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 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. 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.computHash=false 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. Signed-off-by: Derrick Stolee <derrickstolee@xxxxxxxxxx> --- Documentation/config/index.txt | 8 ++++++++ read-cache.c | 22 +++++++++++++++++++++- t/t1600-index.sh | 8 ++++++++ 3 files changed, 37 insertions(+), 1 deletion(-) diff --git a/Documentation/config/index.txt b/Documentation/config/index.txt index 75f3a2d1054..709ba72f622 100644 --- a/Documentation/config/index.txt +++ b/Documentation/config/index.txt @@ -30,3 +30,11 @@ 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.computeHash:: + When enabled, compute the hash of the index file as it is written + and store the hash at the end of the content. This is enabled by + default. ++ +If you disable `index.computHash`, then older Git clients may report that +your index is corrupt during `git fsck`. diff --git a/read-cache.c b/read-cache.c index 32024029274..f24d96de4d3 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; + int all_zeroes = 1; + unsigned char *start, *end; if (hdr->hdr_signature != htonl(CACHE_SIGNATURE)) return error(_("bad signature 0x%08x"), hdr->hdr_signature); @@ -1827,10 +1829,23 @@ 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; + while (start < end) { + if (*start != 0) { + all_zeroes = 0; + break; + } + start++; + } + + if (all_zeroes) + 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; } @@ -2917,9 +2932,14 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile, int ieot_entries = 1; struct index_entry_offset_table *ieot = NULL; int nr, nr_threads; + int compute_hash; f = hashfd(tempfile->fd, tempfile->filename.buf); + if (!git_config_get_maybe_bool("index.computehash", &compute_hash) && + !compute_hash) + f->skip_hash = 1; + 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..24ab90ca047 100755 --- a/t/t1600-index.sh +++ b/t/t1600-index.sh @@ -103,4 +103,12 @@ test_expect_success 'index version config precedence' ' test_index_version 0 true 2 2 ' +test_expect_success 'index.computeHash config option' ' + ( + rm -f .git/index && + git -c index.computeHash=false add a && + git fsck + ) +' + test_done -- gitgitgadget
