rbd:snap limit should't be set smaller than the num of existing snaps

new snap limit must be bigger than the num of existing snaps

Signed-off-by: PCzhangPC <pengcheng.zhang@easystack.cn>

src/cls/rbd/cls_rbd.cc

@@ -2807,6 +2807,33 @@ int snapshot_set_limit(cls_method_context_t hctx, bufferlist *in,
     return -EINVAL;
   }
 
+    int max_read = RBD_MAX_KEYS_READ;
+    vector<snapid_t> snap_ids;
+    string last_read = RBD_SNAP_KEY_PREFIX;
+    bool more;
+
+    do {
+        set<string> keys;
+        rc = cls_cxx_map_get_keys(hctx, last_read, max_read, &keys, &more);
+        if (rc < 0)
+            return rc;
+
+        for (set<string>::const_iterator it = keys.begin();
+             it != keys.end(); ++it) {
+            if ((*it).find(RBD_SNAP_KEY_PREFIX) != 0)
+                break;
+            snapid_t snap_id = snap_id_from_key(*it);
+            snap_ids.push_back(snap_id);
+        }
+        if (!keys.empty())
+            last_read = *(keys.rbegin());
+    } while (more);
+
+    if(new_limit < snap_ids.size()) {
+        CLS_ERR("error setting snapshot limit: %s", cpp_strerror(rc).c_str());
+        return -EINVAL;
+    }
+
   if (new_limit == UINT64_MAX) {
     CLS_LOG(20, "remove snapshot limit\n");
     rc = cls_cxx_map_remove_key(hctx, "snap_limit");
@@ -5305,7 +5332,7 @@ CLS_INIT(rbd)
 			  CLS_METHOD_RD,
 			  snapshot_get_limit, &h_snapshot_get_limit);
   cls_register_cxx_method(h_class, "snapshot_set_limit",
-			  CLS_METHOD_WR,
+			  CLS_METHOD_WR | CLS_METHOD_RD,
 			  snapshot_set_limit, &h_snapshot_set_limit);
 
   /* methods for the rbd_children object */