files-reg: Add try_collect_special_file

The idea is to be able to lookup for special id
which might be not present and we should not
yield the error.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

files-reg.c

@@ -1127,7 +1127,7 @@ static struct file_desc_ops reg_desc_ops = {
 	.collect_fd = collect_reg_fd,
 };
 
-struct file_desc *collect_special_file(u32 id)
+struct file_desc *try_collect_special_file(u32 id, int optional)
 {
 	struct file_desc *fdesc;
 
@@ -1140,6 +1140,7 @@ struct file_desc *collect_special_file(u32 id)
 
 	fdesc = find_file_desc_raw(FD_TYPES__REG, id);
 	if (fdesc == NULL) {
+		if (!optional)
 			pr_err("No entry for reg-file-ID %#x\n", id);
 		return NULL;
 	}

include/files-reg.h

@@ -40,7 +40,9 @@ extern int dump_one_reg_file(int lfd, u32 id, const struct fd_parms *p);
 
 extern struct file_remap *lookup_ghost_remap(u32 dev, u32 ino);
 extern void remap_put(struct file_remap *remap);
-extern struct file_desc *collect_special_file(u32 id);
+
+extern struct file_desc *try_collect_special_file(u32 id, int optional);
+#define collect_special_file(id)	try_collect_special_file(id, 0)
 
 extern struct collect_image_info reg_file_cinfo;
 extern struct collect_image_info remap_cinfo;