commit-graph: implement file format version 2

The commit-graph file format had some shortcomings which we now
correct:

  1. The hash algorithm was determined by a single byte, instead
     of the 4-byte format identifier.

  2. There was no way to update the reachability index we used.
     We currently only support generation numbers, but that will
     change in the future.

  3. Git did not fail with error if the unused eighth byte was
     non-zero, so we could not use that to indicate an incremental
     file format without breaking compatibility across versions.

The new format modifies the header of the commit-graph to solve
these problems. We use the 4-byte hash format id, freeing up a byte
in our 32-bit alignment to introduce a reachability index version.
We can also fail to read the commit-graph if the eighth byte is
non-zero.

Update the 'git commit-graph read' subcommand to display the new
data.

Set the default file format version to 2, and adjust the tests to
expect the new 'git commit-graph read' output.

Add explicit tests for the upgrade path from version 1 to 2. Users
with an existing commit-graph with version 1 will seamlessly
upgrade to version 2 on their next write.

While we converted the existing 'verify' tests to use a version 1
file to avoid recalculating data offsets, add explicit 'verify'
tests on a version 2 file that corrupt the new header values.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: derrickstolee

Documentation/technical/commit-graph-format.txt

@@ -31,13 +31,22 @@ and hash type.
 
 All 4-byte numbers are in network order.
 
+There are two versions available, 1 and 2. These currently differ only in
+the header.
+
 HEADER:
 
+All commit-graph files use the first five bytes for the same purpose.
+
   4-byte signature:
       The signature is: {'C', 'G', 'P', 'H'}
 
   1-byte version number:
-      Currently, the only valid version is 1.
+      Currently, the valid version numbers are 1 and 2.
+
+The remainder of the header changes depending on the version.
+
+Version 1:
 
   1-byte Hash Version (1 = SHA-1)
       We infer the hash length (H) from this value.
@@ -47,6 +56,21 @@ HEADER:
   1-byte (reserved for later use)
      Current clients should ignore this value.
 
+Version 2:
+
+  1-byte number (C) of "chunks"
+
+  1-byte reachability index version number:
+      Currently, the only valid number is 1.
+
+  1-byte (reserved for later use)
+      Current clients expect this value to be zero, and will not
+      try to read the commit-graph file if it is non-zero.
+
+  4-byte format identifier for the hash algorithm:
+      If this identifier does not agree with the repository's current
+      hash algorithm, then the client will not read the commit graph.
+
 CHUNK LOOKUP:
 
   (C + 1) * 12 bytes listing the table of contents for the chunks:

builtin/commit-graph.c

@@ -117,6 +117,11 @@ static int graph_read(int argc, const char **argv)
 		*(unsigned char*)(graph->data + 5),
 		*(unsigned char*)(graph->data + 6),
 		*(unsigned char*)(graph->data + 7));
+
+	if (*(unsigned char *)(graph->data + 4) == 2)
+		printf("hash algorithm: %X\n",
+		       get_be32(graph->data + 8));
+
 	printf("num_commits: %u\n", graph->num_commits);
 	printf("chunks:");
 
@@ -177,6 +182,10 @@ static int graph_write(int argc, const char **argv)
 	case 1:
 		flags |= COMMIT_GRAPH_VERSION_1;
 		break;
+
+	case 2:
+		flags |= COMMIT_GRAPH_VERSION_2;
+		break;
 	}
 
 	read_replace_refs = 0;

commit-graph.c

@@ -152,7 +152,8 @@ struct commit_graph *parse_commit_graph(void *graph_map, int fd,
 	uint64_t last_chunk_offset;
 	uint32_t last_chunk_id;
 	uint32_t graph_signature;
-	unsigned char graph_version, hash_version;
+	unsigned char graph_version, hash_version, reach_index_version;
+	uint32_t hash_id;
 
 	if (!graph_map)
 		return NULL;
@@ -170,7 +171,7 @@ struct commit_graph *parse_commit_graph(void *graph_map, int fd,
 	}
 
