From: Derrick Stolee <derrickstolee@xxxxxxxxxx> The previous change allowed skipping the hashing portion of the hashwrite API, using it instead as a buffered write API. Disabling the hashwrite can be particularly helpful when the write operation is in a critical path. One such critical path is the writing of the index. This operation is so critical that the sparse index was created specifically to reduce the size of the index to make these writes (and reads) faster. Following a similar approach to one used in the microsoft/git fork [1], add a new config option (index.skipHash) that allows disabling this hashing during the index write. The cost is that we can no longer validate the contents for corruption-at-rest using the trailing hash. [1] https://github.com/microsoft/git/commit/21fed2d91410f45d85279467f21d717a2db45201 While older Git versions will not recognize the null hash as a special case, the file format itself is still being met in terms of its structure. Using this null hash will still allow Git operations to function across older versions. The one exception is 'git fsck' which checks the hash of the index file. This used to be a check on every index read, but was split out to just the index in a33fc72fe91 (read-cache: force_verify_index_checksum, 2017-04-14) and released first in Git 2.13.0. Document the versions that relaxed these restrictions, with the optimistic expectation that this change will be included in Git 2.40.0. Here, we disable this check if the trailing hash is all zeroes. We add a warning to the config option that this may cause undesirable behavior with older Git versions. As a quick comparison, I tested 'git update-index --force-write' with and without index.skipHash=true on a copy of the Linux kernel repository. Benchmark 1: with hash Time (mean ± σ): 46.3 ms ± 13.8 ms [User: 34.3 ms, System: 11.9 ms] Range (min … max): 34.3 ms … 79.1 ms 82 runs Benchmark 2: without hash Time (mean ± σ): 26.0 ms ± 7.9 ms [User: 11.8 ms, System: 14.2 ms] Range (min … max): 16.3 ms … 42.0 ms 69 runs Summary 'without hash' ran 1.78 ± 0.76 times faster than 'with hash' These performance benefits are substantial enough to allow users the ability to opt-in to this feature, even with the potential confusion with older 'git fsck' versions. It is critical that this test is placed before the test_index_version tests, since those tests obliterate the .git/config file and hence lose the setting from GIT_TEST_DEFAULT_HASH, if set. Signed-off-by: Derrick Stolee <derrickstolee@xxxxxxxxxx> --- Documentation/config/index.txt | 9 +++++++++ read-cache.c | 12 +++++++++++- t/t1600-index.sh | 6 ++++++ 3 files changed, 26 insertions(+), 1 deletion(-) diff --git a/Documentation/config/index.txt b/Documentation/config/index.txt index 75f3a2d1054..5d62489c302 100644 --- a/Documentation/config/index.txt +++ b/Documentation/config/index.txt @@ -30,3 +30,12 @@ index.version:: Specify the version with which new index files should be initialized. This does not affect existing repositories. If `feature.manyFiles` is enabled, then the default is 4. + +index.skipHash:: + When enabled, do not compute the trailing hash for the index file. + Instead, write a trailing set of bytes with value zero, indicating + that the computation was skipped. ++ +If you enable `index.skipHash`, then Git clients older than 2.13.0 will +refuse to parse the index and Git clients older than 2.40.0 will report an +error during `git fsck`. diff --git a/read-cache.c b/read-cache.c index 46f5e497b14..3f7de8b2e20 100644 --- a/read-cache.c +++ b/read-cache.c @@ -1817,6 +1817,8 @@ static int verify_hdr(const struct cache_header *hdr, unsigned long size) git_hash_ctx c; unsigned char hash[GIT_MAX_RAWSZ]; int hdr_version; + unsigned char *start, *end; + struct object_id oid; if (hdr->hdr_signature != htonl(CACHE_SIGNATURE)) return error(_("bad signature 0x%08x"), hdr->hdr_signature); @@ -1827,10 +1829,16 @@ static int verify_hdr(const struct cache_header *hdr, unsigned long size) if (!verify_index_checksum) return 0; + end = (unsigned char *)hdr + size; + start = end - the_hash_algo->rawsz; + oidread(&oid, start); + if (oideq(&oid, null_oid())) + return 0; + the_hash_algo->init_fn(&c); the_hash_algo->update_fn(&c, hdr, size - the_hash_algo->rawsz); the_hash_algo->final_fn(hash, &c); - if (!hasheq(hash, (unsigned char *)hdr + size - the_hash_algo->rawsz)) + if (!hasheq(hash, end - the_hash_algo->rawsz)) return error(_("bad index file sha1 signature")); return 0; } @@ -2918,6 +2926,8 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile, f = hashfd(tempfile->fd, tempfile->filename.buf); + git_config_get_maybe_bool("index.skiphash", (int *)&f->skip_hash); + for (i = removed = extended = 0; i < entries; i++) { if (cache[i]->ce_flags & CE_REMOVE) removed++; diff --git a/t/t1600-index.sh b/t/t1600-index.sh index 010989f90e6..45feb0fc5d8 100755 --- a/t/t1600-index.sh +++ b/t/t1600-index.sh @@ -65,6 +65,12 @@ test_expect_success 'out of bounds index.version issues warning' ' ) ' +test_expect_success 'index.skipHash config option' ' + rm -f .git/index && + git -c index.skipHash=true add a && + git fsck +' + test_index_version () { INDEX_VERSION_CONFIG=$1 && FEATURE_MANY_FILES=$2 && -- gitgitgadget
