX layers in the above configuration without permuting the object-to-bit-position mapping. Changing this mapping would affect all bitmaps belonging to newer layers, rendering the bitmaps associated with MIDX #2 unreadable. One of the goals of MIDX compaction is that we are able to shrink the length of the MIDX chain *without* invalidating bitmaps that belong to newer layers, and the lexicographic ordering constraint is at odds with this goal. However, packs do not *need* to be lexicographically ordered within the MIDX. As far as I can gather, the only reason they are sorted lexically is to make it possible to perform a binary search over the pack names in a MIDX, necessary to make `midx_contains_pack()`'s performance logarithmic in the number of packs rather than linear. Relax this constraint by allowing MIDX writes to proceed with packs that are not arranged in lexicographic order. `midx_contains_pack()` will lazily instantiate a `pack_names_sorted` array on the MIDX, which will be used to implement the binary search over pack names. Because this change produces MIDXs which may not be correctly read with external tools or older versions of Git. Though older versions of Git know how to gracefully degrade and ignore any MIDX(s) they consider corrupt, external tools may not be as robust. To avoid unintentionally breaking any such tools, guard this change behind a version bump in the MIDX's on-disk format. Signed-off-by: Taylor Blau Signed-off-by: Junio C Hamano --- diff --git a/Documentation/gitformat-pack.adoc b/Documentation/gitformat-pack.adoc index 1b4db4aa61..5be2206b20 100644 --- a/Documentation/gitformat-pack.adoc +++ b/Documentation/gitformat-pack.adoc @@ -374,7 +374,7 @@ HEADER: The signature is: {'M', 'I', 'D', 'X'} 1-byte version number: - Git only writes or recognizes version 1. + Git only writes version 2, but recognizes versions 1 and 2. 1-byte Object Id Version We infer the length of object IDs (OIDs) from this value: @@ -413,7 +413,9 @@ CHUNK DATA: strings. There is no extra padding between the filenames, and they are listed in lexicographic order. The chunk itself is padded at the end with between 0 and 3 NUL bytes to make the - chunk size a multiple of 4 bytes. + chunk size a multiple of 4 bytes. Version 1 MIDXs are required to + list their packs in lexicographic order, but version 2 MIDXs may + list their packs in any arbitrary order. Bitmapped Packfiles (ID: {'B', 'T', 'M', 'P'}) Stores a table of two 4-byte unsigned integers in network order. diff --git a/midx-write.c b/midx-write.c index 8a54644e42..5c8700065a 100644 --- a/midx-write.c +++ b/midx-write.c @@ -36,10 +36,13 @@ extern int cmp_idx_or_pack_name(const char *idx_or_pack_name, static size_t write_midx_header(const struct git_hash_algo *hash_algo, struct hashfile *f, unsigned char num_chunks, - uint32_t num_packs) + uint32_t num_packs, int version) { + if (version != MIDX_VERSION_V1 && version != MIDX_VERSION_V2) + BUG("unexpected MIDX version: %d", version); + hashwrite_be32(f, MIDX_SIGNATURE); - hashwrite_u8(f, MIDX_VERSION); + hashwrite_u8(f, version); hashwrite_u8(f, oid_version(hash_algo)); hashwrite_u8(f, num_chunks); hashwrite_u8(f, 0); /* unused */ @@ -105,6 +108,8 @@ struct write_midx_context { uint32_t preferred_pack_idx; + int version; /* must be MIDX_VERSION_V1 or _V2 */ + int incremental; uint32_t num_multi_pack_indexes_before; @@ -410,7 +415,9 @@ static int write_midx_pack_names(struct hashfile *f, void *data) if (ctx->info[i].expired) continue; - if (i && strcmp(ctx->info[i].pack_name, ctx->info[i - 1].pack_name) <= 0) + if (ctx->version == MIDX_VERSION_V1 && + i && strcmp(ctx->info[i].pack_name, + ctx->info[i - 1].pack_name) <= 0) BUG("incorrect pack-file order: %s before %s", ctx->info[i - 1].pack_name, ctx->info[i].pack_name); @@ -1025,6 +1032,12 @@ static bool midx_needs_update(struct multi_pack_index *midx, struct write_midx_c if (!midx_checksum_valid(midx)) goto out; + /* + * If the version differs, we need to update. + */ + if (midx->version != ctx->version) + goto out; + /* * Ignore incremental updates for now. The assumption is that any * incremental update would be either empty (in which case we will bail @@ -1100,6 +1113,7 @@ static int write_midx_internal(struct write_midx_opts *opts) struct tempfile *incr; struct write_midx_context ctx = { .preferred_pack_idx = NO_PREFERRED_PACK, + .version = MIDX_VERSION_V2, }; struct multi_pack_index *midx_to_free = NULL; int bitmapped_packs_concat_len = 0; @@ -1114,6 +1128,10 @@ static int write_midx_internal(struct write_midx_opts *opts) ctx.repo = r; ctx.source = opts->source; + repo_config_get_int(ctx.repo, "midx.version", &ctx.version); + if (ctx.version != MIDX_VERSION_V1 && ctx.version != MIDX_VERSION_V2) + die(_("unknown MIDX version: %d"), ctx.version); + ctx.incremental = !!