Re: [PATCH] [Outreachy] patch-ids: fix const correctness

[Junio C Hamano <gitster@pobox.com>]

Okhuomon Ajayi <okhuomonajayi54@gmail.com> writes:

> The `patch_id_neq()` function received a pointer to diff options via
> `cmpfn_data` but cast it to a non-const type. This caused a const
> correctness warning and could potentially allow unintended modification
> of read-only data.
>
> Fix this by casting to `const struct diff_options *` instead, removing
> the outdated NEEDSWORK comment in the process.
>
> Signed-off-by: Okhuomon Ajayi <okhuomonajayi54@gmail.com>
> ---
>  patch-ids.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/patch-ids.c b/patch-ids.c
> index a5683b462c..b6b808332f 100644
> --- a/patch-ids.c
> +++ b/patch-ids.c
> @@ -41,8 +41,8 @@ static int patch_id_neq(const void *cmpfn_data,
>  			const struct hashmap_entry *entry_or_key,
>  			const void *keydata UNUSED)
>  {
> -	/* NEEDSWORK: const correctness? */
> -	struct diff_options *opt = (void *)cmpfn_data;
> +	

Trailing whitespace on this line.  Remove the entire line instead.

> +	const struct diff_options *opt = (void *)cmpfn_data;
>  	struct patch_id *a, *b;
>  
>  	a = container_of(eptr, struct patch_id, ent);

I do not think this is correct.  Have you even compile-tested this
patch?

Later in this same function, opt is passed to commit_patch_id()
function (twice), which takes non-const "struct diff_options *" that
is given to diffcore_std().  And the last function in this callchain
has to modify the structure to record various findings (like "did we
see any changes in the diff?"), so it cannot be "const" at all.

I think patch_id_neq() that says cmpfn_data is const is the source
of the problem, but that function signature is mandated by the
hashmap API.  I do not know if we can loosen it there in the hashmap
API and if so what the argument would be, but thinking about these
things is what the NEEDSWORK comment is about ;-).