From: René Scharfe <l.s.r@web.de>
Date: Wed, 26 Oct 2016 17:08:41 GMT
Subject: Re: [PATCH] hex: use unsigned index for ring buffer
Message-ID: <3c95a89a-2a9b-2856-42a1-6b994f2e31cd@web.de>
In-Reply-To: <xmqqd1ios2p3.fsf@gitster.mtv.corp.google.com>

Am 25.10.2016 um 20:28 schrieb Junio C Hamano:
> Jeff King <peff@peff.net> writes:
>
>> On Mon, Oct 24, 2016 at 04:53:50PM -0700, Junio C Hamano wrote:
>>
>>>> So how about this?  It gets rid of magic number 3 and works for array
>>>> size that's not a power of two.  And as a nice side effect it can't
>>>> trigger a signed overflow anymore.
>>>
>>> Looks good to me.  Peff?
>>
>> Any of the variants discussed in this thread is fine by me.
>
> OK, here is what I'll queue then.
> I assumed that René wants to sign it off ;-).

Actually I didn't sign-off on purpose originally.  But OK, let's keep
the version below.  I just feel strangely sad seeing that concise magic
go.  Nevermind.

Signed-off-by: Rene Scharfe <l.s.r@web.de>

> -- >8 --
> From: René Scharfe <l.s.r@web.de>
> Date: Sun, 23 Oct 2016 19:57:30 +0200
> Subject: [PATCH] hex: make wraparound of the index into ring-buffer explicit
>
> Overflow is defined for unsigned integers, but not for signed ones.
>
> We could make the ring-buffer index in sha1_to_hex() and
> get_pathname() unsigned to be on the safe side to resolve this, but
> let's make it explicit that we are wrapping around at whatever the
> number of elements the ring-buffer has.  The compiler is smart enough
> to turn modulus into bitmask for these codepaths that use
> ring-buffers of a size that is a power of 2.
>
> Signed-off-by: René Scharfe <l.s.r@web.de>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> ---
>  hex.c  | 3 ++-
>  path.c | 3 ++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/hex.c b/hex.c
> index ab2610e498..845b01a874 100644
> --- a/hex.c
> +++ b/hex.c
> @@ -78,7 +78,8 @@ char *sha1_to_hex(const unsigned char *sha1)
>  {
>  	static int bufno;
>  	static char hexbuffer[4][GIT_SHA1_HEXSZ + 1];
> -	return sha1_to_hex_r(hexbuffer[3 & ++bufno], sha1);
> +	bufno = (bufno + 1) % ARRAY_SIZE(hexbuffer);
> +	return sha1_to_hex_r(hexbuffer[bufno], sha1);
>  }
>
>  char *oid_to_hex(const struct object_id *oid)
> diff --git a/path.c b/path.c
> index fe3c4d96c6..9bfaeda207 100644
> --- a/path.c
> +++ b/path.c
> @@ -24,7 +24,8 @@ static struct strbuf *get_pathname(void)
>  		STRBUF_INIT, STRBUF_INIT, STRBUF_INIT, STRBUF_INIT
>  	};
>  	static int index;
> -	struct strbuf *sb = &pathname_array[3 & ++index];
> +	struct strbuf *sb = &pathname_array[index];
> +	index = (index + 1) % ARRAY_SIZE(pathname_array);
>  	strbuf_reset(sb);
>  	return sb;
>  }
>