Re: [PATCH 2/3] commit-graph: use the hash version byte

From: "brian m. carlson" <sandals@crustytoothpaste.net>
To: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Cc: git@vger.kernel.org, martin.agren@gmail.com, me@ttaylorr.com,
	abhishekkumar8222@gmail.com,
	Derrick Stolee <derrickstolee@github.com>,
	Derrick Stolee <dstolee@microsoft.com>
Subject: Re: [PATCH 2/3] commit-graph: use the hash version byte
Date: Fri, 14 Aug 2020 20:11:53 +0000	[thread overview]
Message-ID: <20200814201153.GK8085@camp.crustytoothpaste.net> (raw)
In-Reply-To: <4bbfd345d16da4604dd20decda8ecb12372e4223.1597428440.git.gitgitgadget@gmail.com>

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On 2020-08-14 at 18:07:19, Derrick Stolee via GitGitGadget wrote:
> diff --git a/commit-graph.c b/commit-graph.c
> index e51c91dd5b..d03328d64c 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -179,7 +179,11 @@ static char *get_chain_filename(struct object_directory *odb)
>  
>  static uint8_t oid_version(void)
>  {
> -	return 1;
> +	if (the_hash_algo->rawsz == GIT_SHA1_RAWSZ)
> +		return 1;
> +	if (the_hash_algo->rawsz == GIT_SHA256_RAWSZ)
> +		return 2;
> +	die(_("invalid hash version"));
>  }

Can we maybe say something like this?

  switch (hash_algo_by_ptr(the_hash_algo)) {
    case GIT_HASH_SHA1:
      return 1;
    case GIT_HASH_SHA256:
      return 2;
    default:
      die(_("invalid hash version"));
  }

That way if SHA-256 gets broken and we switch to another 256-bit hash
(SHA-3-256?), we'll be forced to update this properly.

>  static struct commit_graph *alloc_commit_graph(void)
> diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh
> index c21cc160f3..906af2799d 100755
> --- a/t/t4216-log-bloom.sh
> +++ b/t/t4216-log-bloom.sh
> @@ -6,6 +6,12 @@ test_description='git log for a path with Bloom filters'
>  GIT_TEST_COMMIT_GRAPH=0
>  GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS=0
>  
> +OID_VERSION=1
> +if [ "$GIT_DEFAULT_HASH" = "sha256" ]
> +then
> +	OID_VERSION=2
> +fi

Can we do something like this in the setup test instead?

  test_oid_cache <<-EOF
  oid_version sha1:1
  oid_version sha256:2
  EOF

  OID_VERSION=$(test_oid oid_version)
  # or, inline
  $(test_oid oid_version)

That uses the existing test framework to ensure we produce the right
results if someone adds another hash algorithm and that we produce a BUG
if the value is missing.  It will also make it easier to write tests if
we end up creating SHA-1- or SHA-256-specific tests, since we can look
up those values directly with test_oid.

Since you're using this across several tests, you could even just add
this to t/oid-info/hash-info like so:

  commit_graph_oid_version sha1:1
  commit_graph_oid_version sha256:2

and then it's set in one place and can be used without any required
test_oid_cache invocations at all.  I think three tests is sufficient
basis for us to add an entry there.
-- 
brian m. carlson: Houston, Texas, US

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