commit-graph: rearrange chunk count logic

The number of chunks in a commit-graph file can change depending on
whether we need the Extra Edges Chunk. We are going to add more optional
chunks, and it will be helpful to rearrange this logic around the chunk
count before doing so.

Specifically, we need to finalize the number of chunks before writing
the commit-graph header. Further, we also need to fill out the chunk
lookup table dynamically and using "num_chunks" as we add optional
chunks is useful for adding optional chunks in the future.

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

1 files changed, 21 insertions, 14 deletions

diff --git a/commit-graph.c b/commit-graph.c
index 6f7961d071..f2163e109f 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1213,7 +1213,7 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 	uint64_t chunk_offsets[5];
 	const unsigned hashsz = the_hash_algo->rawsz;
 	struct strbuf progress_title = STRBUF_INIT;
-	int num_chunks = ctx->num_extra_edges ? 4 : 3;
+	int num_chunks = 3;
 
 	ctx->graph_name = get_commit_graph_filename(ctx->obj_dir);
 	if (safe_create_leading_directories(ctx->graph_name)) {
@@ -1226,27 +1226,34 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 	hold_lock_file_for_update(&lk, ctx->graph_name, LOCK_DIE_ON_ERROR);
 	f = hashfd(lk.tempfile->fd, lk.tempfile->filename.buf);
 
-	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 */
-
 	chunk_ids[0] = GRAPH_CHUNKID_OIDFANOUT;
 	chunk_ids[1] = GRAPH_CHUNKID_OIDLOOKUP;
 	chunk_ids[2] = GRAPH_CHUNKID_DATA;
-	if (ctx->num_extra_edges)
-		chunk_ids[3] = GRAPH_CHUNKID_EXTRAEDGES;
-	else
-		chunk_ids[3] = 0;
-	chunk_ids[4] = 0;
+	if (ctx->num_extra_edges) {
+		chunk_ids[num_chunks] = GRAPH_CHUNKID_EXTRAEDGES;
+		num_chunks++;
+	}
+
+	chunk_ids[num_chunks] = 0;
 
 	chunk_offsets[0] = 8 + (num_chunks + 1) * GRAPH_CHUNKLOOKUP_WIDTH;
 	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] + 4 * ctx->num_extra_edges;
+
+	num_chunks = 3;
+	if (ctx->num_extra_edges) {
+		chunk_offsets[num_chunks + 1] = chunk_offsets[num_chunks] +
+						4 * ctx->num_extra_edges;
+		num_chunks++;
+	}
+
+	hashwrite_be32(f, GRAPH_SIGNATURE);
+
+	hashwrite_u8(f, GRAPH_VERSION);
+	hashwrite_u8(f, oid_version());
+	hashwrite_u8(f, num_chunks);
+	hashwrite_u8(f, 0);
 
 	for (i = 0; i <= num_chunks; i++) {
 		uint32_t chunk_write[3];