Subject: Re: [PATCH v4 03/10] commit-graph: consolidate fill_commit_graph_info

From: [email-masked] (Jakub Narębski)
Date: Sun, 25 Oct 2020 11:52:42 +0100

Hi Abhishek,

In short: everything is all right, except for the now duplicated test
names in t5000 after this commit.

"Abhishek Kumar via GitGitGadget" <[email-masked]> writes:

> From: Abhishek Kumar <[email-masked]>
>
> Both fill_commit_graph_info() and fill_commit_in_graph() parse
> information present in commit data chunk. Let's simplify the
> implementation by calling fill_commit_graph_info() within
> fill_commit_in_graph().
>
> fill_commit_graph_info() used to not load committer data from commit data
> chunk. However, with the corrected committer date, we have to load
> committer date to calculate generation number value.

Nice writeup, however the last sentence would in my opinion read better
in the future tense: we don't use generation number v2 yet.  For
example:

  However, with upcoming switch to using corrected committer date as
  generation number v2, we will have to load committer date to compute
  generation number value anyway.

Or something like that - notice the minor addition and changes.

The following is slightly unrelated change, but we agreed that it would
be better to not separate them; the need for change to the t5000 test is
caused by the change described above.

>
> e51217e15 (t5000: test tar files that overflow ustar headers,
> 30-06-2016) introduced a test 'generate tar with future mtime' that
> creates a commit with committer date of (2 ^ 36 + 1) seconds since
> EPOCH. The CDAT chunk provides 34-bits for storing committer date, thus
> committer time overflows into generation number (within CDAT chunk) and
> has undefined behavior.
>
> The test used to pass as fill_commit_graph_info() would not set struct
> member `date` of struct commit and loads committer date from the object
> database, generating a tar file with the expected mtime.

I think it should be s/loads/load/, as in "would load", but I am not a
native English speaker.

>
> However, with corrected commit date, we will load the committer date
> from CDAT chunk (truncated to lower 34-bits to populate the generation
> number. Thus, Git sets date and generates tar file with the truncated
> mtime.
>
> The ustar format (the header format used by most modern tar programs)
> only has room for 11 (or 12, depending om some implementations) octal
> digits for the size and mtime of each files.
>
> Thus, setting a timestamp of 2 ^ 33 + 1 would overflow the 11-octal
> digit implementations while still fitting into commit data chunk.
>
> Since we want to test 12-octal digit implementations of ustar as well,
> let's modify the existing test to no longer use commit-graph file.

The description above is for me does not make it entirely clear that we
add new test for handling possible 11-octal digit overflow nearly
identical to the existing one, and turn off use of commit-graph file for
test that checks handling 12-octal digit overflow.

