prune-packed: use strbuf to avoid having to worry about PATH_MAX

A/very/long/path/to/.git that becomes exactly PATH_MAX bytes long
after suffixed with /objects/??/??38-hex??, would have overflown
the on-stack pathname[] buffer.

Noticed-by:  Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/prune-packed.c

@@ -10,10 +10,11 @@ static const char * const prune_packed_usage[] = {
 
 static struct progress *progress;
 
-static void prune_dir(int i, DIR *dir, char *pathname, int len, int opts)
+static void prune_dir(int i, DIR *dir, struct strbuf *pathname, int opts)
 {
 	struct dirent *de;
 	char hex[40];
+	int top_len = pathname->len;
 
 	sprintf(hex, "%02x", i);
 	while ((de = readdir(dir)) != NULL) {
@@ -25,43 +26,46 @@ static void prune_dir(int i, DIR *dir, char *pathname, int len, int opts)
 			continue;
 		if (!has_sha1_pack(sha1))
 			continue;
-		memcpy(pathname + len, de->d_name, 38);
+
+		strbuf_add(pathname, de->d_name, 38);
 		if (opts & PRUNE_PACKED_DRY_RUN)
-			printf("rm -f %s\n", pathname);
+			printf("rm -f %s\n", pathname->buf);
 		else
-			unlink_or_warn(pathname);
+			unlink_or_warn(pathname->buf);
 		display_progress(progress, i + 1);
+		strbuf_setlen(pathname, top_len);
 	}
 }
 
 void prune_packed_objects(int opts)
 {
 	int i;
-	static char pathname[PATH_MAX];
 	const char *dir = get_object_directory();
-	int len = strlen(dir);
+	struct strbuf pathname = STRBUF_INIT;
+	int top_len;
 
+	strbuf_addstr(&pathname, dir);
 	if (opts & PRUNE_PACKED_VERBOSE)
 		progress = start_progress_delay("Removing duplicate objects",
 			256, 95, 2);
 
-	if (len > PATH_MAX - 42)
-		die("impossible object directory");
-	memcpy(pathname, dir, len);
-	if (len && pathname[len-1] != '/')
-		pathname[len++] = '/';
+	if (pathname.len && pathname.buf[pathname.len - 1] != '/')
+		strbuf_addch(&pathname, '/');
+
+	top_len = pathname.len;
 	for (i = 0; i < 256; i++) {
 		DIR *d;
 
 		display_progress(progress, i + 1);
-		sprintf(pathname + len, "%02x/", i);
-		d = opendir(pathname);
+		strbuf_setlen(&pathname, top_len);
+		strbuf_addf(&pathname, "%02x/", i);
+		d = opendir(pathname.buf);
 		if (!d)
 			continue;
-		prune_dir(i, d, pathname, len + 3, opts);
+		prune_dir(i, d, &pathname, opts);
 		closedir(d);
-		pathname[len + 2] = '\0';
-		rmdir(pathname);
+		strbuf_setlen(&pathname, top_len + 2);
+		rmdir(pathname.buf);
 	}
 	stop_progress(&progress);
 }