add_packed_git: convert strcpy into xsnprintf

We have the path "foo.idx", and we create a buffer big
enough to hold "foo.pack" and "foo.keep", and then strcpy
straight into it. This isn't a bug (we have enough space),
but it's very hard to tell from the strcpy that this is so.

Let's instead use strip_suffix to take off the ".idx",
record the size of our allocation, and use xsnprintf to make
sure we don't violate our assumptions.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -1309,7 +1309,7 @@ extern void close_pack_windows(struct packed_git *);
 extern void unuse_pack(struct pack_window **);
 extern void free_pack_by_name(const char *);
 extern void clear_delta_base_cache(void);
-extern struct packed_git *add_packed_git(const char *, int, int);
+extern struct packed_git *add_packed_git(const char *path, size_t path_len, int local);
 
 /*
  * Return the SHA-1 of the nth object within the specified packfile.

sha1_file.c

@@ -1146,11 +1146,12 @@ static void try_to_free_pack_memory(size_t size)
 	release_pack_memory(size);
 }
 
-struct packed_git *add_packed_git(const char *path, int path_len, int local)
+struct packed_git *add_packed_git(const char *path, size_t path_len, int local)
 {
 	static int have_set_try_to_free_routine;
 	struct stat st;
-	struct packed_git *p = alloc_packed_git(path_len + 2);
+	size_t alloc;
+	struct packed_git *p;
 
 	if (!have_set_try_to_free_routine) {
 		have_set_try_to_free_routine = 1;
@@ -1161,18 +1162,22 @@ struct packed_git *add_packed_git(const char *path, int path_len, int local)
 	 * Make sure a corresponding .pack file exists and that
 	 * the index looks sane.
 	 */
-	path_len -= strlen(".idx");
-	if (path_len < 1) {
-		free(p);
+	if (!strip_suffix_mem(path, &path_len, ".idx"))
 		return NULL;
-	}
+
+	/*
+	 * ".pack" is long enough to hold any suffix we're adding (and
+	 * the use xsnprintf double-checks that)
+	 */
+	alloc = path_len + strlen(".pack") + 1;
+	p = alloc_packed_git(alloc);
 	memcpy(p->pack_name, path, path_len);
 
-	strcpy(p->pack_name + path_len, ".keep");
+	xsnprintf(p->pack_name + path_len, alloc - path_len, ".keep");
 	if (!access(p->pack_name, F_OK))
 		p->pack_keep = 1;
 
-	strcpy(p->pack_name + path_len, ".pack");
+	xsnprintf(p->pack_name + path_len, alloc - path_len, ".pack");
 	if (stat(p->pack_name, &st) || !S_ISREG(st.st_mode)) {
 		free(p);
 		return NULL;