algo->final_oid_fn(oid, &ctx);
}
+/*
+ * Fast path: precomputed pack data for a 4 GiB + 1 all-NUL blob.
+ *
+ * The generated pack is almost entirely zeros with a small constant
+ * prefix, periodic deflate block headers, and a constant suffix
+ * containing the tree, two commits, and the pack checksum. Because
+ * every byte is deterministic for a given blob size and hash algorithm,
+ * we can write the pack without computing any hashes at all, reducing
+ * runtime from minutes of hash computation to seconds of pure I/O.
+ *
+ * The blob is stored as an uncompressed deflate stream: a two-byte
+ * zlib header, then 65538 blocks of up to 0xffff bytes each, followed
+ * by an adler32 checksum. The pack header and deflate framing are
+ * shared across hash algorithms; only the suffix (which contains OIDs
+ * and the pack checksum) differs.
+ *
+ * Constants were generated by running the generic path and extracting
+ * the non-zero bytes from the resulting pack file.
+ */
+
+#define FAST_PACK_4G1_BLOB_SIZE ((size_t)4 * 1024 * 1024 * 1024 + 1)
+#define FAST_PACK_4G1_N_FULL_BLOCKS 65537
+
+/*
+ * Per-hash-algorithm constants for the fast path. The prefix and
+ * deflate block structure are identical across algorithms; only the
+ * suffix (tree, commits, pack checksum) and the commit OID differ.
+ */
+struct fast_pack {
+ uint32_t format_id;
+ const unsigned char *suffix;
+ size_t suffix_len;
+ const char *commit_oid;
+};
+
+/* Pack header + pack object header + zlib header + first block header */
+static const unsigned char fast_pack_prefix[] = {
+ /* PACK header: signature, version 2, 5 objects */
+ 0x50, 0x41, 0x43, 0x4b, 0x00, 0x00, 0x00, 0x02,
+ 0x00, 0x00, 0x00, 0x05,
+ /* pack object header: blob, size = 4294967297 */
+ 0xb1, 0x80, 0x80, 0x80, 0x80, 0x01,
+ /* zlib header: CMF=0x78, FLG=0x01 */
+ 0x78, 0x01,
+ /* first non-final block header: BFINAL=0, LEN=0xffff, NLEN=0x0000 */
+ 0x00, 0xff, 0xff, 0x00, 0x00
+};
+
+/* Every non-final deflate block header is identical */
+static const unsigned char fast_pack_block_header[] = {
+ 0x00, 0xff, 0xff, 0x00, 0x00
+};
+
+/* Final block (2 data bytes) + adler32 of 4294967297 NUL bytes */
+static const unsigned char fast_pack_final_block[] = {
+ /* BFINAL=1, LEN=2, NLEN=0xfffd */
+ 0x01, 0x02, 0x00, 0xfd, 0xff,
+ /* 2 NUL data bytes */
+ 0x00, 0x00,
+ /* adler32 */
+ 0x00, 0xe2, 0x00, 0x01
+};
+
+/*
+ * SHA-1 suffix: tree, commit, empty tree, final commit, pack checksum.
