[PATCH 5/6] commit-graph: implement generation data chunk

["Abhishek Kumar via GitGitGadget" <gitgitgadget@gmail.com>]

From: Abhishek Kumar <abhishekkumar8222@gmail.com>

One of the essential pre-requisites before implementing generation
number as to distinguish between generation numbers v1 and v2 while
still being compatible with old Git.

We are going to introduce a new chunk called Generation Data chunk (or
GDAT). GDAT stores generation number v2 (and any subsequent versions),
whereas CDAT will still store topological level.

Old Git does not understand GDAT chunk and would ignore it, reading
topological levels from CDAT. Newer versions of Git can parse GDAT and
take advantage of newer generation numbers, falling back to topological
levels when GDAT chunk is missing (as it would happen with a commit
graph written by old Git).

Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
---
 commit-graph.c                | 33 +++++++++++++++++++++++++++++----
 commit-graph.h                |  1 +
 t/helper/test-read-graph.c    |  2 ++
 t/t4216-log-bloom.sh          |  4 ++--
 t/t5318-commit-graph.sh       | 19 +++++++++++--------
 t/t5324-split-commit-graph.sh | 12 ++++++------
 6 files changed, 51 insertions(+), 20 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index 1c98f38d69..ab714f4a76 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -38,11 +38,12 @@ void git_test_write_commit_graph_or_die(void)
 #define GRAPH_CHUNKID_OIDFANOUT 0x4f494446 /* "OIDF" */
 #define GRAPH_CHUNKID_OIDLOOKUP 0x4f49444c /* "OIDL" */
 #define GRAPH_CHUNKID_DATA 0x43444154 /* "CDAT" */
+#define GRAPH_CHUNKID_GENERATION_DATA 0x47444154 /* "GDAT" */
 #define GRAPH_CHUNKID_EXTRAEDGES 0x45444745 /* "EDGE" */
 #define GRAPH_CHUNKID_BLOOMINDEXES 0x42494458 /* "BIDX" */
 #define GRAPH_CHUNKID_BLOOMDATA 0x42444154 /* "BDAT" */
 #define GRAPH_CHUNKID_BASE 0x42415345 /* "BASE" */
-#define MAX_NUM_CHUNKS 7
+#define MAX_NUM_CHUNKS 8
 
 #define GRAPH_DATA_WIDTH (the_hash_algo->rawsz + 16)
 
@@ -389,6 +390,13 @@ struct commit_graph *parse_commit_graph(void *graph_map, size_t graph_size)
 				graph->chunk_commit_data = data + chunk_offset;
 			break;
 
+		case GRAPH_CHUNKID_GENERATION_DATA:
+			if (graph->chunk_generation_data)
+				chunk_repeated = 1;
+			else
+				graph->chunk_generation_data = data + chunk_offset;
+			break;
+
 		case GRAPH_CHUNKID_EXTRAEDGES:
 			if (graph->chunk_extra_edges)
 				chunk_repeated = 1;
@@ -768,7 +776,10 @@ static void fill_commit_graph_info(struct commit *item, struct commit_graph *g,
 	date_low = get_be32(commit_data + g->hash_len + 12);
 	item->date = (timestamp_t)((date_high << 32) | date_low);
 
-	graph_data->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
+	if (g->chunk_generation_data)
+		graph_data->generation = get_be32(g->chunk_generation_data + sizeof(uint32_t) * lex_index);
+	else
+		graph_data->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
 }
 
 static inline void set_commit_tree(struct commit *c, struct tree *t)
@@ -1100,6 +1111,17 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
 	}
 }
 
+static void write_graph_chunk_generation_data(struct hashfile *f,
+					      struct write_commit_graph_context *ctx)
+{
+	struct commit **list = ctx->commits.list;
+	int count;
+	for (count = 0; count < ctx->commits.nr; count++, list++) {
+		display_progress(ctx->progress, ++ctx->progress_cnt);
+		hashwrite_be32(f, commit_graph_data_at(*list)->generation);
+	}
+}
+
 static void write_graph_chunk_extra_edges(struct hashfile *f,
 					  struct write_commit_graph_context *ctx)
 {
@@ -1605,7 +1627,7 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 	uint64_t chunk_offsets[MAX_NUM_CHUNKS + 1];
 	const unsigned hashsz = the_hash_algo->rawsz;
 	struct strbuf progress_title = STRBUF_INIT;
-	int num_chunks = 3;
+	int num_chunks = 4;
 	struct object_id file_hash;
 	const struct bloom_filter_settings bloom_settings = DEFAULT_BLOOM_FILTER_SETTINGS;
 
