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From: "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>
To: git@vger.kernel.org
Cc: jrnieder@gmail.com, Derrick Stolee <derrickstolee@github.com>,
	Derrick Stolee <derrickstolee@github.com>
Subject: [PATCH 02/30] read-cache: add index.computeHash config option
Date: Mon, 07 Nov 2022 18:35:36 +0000	[thread overview]
Message-ID: <030d76f52af654470026b0c4b1dfba2b6c996885.1667846164.git.gitgitgadget@gmail.com> (raw)
In-Reply-To: <pull.1408.git.1667846164.gitgitgadget@gmail.com>

From: Derrick Stolee <derrickstolee@github.com>

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@github.com>
---
 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


  parent reply	other threads:[~2022-11-07 18:36 UTC|newest]

Thread overview: 56+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-11-07 18:35 [PATCH 00/30] [RFC] extensions.refFormat and packed-refs v2 file format Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 01/30] hashfile: allow skipping the hash function Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` Derrick Stolee via GitGitGadget [this message]
2022-11-11 23:31   ` [PATCH 02/30] read-cache: add index.computeHash config option Elijah Newren
2022-11-14 16:30     ` Derrick Stolee
2022-11-17 16:13   ` Ævar Arnfjörð Bjarmason
2022-11-07 18:35 ` [PATCH 03/30] extensions: add refFormat extension Derrick Stolee via GitGitGadget
2022-11-11 23:39   ` Elijah Newren
2022-11-16 14:37     ` Derrick Stolee
2022-11-07 18:35 ` [PATCH 04/30] config: fix multi-level bulleted list Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 05/30] repository: wire ref extensions to ref backends Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 06/30] refs: allow loose files without packed-refs Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 07/30] chunk-format: number of chunks is optional Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 08/30] chunk-format: document trailing table of contents Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 09/30] chunk-format: store chunk offset during write Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 10/30] chunk-format: allow trailing table of contents Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 11/30] chunk-format: parse " Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 12/30] refs: extract packfile format to new file Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 13/30] packed-backend: extract add_write_error() Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 14/30] packed-backend: extract iterator/updates merge Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 15/30] packed-backend: create abstraction for writing refs Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 16/30] config: add config values for packed-refs v2 Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 17/30] packed-backend: create shell of v2 writes Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 18/30] packed-refs: write file format version 2 Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 19/30] packed-refs: read file format v2 Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 20/30] packed-refs: read optional prefix chunks Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 21/30] packed-refs: write " Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 22/30] packed-backend: create GIT_TEST_PACKED_REFS_VERSION Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 23/30] t1409: test with packed-refs v2 Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 24/30] t5312: allow packed-refs v2 format Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 25/30] t5502: add PACKED_REFS_V1 prerequisite Derrick Stolee via GitGitGadget
2022-11-07 18:36 ` [PATCH 26/30] t3210: require packed-refs v1 for some tests Derrick Stolee via GitGitGadget
2022-11-07 18:36 ` [PATCH 27/30] t*: skip packed-refs v2 over http tests Derrick Stolee via GitGitGadget
2022-11-07 18:36 ` [PATCH 28/30] ci: run GIT_TEST_PACKED_REFS_VERSION=2 in some builds Derrick Stolee via GitGitGadget
2022-11-07 18:36 ` [PATCH 29/30] p1401: create performance test for ref operations Derrick Stolee via GitGitGadget
2022-11-07 18:36 ` [PATCH 30/30] refs: skip hashing when writing packed-refs v2 Derrick Stolee via GitGitGadget
2022-11-09 15:15 ` [PATCH 00/30] [RFC] extensions.refFormat and packed-refs v2 file format Derrick Stolee
2022-11-11 23:28 ` Elijah Newren
2022-11-14  0:07   ` Derrick Stolee
2022-11-15  2:47     ` Elijah Newren
2022-11-16 14:45       ` Derrick Stolee
2022-11-17  4:28         ` Elijah Newren
2022-11-18 23:31     ` Junio C Hamano
2022-11-19  0:41       ` Elijah Newren
2022-11-19  3:00         ` Taylor Blau
2022-11-30 15:31       ` Derrick Stolee
2022-11-28 18:56 ` Han-Wen Nienhuys
2022-11-30 15:16   ` Derrick Stolee
2022-11-30 15:38     ` Phillip Wood
2022-11-30 16:37     ` Taylor Blau
2022-11-30 18:30     ` Han-Wen Nienhuys
2022-11-30 18:37       ` Sean Allred
2022-12-01 20:18       ` Derrick Stolee
2022-12-02 16:46         ` Han-Wen Nienhuys
2022-12-02 18:24           ` Ævar Arnfjörð Bjarmason
2022-11-30 22:55     ` Junio C Hamano

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