Wait for work to complete before returning from Run. (#259)

Partially addresses https://github.com/knative/serving/issues/3074

controller/controller.go

@@ -19,6 +19,7 @@ package controller
 import (
 	"context"
 	"fmt"
+	"sync"
 	"time"
 
 	"go.uber.org/zap"
@@ -208,13 +209,17 @@ func (c *Impl) EnqueueKey(key string) {
 // work queue and waits for workers to finish processing their current work items.
 func (c *Impl) Run(threadiness int, stopCh <-chan struct{}) error {
 	defer runtime.HandleCrash()
+	sg := sync.WaitGroup{}
+	defer sg.Wait()
 	defer c.WorkQueue.ShutDown()
 
 	// Launch workers to process resources that get enqueued to our workqueue.
 	logger := c.logger
 	logger.Info("Starting controller and workers")
 	for i := 0; i < threadiness; i++ {
+		sg.Add(1)
 		go func() {
+			defer sg.Done()
 			for c.processNextWorkItem() {
 			}
 		}()