 	graph_version = *(unsigned char*)(data + 4);
-	if (graph_version != 1) {
+	if (!graph_version || graph_version > 2) {
 		error(_("commit-graph version %X does not match version %X"),
 		      graph_version, 1);
 		return NULL;
@@ -190,6 +191,30 @@ struct commit_graph *parse_commit_graph(void *graph_map, int fd,
 		graph->num_chunks = *(unsigned char*)(data + 6);
 		chunk_lookup = data + 8;
 		break;
+
+	case 2:
+		graph->num_chunks = *(unsigned char *)(data + 5);
+
+		reach_index_version = *(unsigned char *)(data + 6);
+		if (reach_index_version != 1) {
+			error(_("unsupported reachability index version %d"),
+			      reach_index_version);
+			return NULL;
+		}
+
+		if (*(unsigned char*)(data + 7)) {
+			error(_("unsupported value in commit-graph header"));
+			return NULL;
+		}
+
+		hash_id = get_be32(data + 8);
+		if (hash_id != the_hash_algo->format_id) {
+			error(_("commit-graph hash algorithm does not match current algorithm"));
+			return NULL;
+		}
+
+		chunk_lookup = data + 12;
+		break;
 	}
 
 	graph->hash_len = the_hash_algo->rawsz;
@@ -898,9 +923,11 @@ int write_commit_graph(const char *obj_dir,
 
 	if (flags & COMMIT_GRAPH_VERSION_1)
 		version = 1;
+	if (flags & COMMIT_GRAPH_VERSION_2)
+		version = 2;
 	if (!version)
-		version = 1;
-	if (version != 1) {
+		version = 2;
+	if (version <= 0 || version > 2) {
 		error(_("unsupported commit-graph version %d"),
 		      version);
 		return 1;
@@ -1099,6 +1126,14 @@ int write_commit_graph(const char *obj_dir,
 		hashwrite_u8(f, 0); /* unused padding byte */
 		header_size = 8;
 		break;
+
+	case 2:
+		hashwrite_u8(f, num_chunks);
+		hashwrite_u8(f, 1); /* reachability index version */
+		hashwrite_u8(f, 0); /* unused padding byte */
+		hashwrite_be32(f, the_hash_algo->format_id);
+		header_size = 12;
+		break;
 	}
 
 	chunk_ids[0] = GRAPH_CHUNKID_OIDFANOUT;

commit-graph.h

@@ -68,6 +68,7 @@ int generation_numbers_enabled(struct repository *r);
 #define COMMIT_GRAPH_APPEND     (1 << 0)
 #define COMMIT_GRAPH_PROGRESS   (1 << 1)
 #define COMMIT_GRAPH_VERSION_1  (1 << 2)
+#define COMMIT_GRAPH_VERSION_2  (1 << 3)
 
 int write_commit_graph_reachable(const char *obj_dir, int flags);
 int write_commit_graph(const char *obj_dir,

t/t5318-commit-graph.sh

@@ -33,13 +33,13 @@ test_expect_success 'create commits and repack' '
 	git repack
 '
 
-graph_git_two_modes() {
+graph_git_two_modes () {
 	git -c core.commitGraph=true $1 >output
 	git -c core.commitGraph=false $1 >expect
 	test_cmp expect output
 }
 
-graph_git_behavior() {
+graph_git_behavior () {
 	MSG=$1
 	DIR=$2
 	BRANCH=$3
@@ -56,7 +56,7 @@ graph_git_behavior() {
 
 graph_git_behavior 'no graph' full commits/3 commits/1
 
-graph_read_expect() {
+graph_read_expect () {
 	OPTIONAL=""
 	NUM_CHUNKS=3
 	if test ! -z $2
@@ -65,7 +65,8 @@ graph_read_expect() {
 		NUM_CHUNKS=$((3 + $(echo "$2" | wc -w)))
 	fi
 	cat >expect <<- EOF
-	header: 43475048 1 1 $NUM_CHUNKS 0
+	header: 43475048 2 $NUM_CHUNKS 1 0
+	hash algorithm: 73686131
 	num_commits: $1
 	chunks: oid_fanout oid_lookup commit_metadata$OPTIONAL
 	EOF
@@ -320,6 +321,24 @@ test_expect_success 'replace-objects invalidates commit-graph' '
 	)
 '
 