> Signed-off-by: Abhishek Kumar <[email-masked]>
> ---
>  commit-graph.c      | 27 ++++++++++-----------------
>  t/t5000-tar-tree.sh | 20 +++++++++++++++++++-
>  2 files changed, 29 insertions(+), 18 deletions(-)
>
> diff --git a/commit-graph.c b/commit-graph.c
> index 94503e584b..e8362e144e 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -749,15 +749,24 @@ static void fill_commit_graph_info(struct commit *item, struct commit_graph *g,
>  	const unsigned char *commit_data;
>  	struct commit_graph_data *graph_data;
>  	uint32_t lex_index;
> +	uint64_t date_high, date_low;
>  
>  	while (pos < g->num_commits_in_base)
>  		g = g->base_graph;
>  
> +	if (pos >= g->num_commits + g->num_commits_in_base)
> +		die(_("invalid commit position. commit-graph is likely corrupt"));
> +
>  	lex_index = pos - g->num_commits_in_base;
>  	commit_data = g->chunk_commit_data + GRAPH_DATA_WIDTH * lex_index;
>  
>  	graph_data = commit_graph_data_at(item);
>  	graph_data->graph_pos = pos;
> +
> +	date_high = get_be32(commit_data + g->hash_len + 8) & 0x3;
> +	date_low = get_be32(commit_data + g->hash_len + 12);
> +	item->date = (timestamp_t)((date_high << 32) | date_low);
> +
>  	graph_data->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
>  }
>  
> @@ -772,38 +781,22 @@ static int fill_commit_in_graph(struct repository *r,
>  {
>  	uint32_t edge_value;
>  	uint32_t *parent_data_ptr;
> -	uint64_t date_low, date_high;
>  	struct commit_list **pptr;
> -	struct commit_graph_data *graph_data;
>  	const unsigned char *commit_data;
>  	uint32_t lex_index;
>  
>  	while (pos < g->num_commits_in_base)
>  		g = g->base_graph;
>  
> -	if (pos >= g->num_commits + g->num_commits_in_base)
> -		die(_("invalid commit position. commit-graph is likely corrupt"));
> +	fill_commit_graph_info(item, g, pos);
>  
> -	/*
> -	 * Store the "full" position, but then use the
> -	 * "local" position for the rest of the calculation.
> -	 */
> -	graph_data = commit_graph_data_at(item);
> -	graph_data->graph_pos = pos;
>  	lex_index = pos - g->num_commits_in_base;
> -
>  	commit_data = g->chunk_commit_data + (g->hash_len + 16) * lex_index;
>  
>  	item->object.parsed = 1;
>  
>  	set_commit_tree(item, NULL);
>  
> -	date_high = get_be32(commit_data + g->hash_len + 8) & 0x3;
> -	date_low = get_be32(commit_data + g->hash_len + 12);
> -	item->date = (timestamp_t)((date_high << 32) | date_low);
> -
> -	graph_data->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
> -
>  	pptr = &item->parents;
>  
>  	edge_value = get_be32(commit_data + g->hash_len);

All right, looks good for me.

Here second change begins.

> diff --git a/t/t5000-tar-tree.sh b/t/t5000-tar-tree.sh
> index 3ebb0d3b65..8f41cdc509 100755
> --- a/t/t5000-tar-tree.sh
> +++ b/t/t5000-tar-tree.sh
> @@ -431,11 +431,29 @@ test_expect_success TAR_HUGE,LONG_IS_64BIT 'system tar can read our huge size' '
>  	test_cmp expect actual
>  '
>  
> +test_expect_success TIME_IS_64BIT 'set up repository with far-future commit' '
> +	rm -f .git/index &&
> +	echo foo >file &&
> +	git add file &&
> +	GIT_COMMITTER_DATE="@17179869183 +0000" \
> +		git commit -m "tempori parendum"
> +'
> +
> +test_expect_success TIME_IS_64BIT 'generate tar with future mtime' '
> +	git archive HEAD >future.tar
> +'
> +
> +test_expect_success TAR_HUGE,TIME_IS_64BIT,TIME_T_IS_64BIT 'system tar can read our future mtime' '
> +	echo 2514 >expect &&
> +	tar_info future.tar | cut -d" " -f2 >actual &&
> +	test_cmp expect actual
> +'
> +

Everything is all right, except we now have duplicated test names.

Perhaps in the three following tests we should use 'far-far-future
commit' and 'far future mtime' in place of current 'far-future commit'
and 'future mtime' for tests checking handling 12-digital ditgits
overflow, or add description how far the future is, for example
'far-future commit (2^11 + 1)', etc.

>  test_expect_success TIME_IS_64BIT 'set up repository with far-future commit' '
>  	rm -f .git/index &&
>  	echo content >file &&
>  	git add file &&
> -	GIT_COMMITTER_DATE="@68719476737 +0000" \
> +	GIT_TEST_COMMIT_GRAPH=0 GIT_COMMITTER_DATE="@68719476737 +0000" \
>  		git commit -m "tempori parendum"
>  '

Best,
-- 
Jakub Narębski