Re: [PATCH v2 4/4] commit-graph: fix generation number v2 overflow values

> From: Derrick Stolee <derrickstolee@github.com>
>
> The Generation Data Chunk was implemented and tested in e8b63005c
> (commit-graph: implement generation data chunk, 2021-01-16), but the
> test was carefully constructed to work on systems with 32-bit dates.
> Since the corrected commit date offsets still required more than 31
> bits, this triggered writing the generation_data_overflow chunk.
>
> However, upon closer look, the
> write_graph_chunk_generation_data_overflow() method writes the offsets
> to the chunk (as dictated by the format) but fill_commit_graph_info()
> treats the value in the chunk as if it is the full corrected commit date
> (not an offset). For some reason, this does not cause an issue when
> using the FUTURE_DATE specified in t5318-commit-graph.sh, but it does
> show up as a failure in 'git commit-graph verify' if we increase that
> FUTURE_DATE to be above four billion.
>
> Fix this error and update the test to require 64-bit dates so we can
> safely use this large value in our test.

> diff --git a/commit-graph.c b/commit-graph.c
> index 8e52bb09552..b86a6a634fe 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -806,7 +806,7 @@ static void fill_commit_graph_info(struct commit *item, struct commit_graph *g,
>  				die(_("commit-graph requires overflow generation data but has none"));
>  
>  			offset_pos = offset ^ CORRECTED_COMMIT_DATE_OFFSET_OVERFLOW;
> -			graph_data->generation = get_be64(g->chunk_generation_data_overflow + 8 * offset_pos);
> +			graph_data->generation = item->date + get_be64(g->chunk_generation_data_overflow + 8 * offset_pos);
>  		} else
>  			graph_data->generation = item->date + offset;
>  	} else
> diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
> index 1afee1c2705..f4ffaad661d 100755
> --- a/t/t5318-commit-graph.sh
> +++ b/t/t5318-commit-graph.sh
> @@ -815,6 +815,15 @@ test_expect_success 'corrupt commit-graph write (missing tree)' '
>  	)
>  '

> +# The remaining tests check timestamps that flow over
> +# 32-bits. The graph_git_behavior checks can't take a
> +# prereq, so just stop here if we are on a 32-bit machine.
> +
> +if ! test_have_prereq TIME_IS_64BIT || ! test_have_prereq TIME_T_IS_64BIT
> +then
> +	test_done
> +fi
> +

>  # We test the overflow-related code with the following repo history:
>  #
>  #               4:F - 5:N - 6:U
> @@ -832,10 +841,10 @@ test_expect_success 'corrupt commit-graph write (missing tree)' '
>  # The largest offset observed is 2 ^ 31, just large enough to overflow.
>  #
>  
> -test_expect_success 'set up and verify repo with generation data overflow chunk' '
> +test_expect_success TIME_IS_64BIT,TIME_T_IS_64BIT 'set up and verify repo with generation data overflow chunk' '
>  	objdir=".git/objects" &&
>  	UNIX_EPOCH_ZERO="@0 +0000" &&
> -	FUTURE_DATE="@2147483646 +0000" &&
> +	FUTURE_DATE="@4000000000 +0000" &&

>  	test_oid_cache <<-EOF &&
>  	oid_version sha1:1
>  	oid_version sha256:2
> @@ -867,4 +876,8 @@ test_expect_success 'set up and verify repo with generation data overflow chunk'
>  
>  graph_git_behavior 'generation data overflow chunk repo' repo left right
>  
> +# Do not add tests at the end of this file, unless they require 64-bit
> +# timestamps, since this portion of the script is only executed when
> +# time data types have 64 bits.
> +
>  test_done