+test_expect_success 'write v1 graph' '
+	git commit-graph write --reachable --version=1 &&
+	git commit-graph verify
+'
+
+graph_git_behavior 'version 1 graph, commit 8 vs merge 2' full commits/8 merge/2
+graph_git_behavior 'version 1 graph, commit 8 vs merge 2' full commits/8 merge/2
+
+test_expect_success 'upgrade from v1 to v2' '
+	git checkout -b new-commit-for-upgrade &&
+	test_commit force-upgrade &&
+	git commit-graph write --reachable --version=2 &&
+	git commit-graph verify
+'
+
+graph_git_behavior 'upgraded graph, commit 8 vs merge 2' full commits/8 merge/2
+graph_git_behavior 'upgraded graph, commit 8 vs merge 2' full commits/8 merge/2
+
 # the verify tests below expect the commit-graph to contain
 # exactly the commits reachable from the commits/8 branch.
 # If the file changes the set of commits in the list, then the
@@ -392,7 +411,7 @@ corrupt_graph_verify() {
 # starting at <zero_pos>, then runs 'git commit-graph verify'
 # and places the output in the file 'err'. Test 'err' for
 # the given string.
-corrupt_graph_and_verify() {
+corrupt_graph_and_verify () {
 	pos=$1
 	data="${2:-\0}"
 	grepstr=$3
@@ -424,10 +443,14 @@ test_expect_success 'detect bad signature' '
 '
 
 test_expect_success 'detect bad version' '
-	corrupt_graph_and_verify $GRAPH_BYTE_VERSION "\02" \
+	corrupt_graph_and_verify $GRAPH_BYTE_VERSION "\03" \
 		"graph version"
 '
 
+test_expect_success 'detect version 2 with version 1 data' '
+	corrupt_graph_and_verify $GRAPH_BYTE_VERSION "\02" \
+		"reachability index version"
+'
 test_expect_success 'detect bad hash version' '
 	corrupt_graph_and_verify $GRAPH_BYTE_HASH "\02" \
 		"hash version"
@@ -532,6 +555,37 @@ test_expect_success 'git fsck (checks commit-graph)' '
 	test_must_fail git fsck
 '
 
+test_expect_success 'rewrite commmit-graph with version 2' '
+	rm -f .git/objects/info/commit-graph &&
+	git commit-graph write --reachable --version=2 &&
+	git commit-graph verify
+'
+
+GRAPH_BYTE_CHUNK_COUNT=5
+GRAPH_BYTE_REACH_INDEX=6
+GRAPH_BYTE_UNUSED=7
+GRAPH_BYTE_HASH=8
+
+test_expect_success 'detect low chunk count (v2)' '
+	corrupt_graph_and_verify $GRAPH_BYTE_CHUNK_COUNT "\02" \
+		"missing the .* chunk"
+'
+
+test_expect_success 'detect incorrect reachability index' '
+	corrupt_graph_and_verify $GRAPH_BYTE_REACH_INDEX "\03" \
+		"reachability index version"
+'
+
+test_expect_success 'detect non-zero unused byte' '
+	corrupt_graph_and_verify $GRAPH_BYTE_UNUSED "\01" \
+		"unsupported value"
+'
+
+test_expect_success 'detect bad hash version (v2)' '
+	corrupt_graph_and_verify $GRAPH_BYTE_HASH "\00" \
+		"hash algorithm"
+'
+
 test_expect_success 'setup non-the_repository tests' '
 	rm -rf repo &&
 	git init repo &&