(opts->flags & MIDX_WRITE_INCREMENTAL); if (ctx.incremental) @@ -1445,7 +1463,7 @@ static int write_midx_internal(struct write_midx_opts *opts) } write_midx_header(r->hash_algo, f, get_num_chunks(cf), - ctx.nr - dropped_packs); + ctx.nr - dropped_packs, ctx.version); write_chunkfile(cf, &ctx); finalize_hashfile(f, midx_hash, FSYNC_COMPONENT_PACK_METADATA, diff --git a/midx.c b/midx.c index 19ef230d3f..1327d0a369 100644 --- a/midx.c +++ b/midx.c @@ -149,7 +149,7 @@ static struct multi_pack_index *load_multi_pack_index_one(struct odb_source *sou m->signature, MIDX_SIGNATURE); m->version = m->data[MIDX_BYTE_FILE_VERSION]; - if (m->version != MIDX_VERSION) + if (m->version != MIDX_VERSION_V1 && m->version != MIDX_VERSION_V2) die(_("multi-pack-index version %d not recognized"), m->version); @@ -210,7 +210,8 @@ static struct multi_pack_index *load_multi_pack_index_one(struct odb_source *sou die(_("multi-pack-index pack-name chunk is too short")); cur_pack_name = end + 1; - if (i && strcmp(m->pack_names[i], m->pack_names[i - 1]) <= 0) + if (m->version == MIDX_VERSION_V1 && + i && strcmp(m->pack_names[i], m->pack_names[i - 1]) <= 0) die(_("multi-pack-index pack names out of order: '%s' before '%s'"), m->pack_names[i - 1], m->pack_names[i]); @@ -411,6 +412,7 @@ void close_midx(struct multi_pack_index *m) } FREE_AND_NULL(m->packs); FREE_AND_NULL(m->pack_names); + FREE_AND_NULL(m->pack_names_sorted); free(m); } @@ -656,17 +658,40 @@ int cmp_idx_or_pack_name(const char *idx_or_pack_name, return strcmp(idx_or_pack_name, idx_name); } + +static int midx_pack_names_cmp(const void *a, const void *b, void *m_) +{ + struct multi_pack_index *m = m_; + return strcmp(m->pack_names[*(const size_t *)a], + m->pack_names[*(const size_t *)b]); +} + static int midx_contains_pack_1(struct multi_pack_index *m, const char *idx_or_pack_name) { uint32_t first = 0, last = m->num_packs; + if (m->version == MIDX_VERSION_V2 && !m->pack_names_sorted) { + uint32_t i; + + ALLOC_ARRAY(m->pack_names_sorted, m->num_packs); + + for (i = 0; i < m->num_packs; i++) + m->pack_names_sorted[i] = i; + + QSORT_S(m->pack_names_sorted, m->num_packs, midx_pack_names_cmp, + m); + } + while (first < last) { uint32_t mid = first + (last - first) / 2; const char *current; int cmp; - current = m->pack_names[mid]; + if (m->pack_names_sorted) + current = m->pack_names[m->pack_names_sorted[mid]]; + else + current = m->pack_names[mid]; cmp = cmp_idx_or_pack_name(idx_or_pack_name, current); if (!cmp) return 1; diff --git a/midx.h b/midx.h index a39bcc9d03..aa99a6cb21 100644 --- a/midx.h +++ b/midx.h @@ -11,7 +11,8 @@ struct git_hash_algo; struct odb_source; #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */ -#define MIDX_VERSION 1 +#define MIDX_VERSION_V1 1 +#define MIDX_VERSION_V2 2 #define MIDX_BYTE_FILE_VERSION 4 #define MIDX_BYTE_HASH_VERSION 5 #define MIDX_BYTE_NUM_CHUNKS 6 @@ -71,6 +72,7 @@ struct multi_pack_index { uint32_t num_packs_in_base; const char **pack_names; + size_t *pack_names_sorted; struct packed_git **packs; }; diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh index efeab4d22b..250d21dbd6 100755 --- a/t/t5319-multi-pack-index.sh +++ b/t/t5319-multi-pack-index.sh @@ -21,7 +21,7 @@ midx_read_expect () { EXTRA_CHUNKS="$5" { cat <<-EOF && - header: 4d494458 1 $HASH_LEN $NUM_CHUNKS $NUM_PACKS + header: 4d494458 2 $HASH_LEN $NUM_CHUNKS $NUM_PACKS chunks: pack-names oid-fanout oid-lookup object-offsets$EXTRA_CHUNKS num_objects: $NUM_OBJECTS packs: @@ -512,11 +512,6 @@ test_expect_success 'verify invalid chunk offset' ' "improper chunk offset(s)" ' -test_expect_success 'verify packnames out of order' ' - corrupt_midx_and_verify $MIDX_BYTE_PACKNAME_ORDER "z" $objdir \ - "pack names out of order" -' - test_expect_success 'verify missing pack' ' corrupt_midx_and_verify $MIDX_BYTE_PACKNAME_ORDER "a" $objdir \ "failed to load pack" @@ -578,6 +573,15 @@ test_expect_success 'verify incorrect checksum' ' $objdir "incorrect checksum" ' +test_expect_success 'setup for v1-specific fsck tests' ' + git -c midx.version=1 multi-pack-index write +' + +test_expect_success 'verify packnames out of order (v1)' ' + corrupt_midx_and_verify $MIDX_BYTE_PACKNAME_ORDER "z" $objdir \ + "pack names out of order" +' + test_expect_success 'repack progress off for redirected stderr' ' GIT_PROGRESS_DELAY=0 git multi-pack-index --object-dir=$objdir repack 2>err && test_line_count = 0 err