[PATCH v6 06/18] commit-graph: rearrange chunk count logic

From: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Date: Jun 18, 2019, 18:14 UTC
Message-ID: <6595617bf19f7172cb9983f52a8faff4e463f795.1560881661.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.184.v6.git.gitgitgadget@gmail.com>

From: Derrick Stolee <dstolee@microsoft.com>

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>
---
 commit-graph.c | 35 +++++++++++++++++++++--------------
 1 file 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];

-- 
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