+ */
+static const unsigned char fast_pack_sha1_suffix[] = {
+ 0xa0, 0x02, 0x78, 0x01, 0x01, 0x20, 0x00, 0xdf,
+ 0xff, 0x31, 0x30, 0x30, 0x36, 0x34, 0x34, 0x20,
+ 0x66, 0x69, 0x6c, 0x65, 0x00, 0x3e, 0xb7, 0xfe,
+ 0xb1, 0x41, 0x3c, 0x75, 0x7f, 0x0d, 0x81, 0x81,
+ 0xde, 0xb2, 0x8d, 0x1d, 0xab, 0x03, 0xd6, 0x48,
+ 0x46, 0xb4, 0xb4, 0x0c, 0x60, 0x95, 0x0b, 0x78,
+ 0x01, 0x01, 0xb5, 0x00, 0x4a, 0xff, 0x74, 0x72,
+ 0x65, 0x65, 0x20, 0x63, 0x36, 0x38, 0x33, 0x66,
+ 0x63, 0x63, 0x37, 0x64, 0x31, 0x64, 0x38, 0x33,
+ 0x65, 0x66, 0x32, 0x66, 0x65, 0x31, 0x61, 0x66,
+ 0x35, 0x35, 0x32, 0x31, 0x35, 0x64, 0x30, 0x31,
+ 0x36, 0x38, 0x64, 0x62, 0x35, 0x32, 0x61, 0x33,
+ 0x61, 0x33, 0x62, 0x0a, 0x61, 0x75, 0x74, 0x68,
+ 0x6f, 0x72, 0x20, 0x41, 0x20, 0x55, 0x20, 0x54,
+ 0x68, 0x6f, 0x72, 0x20, 0x3c, 0x61, 0x75, 0x74,
+ 0x68, 0x6f, 0x72, 0x40, 0x65, 0x78, 0x61, 0x6d,
+ 0x70, 0x6c, 0x65, 0x2e, 0x63, 0x6f, 0x6d, 0x3e,
+ 0x20, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
+ 0x38, 0x39, 0x30, 0x20, 0x2b, 0x30, 0x30, 0x30,
+ 0x30, 0x0a, 0x63, 0x6f, 0x6d, 0x6d, 0x69, 0x74,
+ 0x74, 0x65, 0x72, 0x20, 0x43, 0x20, 0x4f, 0x20,
+ 0x4d, 0x69, 0x74, 0x74, 0x65, 0x72, 0x20, 0x3c,
+ 0x63, 0x6f, 0x6d, 0x6d, 0x69, 0x74, 0x74, 0x65,
+ 0x72, 0x40, 0x65, 0x78, 0x61, 0x6d, 0x70, 0x6c,
+ 0x65, 0x2e, 0x63, 0x6f, 0x6d, 0x3e, 0x20, 0x31,
+ 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39,
+ 0x30, 0x20, 0x2b, 0x30, 0x30, 0x30, 0x30, 0x0a,
+ 0x0a, 0x4c, 0x61, 0x72, 0x67, 0x65, 0x20, 0x62,
+ 0x6c, 0x6f, 0x62, 0x20, 0x63, 0x6f, 0x6d, 0x6d,
+ 0x69, 0x74, 0x0a, 0xc6, 0x55, 0x37, 0x6b, 0x20,
+ 0x78, 0x01, 0x01, 0x00, 0x00, 0xff, 0xff, 0x00,
+ 0x00, 0x00, 0x01, 0x95, 0x0e, 0x78, 0x01, 0x01,
+ 0xe5, 0x00, 0x1a, 0xff, 0x74, 0x72, 0x65, 0x65,
+ 0x20, 0x34, 0x62, 0x38, 0x32, 0x35, 0x64, 0x63,
+ 0x36, 0x34, 0x32, 0x63, 0x62, 0x36, 0x65, 0x62,
+ 0x39, 0x61, 0x30, 0x36, 0x30, 0x65, 0x35, 0x34,
+ 0x62, 0x66, 0x38, 0x64, 0x36, 0x39, 0x32, 0x38,
+ 0x38, 0x66, 0x62, 0x65, 0x65, 0x34, 0x39, 0x30,
+ 0x34, 0x0a, 0x70, 0x61, 0x72, 0x65, 0x6e, 0x74,
+ 0x20, 0x63, 0x35, 0x62, 0x32, 0x31, 0x63, 0x36,
+ 0x31, 0x31, 0x61, 0x61, 0x35, 0x39, 0x34, 0x65,
+ 0x63, 0x39, 0x66, 0x64, 0x37, 0x65, 0x39, 0x32,
+ 0x63, 0x66, 0x39, 0x36, 0x34, 0x38, 0x39, 0x31,
+ 0x34, 0x63, 0x61, 0x34, 0x63, 0x32, 0x34, 0x31,
+ 0x32, 0x0a, 0x61, 0x75, 0x74, 0x68, 0x6f, 0x72,
+ 0x20, 0x41, 0x20, 0x55, 0x20, 0x54, 0x68, 0x6f,
+ 0x72, 0x20, 0x3c, 0x61, 0x75, 0x74, 0x68, 0x6f,
+ 0x72, 0x40, 0x65, 0x78, 0x61, 0x6d, 0x70, 0x6c,
+ 0x65, 0x2e, 0x63, 0x6f, 0x6d, 0x3e, 0x20, 0x31,
+ 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39,
+ 0x30, 0x20, 0x2b, 0x30, 0x30, 0x30, 0x30, 0x0a,
+ 0x63, 0x6f, 0x6d, 0x6d, 0x69, 0x74, 0x74, 0x65,
+ 0x72, 0x20, 0x43, 0x20, 0x4f, 0x20, 0x4d, 0x69,
