hex: make wraparound of the index into ring-buffer explicit

rscharfe · gitster · commit bb84735c80db · 2016-10-26T10:54:11.000-07:00
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 &lt;l.s.r@web.de&gt;
Signed-off-by: Junio C Hamano &lt;gitster@pobox.com&gt;
diff --git 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
@@ -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;
 }

Original file line number	Diff line number	Diff line change
`@@ -78,7 +78,8 @@ char sha1_to_hex(const unsigned char sha1)`
`78`	`78`	`{`
`79`	`79`	`static int bufno;`
`80`	`80`	`static char hexbuffer[4][GIT_SHA1_HEXSZ + 1];`
`81`		`- return sha1_to_hex_r(hexbuffer[3 & ++bufno], sha1);`
	`81`	`+ bufno = (bufno + 1) % ARRAY_SIZE(hexbuffer);`
	`82`	`+ return sha1_to_hex_r(hexbuffer[bufno], sha1);`
`82`	`83`	`}`
`83`	`84`
`84`	`85`	`char oid_to_hex(const struct object_id oid)`