Re: [PATCH] hex: use unsigned index for ring buffer

[René Scharfe]

Am 25.10.2016 um 20:28 schrieb Junio C Hamano:

Jeff King <p...@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....@web.de>

-- >8 --
From: René Scharfe <l....@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....@web.de>
Signed-off-by: Junio C Hamano <gits...@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;
 }