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[PATCH 3/3] multi-pack-index: use hash version byte

From
Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Date
Aug 14, 2020, 18:07 UTC
Message-ID
<b4645789adf26e46cea721896867135941fb7654.1597428440.git.gitgitgadget@gmail.com>
In-Reply-To
<pull.703.git.1597428440.gitgitgadget@gmail.com>
From: Derrick Stolee <dstolee@microsoft.com>

Similar to the commit-graph format, the multi-pack-index format has a byte in the header intended to track the hash version used to write the file. This allows one to interpret the hash length without having the context of the repository config specifying the hash length. This was not modified as part of the SHA-256 work because the hash length was automatically up-shifted due to that config.

Since we have this byte available, we can make the file formats more obviously incompatible instead of relying on other context from the repository.

Add a new oid_version() method in midx.c similar to the one in commit-graph.c. This is specifically made separate from that implementation to avoid artificially linking the formats.

The test impact requires a few more things than the corresponding change in the commit-graph format. Specifically, 'test-tool read-midx' was not writing anything about this header value to output. Since the value available in 'struct multi_pack_index' is hash_len instead of a version value, we output "20" or "32" instead of "1" or "2".

Since we want a user to not have their Git commands fail if their multi-pack-index has the incorrect hash version compared to the repository's hash version, we relax the die() to an error() in load_multi_pack_index(). This has some effect on 'git multi-pack-index verify' as we need to check that a failed parse of a file that exists is actually a verify error. For that test that checks the hash version matches, we change the corrupted byte from "2" to "3" to ensure the test fails for both hash algorithms.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 Documentation/technical/pack-format.txt |  7 +++-
 midx.c                                  | 32 ++++++++++++++----
 t/helper/test-read-midx.c               |  8 +++--
 t/t5319-multi-pack-index.sh             | 43 ++++++++++++++++++++++---
 4 files changed, 77 insertions(+), 13 deletions(-)
diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
index d3a142c652..16cf7e83aa 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -273,7 +273,12 @@ HEADER:
 	    Git only writes or recognizes version 1.
 
 	1-byte Object Id Version
-	    Git only writes or recognizes version 1 (SHA1).
+	    We infer the length of object IDs (OIDs) from this value:
+		1 => SHA-1
+		2 => SHA-256
+	    If the hash type does not match the repository's hash algorithm,
+	    the multi-pack-index file should be ignored with a warning
+	    presented to the user.
 
 	1-byte number of "chunks"
 
diff --git a/midx.c b/midx.c
index a5fb797ede..0c165a40f5 100644
--- a/midx.c
+++ b/midx.c
@@ -17,7 +17,6 @@
 #define MIDX_BYTE_HASH_VERSION 5
 #define MIDX_BYTE_NUM_CHUNKS 6
 #define MIDX_BYTE_NUM_PACKS 8
-#define MIDX_HASH_VERSION 1
 #define MIDX_HEADER_SIZE 12
 #define MIDX_MIN_SIZE (MIDX_HEADER_SIZE + the_hash_algo->rawsz)
 
@@ -36,6 +35,15 @@
 
 #define PACK_EXPIRED UINT_MAX
 
+static uint8_t oid_version(void)
+{
+	if (the_hash_algo->rawsz == GIT_SHA1_RAWSZ)
+		return 1;
+	if (the_hash_algo->rawsz == GIT_SHA256_RAWSZ)
+		return 2;
+	die(_("invalid hash version"));
+}
+
 static char *get_midx_filename(const char *object_dir)
 {
 	return xstrfmt("%s/pack/multi-pack-index", object_dir);
@@ -90,8 +98,11 @@ struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local
 		      m->version);
 
 	hash_version = m->data[MIDX_BYTE_HASH_VERSION];
-	if (hash_version != MIDX_HASH_VERSION)
-		die(_("hash version %u does not match"), hash_version);
+	if (hash_version != oid_version()) {
+		error(_("multi-pack-index hash version %u does not match version %u"),
+		      hash_version, oid_version());
+		goto cleanup_fail;
+	}
 	m->hash_len = the_hash_algo->rawsz;
 
 	m->num_chunks = m->data[MIDX_BYTE_NUM_CHUNKS];
@@ -418,7 +429,7 @@ static size_t write_midx_header(struct hashfile *f,
 
 	hashwrite_be32(f, MIDX_SIGNATURE);
 	byte_values[0] = MIDX_VERSION;
-	byte_values[1] = MIDX_HASH_VERSION;
+	byte_values[1] = oid_version();
 	byte_values[2] = num_chunks;
 	byte_values[3] = 0; /* unused */
 	hashwrite(f, byte_values, sizeof(byte_values));
@@ -1105,8 +1116,17 @@ int verify_midx_file(struct repository *r, const char *object_dir, unsigned flag
 	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
 	verify_midx_error = 0;
 
-	if (!m)
-		return 0;
+	if (!m) {
+		int result = 0;
+		struct stat sb;
+		char *filename = get_midx_filename(object_dir);
+		if (!stat(filename, &sb)) {
+			error(_("multi-pack-index file exists, but failed to parse"));
+			result = 1;
+		}
+		free(filename);
+		return result;
+	}
 
 	if (flags & MIDX_PROGRESS)
 		progress = start_progress(_("Looking for referenced packfiles"),
diff --git a/t/helper/test-read-midx.c b/t/helper/test-read-midx.c
index 831b586d02..2430880f78 100644
--- a/t/helper/test-read-midx.c
+++ b/t/helper/test-read-midx.c
@@ -7,14 +7,18 @@
 static int read_midx_file(const char *object_dir)
 {
 	uint32_t i;
-	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+	struct multi_pack_index *m;
+
+	setup_git_directory();
+	m = load_multi_pack_index(object_dir, 1);
 
 	if (!m)
 		return 1;
 
