[PATCH 3/6] commit-graph: create new version flags

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <dstolee@microsoft.com>

In anticipation of a new commit-graph file format version, create
a flag for the write_commit_graph() and write_commit_graph_reachable()
methods to take a version number.

When there is no specified version, the implementation selects a
default value. Currently, the only valid value is 1.

The file format will change the header information, so place the
existing header logic inside a switch statement with only one case.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 commit-graph.c | 58 +++++++++++++++++++++++++++++++++-----------------
 commit-graph.h |  1 +
 2 files changed, 40 insertions(+), 19 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index 28fe2378be..f7f45893fd 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -25,9 +25,6 @@
 
 #define GRAPH_DATA_WIDTH (the_hash_algo->rawsz + 16)
 
-#define GRAPH_VERSION_1 0x1
-#define GRAPH_VERSION GRAPH_VERSION_1
-
 #define GRAPH_EXTRA_EDGES_NEEDED 0x80000000
 #define GRAPH_PARENT_MISSING 0x7fffffff
 #define GRAPH_EDGE_LAST_MASK 0x7fffffff
@@ -118,30 +115,35 @@ struct commit_graph *load_commit_graph_one(const char *graph_file)
 	}
 
 	graph_version = *(unsigned char*)(data + 4);
-	if (graph_version != GRAPH_VERSION) {
+	if (graph_version != 1) {
 		error(_("graph version %X does not match version %X"),
-		      graph_version, GRAPH_VERSION);
-		goto cleanup_fail;
-	}
-
-	hash_version = *(unsigned char*)(data + 5);
-	if (hash_version != oid_version()) {
-		error(_("hash version %X does not match version %X"),
-		      hash_version, oid_version());
+		      graph_version, 1);
 		goto cleanup_fail;
 	}
 
 	graph = alloc_commit_graph();
 
+	switch (graph_version) {
+	case 1:
+		hash_version = *(unsigned char*)(data + 5);
+		if (hash_version != oid_version()) {
+			error(_("hash version %X does not match version %X"),
+			      hash_version, oid_version());
+			goto cleanup_fail;
+		}
+
+		graph->num_chunks = *(unsigned char*)(data + 6);
+		chunk_lookup = data + 8;
+		break;
+	}
+
 	graph->hash_len = the_hash_algo->rawsz;
-	graph->num_chunks = *(unsigned char*)(data + 6);
 	graph->graph_fd = fd;
 	graph->data = graph_map;
 	graph->data_len = graph_size;
 
 	last_chunk_id = 0;
 	last_chunk_offset = 8;
-	chunk_lookup = data + 8;
 	for (i = 0; i < graph->num_chunks; i++) {
 		uint32_t chunk_id = get_be32(chunk_lookup + 0);
 		uint64_t chunk_offset = get_be64(chunk_lookup + 4);
@@ -792,10 +794,22 @@ int write_commit_graph(const char *obj_dir,
 	int res = 0;
 	int append = flags & COMMIT_GRAPH_APPEND;
 	int report_progress = flags & COMMIT_GRAPH_PROGRESS;
+	int version = 0;
+	int header_size = 0;
 
 	if (!commit_graph_compatible(the_repository))
 		return 0;
 
+	if (flags & COMMIT_GRAPH_VERSION_1)
+		version = 1;
+	if (!version)
+		version = 1;
+	if (version != 1) {
+		error(_("unsupported commit-graph version %d"),
+		      version);
+		return 1;
+	}
+
 	oids.nr = 0;
 	approx_nr_objects = approximate_object_count();
 	oids.alloc = approx_nr_objects / 32;
@@ -980,10 +994,16 @@ int write_commit_graph(const char *obj_dir,
 
 	hashwrite_be32(f, GRAPH_SIGNATURE);
 
-	hashwrite_u8(f, GRAPH_VERSION);
-	hashwrite_u8(f, oid_version());
-	hashwrite_u8(f, num_chunks);
-	hashwrite_u8(f, 0); /* unused padding byte */
+	hashwrite_u8(f, version);
+
+	switch (version) {
+	case 1:
+		hashwrite_u8(f, oid_version());
+		hashwrite_u8(f, num_chunks);
+		hashwrite_u8(f, 0); /* unused padding byte */
+		header_size = 8;
+		break;
+	}
 
 	chunk_ids[0] = GRAPH_CHUNKID_OIDFANOUT;
 	chunk_ids[1] = GRAPH_CHUNKID_OIDLOOKUP;
@@ -994,7 +1014,7 @@ int write_commit_graph(const char *obj_dir,
 		chunk_ids[3] = 0;
 	chunk_ids[4] = 0;
 
-	chunk_offsets[0] = 8 + (num_chunks + 1) * GRAPH_CHUNKLOOKUP_WIDTH;
+	chunk_offsets[0] = header_size + (num_chunks + 1) * GRAPH_CHUNKLOOKUP_WIDTH;
 	chunk_offsets[1] = chunk_offsets[0] + GRAPH_FANOUT_SIZE;
 	chunk_offsets[2] = chunk_offsets[1] + hashsz * commits.nr;
 	chunk_offsets[3] = chunk_offsets[2] + (hashsz + 16) * commits.nr;
diff --git a/commit-graph.h b/commit-graph.h
index 83fa548138..e03df54e33 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -62,6 +62,7 @@ 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)
 
 int write_commit_graph_reachable(const char *obj_dir, int flags);
 int write_commit_graph(const char *obj_dir,
-- 
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