compat: helper for detecting unsigned overflow

The idiom (a + b < a) works fine for detecting that an unsigned
integer has overflowed, but a more explicit

	unsigned_add_overflows(a, b)

might be easier to read.

Define such a macro, expanding roughly to ((a) < UINT_MAX - (b)).
Because the expansion uses each argument only once outside of sizeof()
expressions, it is safe to use with arguments that have side effects.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/strbuf.c b/strbuf.c
index 9b3c4457f2..07e8883ceb 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -63,7 +63,8 @@ void strbuf_attach(struct strbuf *sb, void *buf, size_t len, size_t alloc)
 
 void strbuf_grow(struct strbuf *sb, size_t extra)
 {
-	if (sb->len + extra + 1 <= sb->len)
+	if (unsigned_add_overflows(extra, 1) ||
+	    unsigned_add_overflows(sb->len, extra + 1))
 		die("you want to use way too much memory");
 	if (!sb->alloc)
 		sb->buf = NULL;
@@ -152,7 +153,7 @@ int strbuf_cmp(const struct strbuf *a, const struct strbuf *b)
 void strbuf_splice(struct strbuf *sb, size_t pos, size_t len,
 				   const void *data, size_t dlen)
 {
-	if (pos + len < pos)
+	if (unsigned_add_overflows(pos, len))
 		die("you want to use way too much memory");
 	if (pos > sb->len)
 		die("`pos' is too far after the end of the buffer");