Subject: Re: [PATCH v3 13/20] commit-graph: verify generation number

From: Jakub Narebski <[email-masked]>
Date: Sat, 02 Jun 2018 14:23:53 +0200

Derrick Stolee <[email-masked]> writes:

> While iterating through the commit parents, perform the generation
> number calculation and compare against the value stored in the
> commit-graph.

All right, that's good.

What about commit-graph files that have GENERATION_NUMBER_ZERO for all
its commits (because we verify single commit-graph file only, there
wouldn't be GENERATION_NUMBER_ZERO mixed with non-zero generation
numbers)?

Unless we can assume that no commit-graph file in the wild would have
GENERATION_NUMBER_ZERO.

>
> The tests demonstrate that having a different set of parents affects
> the generation number calculation, and this value propagates to
> descendants. Hence, we drop the single-line condition on the output.

I don't understand what part of changes this paragraph of the commit
message refers to.

Anyway, changing parents may not lead to changed generation numbers;
take for example commit with single parent, which we change to other
commit with the same generation number.

>
> Signed-off-by: Derrick Stolee <[email-masked]>
> ---
>  commit-graph.c          | 18 ++++++++++++++++++
>  t/t5318-commit-graph.sh |  6 ++++++

Sidenote: I have just realized that it may be better to put
validation-related tests into different test file.

>  2 files changed, 24 insertions(+)
>
> diff --git a/commit-graph.c b/commit-graph.c
> index fff22dc0c3..ead92460c1 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -922,6 +922,7 @@ int verify_commit_graph(struct commit_graph *g)
>  	for (i = 0; i < g->num_commits; i++) {
>  		struct commit *graph_commit, *odb_commit;
>  		struct commit_list *graph_parents, *odb_parents;
> +		uint32_t max_generation = 0;
>  
>  		hashcpy(cur_oid.hash, g->chunk_oid_lookup + g->hash_len * i);
>  
> @@ -956,6 +957,9 @@ int verify_commit_graph(struct commit_graph *g)
>  					     oid_to_hex(&graph_parents->item->object.oid),
>  					     oid_to_hex(&odb_parents->item->object.oid));
>  
> +			if (graph_parents->item->generation > max_generation)
> +				max_generation = graph_parents->item->generation;
> +

All right, that calculates the maximum of generation number of commit
parents.

>  			graph_parents = graph_parents->next;
>  			odb_parents = odb_parents->next;
>  		}
> @@ -963,6 +967,20 @@ int verify_commit_graph(struct commit_graph *g)
>  		if (odb_parents != NULL)
>  			graph_report("commit-graph parent list for commit %s terminates early",
>  				     oid_to_hex(&cur_oid));
> +
> +		/*
> +		 * If one of our parents has generation GENERATION_NUMBER_MAX, then
> +		 * our generation is also GENERATION_NUMBER_MAX. Decrement to avoid
> +		 * extra logic in the following condition.
> +		 */

Nice trick.

> +		if (max_generation == GENERATION_NUMBER_MAX)
> +			max_generation--;

What about GENERATION_NUMBER_ZERO?

> +
> +		if (graph_commit->generation != max_generation + 1)
> +			graph_report("commit-graph generation for commit %s is %u != %u",
> +				     oid_to_hex(&cur_oid),
> +				     graph_commit->generation,
> +				     max_generation + 1);

I think we should also check that generation numbers do not exceed
GENERATION_NUMBER_MAX... unless it is already taken care of?

>  	}
>  
>  	return verify_commit_graph_error;
> diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
> index 12f0d7f54d..673b0d37d5 100755
> --- a/t/t5318-commit-graph.sh
> +++ b/t/t5318-commit-graph.sh
> @@ -272,6 +272,7 @@ GRAPH_BYTE_COMMIT_TREE=$GRAPH_COMMIT_DATA_OFFSET
>  GRAPH_BYTE_COMMIT_PARENT=`expr $GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN`
>  GRAPH_BYTE_COMMIT_EXTRA_PARENT=`expr $GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN + 4`
>  GRAPH_BYTE_COMMIT_WRONG_PARENT=`expr $GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN + 3`
> +GRAPH_BYTE_COMMIT_GENERATION=`expr $GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN + 8`
>  
>  # usage: corrupt_graph_and_verify <position> <data> <string>
>  # Manipulates the commit-graph file at the position
> @@ -366,4 +367,9 @@ test_expect_success 'detect incorrect tree OID' '
>  		"commit-graph parent for"
>  '
>  
> +test_expect_success 'detect incorrect generation number' '
> +	corrupt_graph_and_verify $GRAPH_BYTE_COMMIT_GENERATION "\01" \

I assume that you have checked that it actually corrupts generation
number (without affecting commit date).

> +		"generation"

A very minor nitpick: Not "generation for commit"?

> +'
> +
>  test_done