@@ -1656,6 +1678,7 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 	chunk_ids[0] = GRAPH_CHUNKID_OIDFANOUT;
 	chunk_ids[1] = GRAPH_CHUNKID_OIDLOOKUP;
 	chunk_ids[2] = GRAPH_CHUNKID_DATA;
+	chunk_ids[3] = GRAPH_CHUNKID_GENERATION_DATA;
 	if (ctx->num_extra_edges) {
 		chunk_ids[num_chunks] = GRAPH_CHUNKID_EXTRAEDGES;
 		num_chunks++;
@@ -1677,8 +1700,9 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 	chunk_offsets[1] = chunk_offsets[0] + GRAPH_FANOUT_SIZE;
 	chunk_offsets[2] = chunk_offsets[1] + hashsz * ctx->commits.nr;
 	chunk_offsets[3] = chunk_offsets[2] + (hashsz + 16) * ctx->commits.nr;
+	chunk_offsets[4] = chunk_offsets[3] + sizeof(uint32_t) * ctx->commits.nr;
 
-	num_chunks = 3;
+	num_chunks = 4;
 	if (ctx->num_extra_edges) {
 		chunk_offsets[num_chunks + 1] = chunk_offsets[num_chunks] +
 						4 * ctx->num_extra_edges;
@@ -1728,6 +1752,7 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 	write_graph_chunk_fanout(f, ctx);
 	write_graph_chunk_oids(f, hashsz, ctx);
 	write_graph_chunk_data(f, hashsz, ctx);
+	write_graph_chunk_generation_data(f, ctx);
 	if (ctx->num_extra_edges)
 		write_graph_chunk_extra_edges(f, ctx);
 	if (ctx->changed_paths) {
diff --git a/commit-graph.h b/commit-graph.h
index 98cc5a3b9d..e3d4ba96f4 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -67,6 +67,7 @@ struct commit_graph {
 	const uint32_t *chunk_oid_fanout;
 	const unsigned char *chunk_oid_lookup;
 	const unsigned char *chunk_commit_data;
+	const unsigned char *chunk_generation_data;
 	const unsigned char *chunk_extra_edges;
 	const unsigned char *chunk_base_graphs;
 	const unsigned char *chunk_bloom_indexes;
diff --git a/t/helper/test-read-graph.c b/t/helper/test-read-graph.c
index 6d0c962438..1c2a5366c7 100644
--- a/t/helper/test-read-graph.c
+++ b/t/helper/test-read-graph.c
@@ -32,6 +32,8 @@ int cmd__read_graph(int argc, const char **argv)
 		printf(" oid_lookup");
 	if (graph->chunk_commit_data)
 		printf(" commit_metadata");
+	if (graph->chunk_generation_data)
+		printf(" generation_data");
 	if (graph->chunk_extra_edges)
 		printf(" extra_edges");
 	if (graph->chunk_bloom_indexes)
diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh
index c855bcd3e7..780855e691 100755
--- a/t/t4216-log-bloom.sh
+++ b/t/t4216-log-bloom.sh
@@ -33,11 +33,11 @@ test_expect_success 'setup test - repo, commits, commit graph, log outputs' '
 	git commit-graph write --reachable --changed-paths
 '
 graph_read_expect () {
-	NUM_CHUNKS=5
+	NUM_CHUNKS=6
 	cat >expect <<- EOF
 	header: 43475048 1 1 $NUM_CHUNKS 0
 	num_commits: $1
-	chunks: oid_fanout oid_lookup commit_metadata bloom_indexes bloom_data
+	chunks: oid_fanout oid_lookup commit_metadata generation_data bloom_indexes bloom_data
 	EOF
 	test-tool read-graph >actual &&
 	test_cmp expect actual
diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index 26f332d6a3..3ec5248d70 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -71,16 +71,16 @@ graph_git_behavior 'no graph' full commits/3 commits/1
 
