Move unix specific logic into _unix.go

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/netns/netns_unix.go

@@ -1,5 +1,7 @@
+// +build !windows
+
 /*
-Copyright 2018 The Containerd Authors.
+Copyright The containerd Authors.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

pkg/netns/netns_windows.go

@@ -0,0 +1,50 @@
+// +build windows
+
+/*
+Copyright The containerd Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package netns
+
+// TODO(windows): Implement netns for windows.
+// NetNS holds network namespace.
+type NetNS struct {
+}
+
+// NewNetNS creates a network namespace.
+func NewNetNS() (*NetNS, error) {
+	return nil, nil
+}
+
+// LoadNetNS loads existing network namespace.
+func LoadNetNS(path string) *NetNS {
+	return nil
+}
+
+// Remove removes network namepace. Remove is idempotent, meaning it might
+// be invoked multiple times and provides consistent result.
+func (n *NetNS) Remove() error {
+	return nil
+}
+
+// Closed checks whether the network namespace has been closed.
+func (n *NetNS) Closed() (bool, error) {
+	return false, nil
+}
+
+// GetPath returns network namespace path for sandbox container
+func (n *NetNS) GetPath() string {
+	return ""
+}