Re: [PATCH v2 07/11] commit-graph: check chunk sizes after writing

> From: =?UTF-8?q?SZEDER=20G=C3=A1bor?= <szeder.dev@gmail.com>
>
> 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_graph_chunk_*() 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 <szeder.dev@gmail.com>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>  commit-graph.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/commit-graph.c b/commit-graph.c
> index 086fc2d070..1de6800d74 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -1683,12 +1683,21 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
>  			num_chunks * ctx->commits.nr);
>  	}
>
> +	chunk_offset = f->total + f->offset;
>  	for (i = 0; i < num_chunks; i++) {
> +		uint64_t end_offset;
> +

>  		if (chunks[i].write_fn(f, ctx)) {
>  			error(_("failed writing chunk with id %"PRIx32""),
>  			      chunks[i].id);
>  			return -1;
>  		}
> +
> +		end_offset = f->total + f->offset;
> +		if (end_offset - chunk_offset != chunks[i].size)
> +			BUG("expected to write %"PRId64" bytes to chunk %"PRIx32", but wrote %"PRId64" instead",
> +			    chunks[i].size, chunks[i].id, end_offset - chunk_offset);
> +		chunk_offset = end_offset;