 graph_read_expect() {
 	OPTIONAL=""
-	NUM_CHUNKS=3
+	NUM_CHUNKS=4
 	if test ! -z $2
 	then
 		OPTIONAL=" $2"
-		NUM_CHUNKS=$((3 + $(echo "$2" | wc -w)))
+		NUM_CHUNKS=$((4 + $(echo "$2" | wc -w)))
 	fi
 	cat >expect <<- EOF
 	header: 43475048 1 1 $NUM_CHUNKS 0
 	num_commits: $1
-	chunks: oid_fanout oid_lookup commit_metadata$OPTIONAL
+	chunks: oid_fanout oid_lookup commit_metadata generation_data$OPTIONAL
 	EOF
 	test-tool read-graph >output &&
 	test_cmp expect output
@@ -433,7 +433,7 @@ GRAPH_BYTE_HASH=5
 GRAPH_BYTE_CHUNK_COUNT=6
 GRAPH_CHUNK_LOOKUP_OFFSET=8
 GRAPH_CHUNK_LOOKUP_WIDTH=12
-GRAPH_CHUNK_LOOKUP_ROWS=5
+GRAPH_CHUNK_LOOKUP_ROWS=6
 GRAPH_BYTE_OID_FANOUT_ID=$GRAPH_CHUNK_LOOKUP_OFFSET
 GRAPH_BYTE_OID_LOOKUP_ID=$(($GRAPH_CHUNK_LOOKUP_OFFSET + \
 			    1 * $GRAPH_CHUNK_LOOKUP_WIDTH))
@@ -451,11 +451,14 @@ GRAPH_BYTE_COMMIT_TREE=$GRAPH_COMMIT_DATA_OFFSET
 GRAPH_BYTE_COMMIT_PARENT=$(($GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN))
 GRAPH_BYTE_COMMIT_EXTRA_PARENT=$(($GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN + 4))
 GRAPH_BYTE_COMMIT_WRONG_PARENT=$(($GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN + 3))
-GRAPH_BYTE_COMMIT_GENERATION=$(($GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN + 11))
 GRAPH_BYTE_COMMIT_DATE=$(($GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN + 12))
 GRAPH_COMMIT_DATA_WIDTH=$(($HASH_LEN + 16))
-GRAPH_OCTOPUS_DATA_OFFSET=$(($GRAPH_COMMIT_DATA_OFFSET + \
-			     $GRAPH_COMMIT_DATA_WIDTH * $NUM_COMMITS))
+GRAPH_GENERATION_DATA_OFFSET=$(($GRAPH_COMMIT_DATA_OFFSET + \
+				$GRAPH_COMMIT_DATA_WIDTH * $NUM_COMMITS))
+GRAPH_GENERATION_DATA_WIDTH=4
+GRAPH_BYTE_COMMIT_GENERATION=$(($GRAPH_GENERATION_DATA_OFFSET + 3))
+GRAPH_OCTOPUS_DATA_OFFSET=$(($GRAPH_GENERATION_DATA_OFFSET + \
+			     $GRAPH_GENERATION_DATA_WIDTH * $NUM_COMMITS))
 GRAPH_BYTE_OCTOPUS=$(($GRAPH_OCTOPUS_DATA_OFFSET + 4))
 GRAPH_BYTE_FOOTER=$(($GRAPH_OCTOPUS_DATA_OFFSET + 4 * $NUM_OCTOPUS_EDGES))
 
@@ -594,7 +597,7 @@ test_expect_success 'detect incorrect generation number' '
 '
 
 test_expect_success 'detect incorrect generation number' '
-	corrupt_graph_and_verify $GRAPH_BYTE_COMMIT_GENERATION "\01" \
+	corrupt_graph_and_verify $GRAPH_BYTE_COMMIT_GENERATION "\00" \
 		"non-zero generation number"
 '
 
diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index 269d0964a3..096a96ec41 100755
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -14,11 +14,11 @@ test_expect_success 'setup repo' '
 	graphdir="$infodir/commit-graphs" &&
 	test_oid_init &&
 	test_oid_cache <<-EOM
-	shallow sha1:1760
-	shallow sha256:2064
+	shallow sha1:2132
+	shallow sha256:2436
 
-	base sha1:1376
-	base sha256:1496
+	base sha1:1408
+	base sha256:1528
 	EOM
 '
 
@@ -29,9 +29,9 @@ graph_read_expect() {
 		NUM_BASE=$2
 	fi
 	cat >expect <<- EOF
-	header: 43475048 1 1 3 $NUM_BASE
+	header: 43475048 1 1 4 $NUM_BASE
 	num_commits: $1
-	chunks: oid_fanout oid_lookup commit_metadata
+	chunks: oid_fanout oid_lookup commit_metadata generation_data
 	EOF
 	test-tool read-graph >output &&
 	test_cmp expect output
-- 
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