-	printf("header: %08x %d %d %d\n",
+	printf("header: %08x %d %d %d %d\n",
 	       m->signature,
 	       m->version,
+	       m->hash_len,
 	       m->num_chunks,
 	       m->num_packs);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 7dfff0f8f4..09cbca4949 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -5,6 +5,8 @@ test_description='multi-pack-indexes'
 
 objdir=.git/objects
 
+HASH_LEN=$(test_oid rawsz)
+
 midx_read_expect () {
 	NUM_PACKS=$1
 	NUM_OBJECTS=$2
@@ -13,7 +15,7 @@ midx_read_expect () {
 	EXTRA_CHUNKS="$5"
 	{
 		cat <<-EOF &&
-		header: 4d494458 1 $NUM_CHUNKS $NUM_PACKS
+		header: 4d494458 1 $HASH_LEN $NUM_CHUNKS $NUM_PACKS
 		chunks: pack-names oid-fanout oid-lookup object-offsets$EXTRA_CHUNKS
 		num_objects: $NUM_OBJECTS
 		packs:
@@ -46,7 +48,7 @@ test_expect_success "don't write midx with no packs" '
 	test_path_is_missing pack/multi-pack-index
 '
 
-test_expect_success "Warn if a midx contains no oid" '
+test_expect_success SHA1 'warn if a midx contains no oid' '
 	cp "$TEST_DIRECTORY"/t5319/no-objects.midx $objdir/pack/multi-pack-index &&
 	test_must_fail git multi-pack-index verify &&
 	rm $objdir/pack/multi-pack-index
@@ -198,6 +200,40 @@ test_expect_success 'write midx with twelve packs' '
 
 compare_results_with_midx "twelve packs"
 
+test_expect_success 'warn on improper hash version' '
+	git init --object-format=sha1 sha1 &&
+	(
+		cd sha1 &&
+		git config core.multiPackIndex true &&
+		test_commit 1 &&
+		git repack -a &&
+		git multi-pack-index write &&
+		mv .git/objects/pack/multi-pack-index ../mpi-sha1
+	) &&
+	git init --object-format=sha256 sha256 &&
+	(
+		cd sha256 &&
+		git config core.multiPackIndex true &&
+		test_commit 1 &&
+		git repack -a &&
+		git multi-pack-index write &&
+		mv .git/objects/pack/multi-pack-index ../mpi-sha256
+	) &&
+	(
+		cd sha1 &&
+		mv ../mpi-sha256 .git/objects/pack/multi-pack-index &&
+		git log -1 2>err &&
+		test_i18ngrep "multi-pack-index hash version 2 does not match version 1" err
+	) &&
+	(
+		cd sha256 &&
+		mv ../mpi-sha1 .git/objects/pack/multi-pack-index &&
+		git log -1 2>err &&
+		test_i18ngrep "multi-pack-index hash version 1 does not match version 2" err
+	)
+'
+
+
 test_expect_success 'verify multi-pack-index success' '
 	git multi-pack-index verify --object-dir=$objdir
 '
@@ -243,7 +279,6 @@ test_expect_success 'verify bad signature' '
 		"multi-pack-index signature"
 '
 
-HASH_LEN=$(test_oid rawsz)
 NUM_OBJECTS=74
 MIDX_BYTE_VERSION=4
 MIDX_BYTE_OID_VERSION=5
@@ -272,7 +307,7 @@ test_expect_success 'verify bad version' '
 '
 
 test_expect_success 'verify bad OID version' '
-	corrupt_midx_and_verify $MIDX_BYTE_OID_VERSION "\02" $objdir \
+	corrupt_midx_and_verify $MIDX_BYTE_OID_VERSION "\03" $objdir \
 		"hash version"
 '
 

-- 
gitgitgadget
Previous: Martin ÅgrenNext: brian m. carlson
Message 13 of 22 in “SHA-256: Update commit-graph and multi-pack-index formats”
  1. 0/3 SHA-256: Update commit-graph and multi-pack-index formatsDerrick Stolee via GitGitGadget, Aug 14, 2020
  2. 1/3 t/README: document GIT_TEST_DEFAULT_HASHDerrick Stolee via GitGitGadget, Aug 14, 2020
  3. Junio C HamanoAug 14, 2020
  4. Eric SunshineAug 14, 2020
  5. Derrick StoleeAug 14, 2020
  6. 2/3 commit-graph: use the hash version byteDerrick Stolee via GitGitGadget, Aug 14, 2020
  7. Junio C HamanoAug 14, 2020
  8. Taylor BlauAug 14, 2020
  9. brian m. carlsonAug 14, 2020
  10. Junio C HamanoAug 14, 2020
  11. Derrick StoleeAug 14, 2020
  12. Martin ÅgrenAug 15, 2020
  13. 3/3 multi-pack-index: use hash version byteDerrick Stolee via GitGitGadget, Aug 14, 2020
  14. brian m. carlsonAug 14, 2020
  15. Junio C HamanoAug 14, 2020
  16. Derrick StoleeAug 14, 2020
  17. Junio C HamanoAug 14, 2020
  18. 0/3 SHA-256: Update commit-graph and multi-pack-index formatsDerrick Stolee via GitGitGadget, Aug 17, 2020
  19. 1/3 t/README: document GIT_TEST_DEFAULT_HASHDerrick Stolee via GitGitGadget, Aug 17, 2020
  20. 2/3 commit-graph: use the "hash version" byteDerrick Stolee via GitGitGadget, Aug 17, 2020
  21. 3/3 multi-pack-index: use hash version byteDerrick Stolee via GitGitGadget, Aug 17, 2020
  22. brian m. carlsonAug 17, 2020

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