read-cache: add index.computeHash config option

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] microsoft@21fed2d 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@github.com>

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`.

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++;

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