Subject: [PATCH 14/34] commit-graph: check chunk sizes after writing

From: SZEDER Gábor <[email-masked]>
Date: Fri, 29 May 2020 10:50:18 +0200

In my experience while experimenting with new commit-graph chunks,
early versions of the corresponding new write_commit_graph_my_chunk()
functions are, sadly but not surprisingly, often buggy, and write more
or less data than they are supposed to, especially if the chunk size
is not directly proportional to the number of commits.  This then
causes all kinds of issues when reading such a bogus commit-graph
file, raising the question of whether the writing or the reading part
happens to be buggy this time.

Let's catch such issues early, already when writing the commit-graph
file, and check that each write_commit_graph_*() function wrote the
amount of data that it was expected to, and what has been encoded in
the Chunk Lookup table.  Now that all commit-graph chunks are written
in a loop we can do this check in a single place for all chunks, and
any chunks added in the future will get checked as well.

Signed-off-by: SZEDER Gábor <[email-masked]>
---
 commit-graph.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/commit-graph.c b/commit-graph.c
index fe26245fad..e3adffd58b 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1474,11 +1474,22 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 			progress_title.buf,
 			chunks_nr * ctx->commits.nr);
 	}
-	for (i = 0; i < chunks_nr; i++)
+	chunk_offset = f->total + f->offset;
+	for (i = 0; i < chunks_nr; i++) {
+		uint64_t end_offset;
+
 		if (chunks[i].write_fn(f, ctx)) {
 			ret = -1;
 			goto cleanup;
 		}
+
+		end_offset = f->total + f->offset;
+		if (end_offset - chunk_offset != chunks[i].size)
+			BUG("expected to write %lu bytes to chunk %08x, but wrote %lu instead",
+			    chunks[i].size, chunks[i].id,
+			    end_offset - chunk_offset);
+		chunk_offset = end_offset;
+	}
 	stop_progress(&ctx->progress);
 	strbuf_release(&progress_title);
 
-- 
2.27.0.rc1.431.g5c813f95dc