darwin: runtime support

This commit attemts to support containerd on darwin platform.  With an
external runtime shim, ctr run should work with, for instance,
--runtime=io.containerd.runu.v1.  An example of runtime and shim is
managed under different repository (github.com/ukontainer/runu/).

Signed-off-by: Hajime Tazaki <thehajime@gmail.com>

services/tasks/local_darwin.go

@@ -0,0 +1,35 @@
+// +build darwin
+
+/*
+   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 tasks
+
+import (
+	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/runtime"
+)
+
+var tasksServiceRequires = []plugin.Type{
+	plugin.RuntimePluginV2,
+	plugin.MetadataPlugin,
+	plugin.TaskMonitorPlugin,
+}
+
+// loadV1Runtimes on darwin returns an empty map. There are no v1 runtimes
+func loadV1Runtimes(ic *plugin.InitContext) (map[string]runtime.PlatformRuntime, error) {
+	return make(map[string]runtime.PlatformRuntime), nil
+}

services/tasks/local_unix.go

@@ -1,5 +1,5 @@
-//go:build !windows && !freebsd
-// +build !windows,!freebsd
+//go:build !windows && !freebsd && !darwin
+// +build !windows,!freebsd,!darwin
 
 /*
    Copyright The containerd Authors.