[RFC PATCH 08/15] strbuf.c: use st_add3(), not unsigned_add_overflows()

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Change the strbuf_grow() function to use st_add3() instead of doing
its own unsigned_add_overflows() checks.  The overflow checking here
was originally added in b449f4cfc97 (Rework strbuf API and semantics.,
2007-09-06) and adjusted in 1368f65002b (compat: helper for detecting
unsigned overflow, 2010-10-10). Instead we compute a "sz" with
st_add3().

That was done at a time when the underlying xrealloc() in
git-compat-util.h didn't use st_mult() yet, that has been the case
since the later e7792a74bcf (harden REALLOC_ARRAY and xcalloc against
size_t overflow, 2016-02-22).

The only behavior change here should be the very obscure edge case
that we'd previously die() in cases where we strictly didn't need to,
as we'd check both "extra + 1" and "sb->len + extra + 1" for
overflow. If we overflowed only on the latter but were doing the
former we'd needlessly die() die. I don't think that difference
mattered, but it's noted here for completeness.

While we're at it add an inline comment about why we're adding 1 to
the values, that's also explained in the API documentation in
strbuf.h, but since we're using that magic constant here...

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 strbuf.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/strbuf.c b/strbuf.c
index 61c4630aeeb..f0a74d2bfb1 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -91,12 +91,12 @@ void strbuf_attach(struct strbuf *sb, void *buf, size_t len, size_t alloc)
 void strbuf_grow(struct strbuf *sb, size_t extra)
 {
 	int new_buf = !sb->alloc;
-	if (unsigned_add_overflows(extra, 1) ||
-	    unsigned_add_overflows(sb->len, extra + 1))
-		die("you want to use way too much memory");
+	const size_t sz_buf = new_buf ? 0 : sb->len;
+	const size_t sz = st_add3(sz_buf, extra, 1 /* for \0 */);
+
 	if (new_buf)
 		sb->buf = NULL;
-	ALLOC_GROW(sb->buf, sb->len + extra + 1, sb->alloc);
+	ALLOC_GROW(sb->buf, sz, sb->alloc);
 	if (new_buf && !sb->buf)
 		BUG("for a new buffer ALLOC_GROW() should always do work!");
 	if (new_buf)
-- 
2.36.1.1124.g577fa9c2ebd