+ 0x74, 0x74, 0x65, 0x72, 0x20, 0x3c, 0x63, 0x6f,
+ 0x6d, 0x6d, 0x69, 0x74, 0x74, 0x65, 0x72, 0x40,
+ 0x65, 0x78, 0x61, 0x6d, 0x70, 0x6c, 0x65, 0x2e,
+ 0x63, 0x6f, 0x6d, 0x3e, 0x20, 0x31, 0x32, 0x33,
+ 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x30, 0x20,
+ 0x2b, 0x30, 0x30, 0x30, 0x30, 0x0a, 0x0a, 0x45,
+ 0x6d, 0x70, 0x74, 0x79, 0x20, 0x74, 0x72, 0x65,
+ 0x65, 0x20, 0x63, 0x6f, 0x6d, 0x6d, 0x69, 0x74,
+ 0x0a, 0xaa, 0xb8, 0x45, 0x01, 0x8e, 0xfc, 0xf0,
+ 0x2f, 0x9c, 0xc5, 0xcc, 0x4f, 0x6a, 0x1a, 0xc9,
+ 0x2b, 0x23, 0xa9, 0xff, 0x91, 0x06, 0xc2, 0x70,
+ 0xe3
+};
+
+static const struct fast_pack fast_packs[] = {
+ {
+ .format_id = GIT_SHA1_FORMAT_ID,
+ .suffix = fast_pack_sha1_suffix,
+ .suffix_len = sizeof(fast_pack_sha1_suffix),
+ .commit_oid = "aac43daf40d0377af31aa9c798a4ae8a31b55c1d",
+ },
+};
+
+/*
+ * Try the fast path for known blob sizes. Returns 1 if the pack was
+ * written from precomputed constants, 0 if the caller should fall
+ * through to the generic path.
+ */
+static int generate_fast_pack(const char *path, size_t blob_size,
+ const struct git_hash_algo *algo)
+{
+ const struct fast_pack *fp = NULL;
+ FILE *f;
+ size_t i;
+
+ if (blob_size != FAST_PACK_4G1_BLOB_SIZE)
+ return 0;
+
+ for (i = 0; i < ARRAY_SIZE(fast_packs); i++) {
+ if (fast_packs[i].format_id == algo->format_id) {
+ fp = &fast_packs[i];
+ break;
+ }
+ }
+ if (!fp)
+ return 0;
+
+ f = xfopen(path, "wb");
+
+ fwrite_or_die(f, fast_pack_prefix, sizeof(fast_pack_prefix));
+
+ /* First full block: 0xffff zero bytes (header already in prefix) */
+ fwrite_or_die(f, zeros, BLOCK_SIZE);
+
+ /* Remaining non-final full blocks */
+ for (i = 1; i < FAST_PACK_4G1_N_FULL_BLOCKS; i++) {
+ fwrite_or_die(f, fast_pack_block_header,
+ sizeof(fast_pack_block_header));
+ fwrite_or_die(f, zeros, BLOCK_SIZE);
+ }
+
+ /* Final block (2 data bytes) + adler32 */
+ fwrite_or_die(f, fast_pack_final_block,
+ sizeof(fast_pack_final_block));
+
+ /* Tree, commits, and pack checksum */
+ fwrite_or_die(f, fp->suffix, fp->suffix_len);
+
+ if (fclose(f))
+ die_errno(_("could not close '%s'"), path);
+
+ printf("%s\n", fp->commit_oid);
+ return 1;
+}
+
/*
* Generate a pack file with a single large (>4GB) reachable object.
*
static int generate_pack_with_large_object(const char *path, size_t blob_size,
const struct git_hash_algo *algo)
{
- FILE *f = xfopen(path, "wb");
+ FILE *f;
struct git_hash_ctx pack_ctx;
unsigned char pack_hash[GIT_MAX_RAWSZ];
struct object_id blob_oid, tree_oid, commit_oid, empty_tree_oid, final_commit_oid;
.hdr_entries = htonl(object_count),
};
+ if (generate_fast_pack(path, blob_size, algo))
+ return 0;
+
+ f = xfopen(path, "wb");
+
algo->init_fn(&pack_ctx);
/* Write pack header */
projects / thirdparty / git.git / commitdiff
