Move unix specific logic into _unix.go

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

pkg/server/io/helpers_unix.go

@@ -0,0 +1,31 @@
+// +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 io
+
+import (
+	"io"
+	"os"
+
+	"github.com/containerd/fifo"
+	"golang.org/x/net/context"
+)
+
+func openFifo(ctx context.Context, fn string, flag int, perm os.FileMode) (io.ReadWriteCloser, error) {
+	return fifo.OpenFifo(ctx, fn, flag, perm)
+}