Add monitor plugin loading

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

plugin/monitor.go

@@ -0,0 +1,54 @@
+package plugin
+
+import "github.com/docker/containerd"
+
+// ContainerMonitor provides an interface for monitoring of containers within containerd
+type ContainerMonitor interface {
+	// Monitor adds the provided container to the monitor
+	Monitor(containerd.Container) error
+	// Stop stops and removes the provided container from the monitor
+	Stop(containerd.Container) error
+}
+
+func NewMultiContainerMonitor(monitors ...ContainerMonitor) ContainerMonitor {
+	return &multiContainerMonitor{
+		monitors: monitors,
+	}
+}
+
+func NewNoopMonitor() ContainerMonitor {
+	return &noopContainerMonitor{}
+}
+
+type noopContainerMonitor struct {
+}
+
+func (mm *noopContainerMonitor) Monitor(c containerd.Container) error {
+	return nil
+}
+
+func (mm *noopContainerMonitor) Stop(c containerd.Container) error {
+	return nil
+}
+
+type multiContainerMonitor struct {
+	monitors []ContainerMonitor
+}
+
+func (mm *multiContainerMonitor) Monitor(c containerd.Container) error {
+	for _, m := range mm.monitors {
+		if err := m.Monitor(c); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (mm *multiContainerMonitor) Stop(c containerd.Container) error {
+	for _, m := range mm.monitors {
+		if err := m.Stop(c); err != nil {
+			return err
+		}
+	}
+	return nil
+}

plugin/plugin.go

@@ -18,6 +18,7 @@ const (
 	RuntimePlugin PluginType = iota + 1
 	GRPCPlugin
 	SnapshotPlugin
+	ContainerMonitorPlugin
 )
 
 type Registration struct {
@@ -26,6 +27,7 @@ type Registration struct {
 	Init   func(*InitContext) (interface{}, error)
 }
 
+// TODO(@crosbymichael): how to we keep this struct from growing but support dependency injection for loaded plugins?
 type InitContext struct {
 	Root        string
 	State       string
@@ -34,6 +36,7 @@ type InitContext struct {
 	Snapshotter snapshot.Snapshotter
 	Config      interface{}
 	Context     context.Context
+	Monitor     ContainerMonitor
 }
 
 type Service interface {