Refactor oom watcher to allow greater test coverage

This diff contains a strict refactor; there are no behavioral changes.

Address a long standing TODO in `oom_watcher_linux_test.go` around test
coverage. We refactor our `oom.Watcher` so it takes in a struct
fulfulling the `streamer` interface (i.e. defines `StreamOoms` method).
In production, we will continue to use the `oomparser` from `cadvisor`.
However, for testing purposes, we can now create our own `fakeStreamer`,
and control how it streams `oomparser.OomInstance`. With this fake, we
can implement richer unit testing for the `oom.Watcher` itself.

Actually adding the additional unit tests will come in a later commit.

pkg/kubelet/oom/oom_watcher_linux.go

@@ -30,29 +30,41 @@ import (
 	"github.com/google/cadvisor/utils/oomparser"
 )
 
+type streamer interface {
+	StreamOoms(chan<- *oomparser.OomInstance)
+}
+
+var _ streamer = &oomparser.OomParser{}
+
 type realWatcher struct {
-	recorder record.EventRecorder
+	recorder    record.EventRecorder
+	oomStreamer streamer
 }
 
 var _ Watcher = &realWatcher{}
 
-// NewWatcher creates and initializes a OOMWatcher based on parameters.
-func NewWatcher(recorder record.EventRecorder) Watcher {
-	return &realWatcher{
-		recorder: recorder,
+// NewWatcher creates and initializes a OOMWatcher backed by Cadvisor as
+// the oom streamer.
+func NewWatcher(recorder record.EventRecorder) (Watcher, error) {
+	oomStreamer, err := oomparser.New()
+	if err != nil {
+		return nil, err
 	}
+
+	watcher := &realWatcher{
+		recorder:    recorder,
+		oomStreamer: oomStreamer,
+	}
+
+	return watcher, nil
 }
 
 const systemOOMEvent = "SystemOOM"
 
 // Start watches for system oom's and records an event for every system oom encountered.
 func (ow *realWatcher) Start(ref *v1.ObjectReference) error {
-	oomLog, err := oomparser.New()
-	if err != nil {
-		return err
-	}
 	outStream := make(chan *oomparser.OomInstance, 10)
-	go oomLog.StreamOoms(outStream)
+	go ow.oomStreamer.StreamOoms(outStream)
 
 	go func() {
 		defer runtime.HandleCrash()