use strip_suffix and xstrfmt to replace suffix

When we want to convert "foo.pack" to "foo.idx", we do it by
duplicating the original string and then munging the bytes
in place. Let's use strip_suffix and xstrfmt instead, which
has several advantages:

  1. It's more clear what the intent is.

  2. It does not implicitly rely on the fact that
     strlen(".idx") <= strlen(".pack") to avoid an overflow.

  3. We communicate the assumption that the input file ends
     with ".pack" (and get a run-time check that this is so).

  4. We drop calls to strcpy, which makes auditing the code
     base easier.

Likewise, we can do this to convert ".pack" to ".bitmap",
avoiding some manual memory computation.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/sha1_file.c b/sha1_file.c
index 592226eb7b..2be1afdde0 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -671,13 +671,15 @@ static int check_packed_git_idx(const char *path, struct packed_git *p)
 int open_pack_index(struct packed_git *p)
 {
 	char *idx_name;
+	size_t len;
 	int ret;
 
 	if (p->index_data)
 		return 0;
 
-	idx_name = xstrdup(p->pack_name);
-	strcpy(idx_name + strlen(idx_name) - strlen(".pack"), ".idx");
+	if (!strip_suffix(p->pack_name, ".pack", &len))
+		die("BUG: pack_name does not end in .pack");
+	idx_name = xstrfmt("%.*s.idx", (int)len, p->pack_name);
 	ret = check_packed_git_idx(idx_name, p);
 	free(idx_name);
 	return ret;