🐛 Prevent race when informers are started more than once

If `Informers` are started a second time, there is a possibility for a
data race because it sets a `ctx` field on itself. This write is
protected by a mutex, but reads from that field are not.

Author: alvaroaleman

pkg/cache/cache_test.go

@@ -1849,6 +1849,12 @@ func CacheTest(createCacheFunc func(config *rest.Config, opts cache.Options) (ca
 			)
 		})
 		Describe("as an Informer", func() {
+			It("should error when starting the cache a second time", func() {
+				err := informerCache.Start(context.Background())
+				Expect(err).To(HaveOccurred())
+				Expect(err.Error()).To(ContainSubstring("Informer already started"))
+			})
+
 			Context("with structured objects", func() {
 				It("should be able to get informer for the object", func() {
 					By("getting a shared index informer for a pod")

pkg/cache/internal/informers.go

@@ -18,6 +18,7 @@ package internal
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"math/rand"
 	"net/http"
@@ -186,6 +187,12 @@ type Informers struct {
 // Start calls Run on each of the informers and sets started to true. Blocks on the context.
 // It doesn't return start because it can't return an error, and it's not a runnable directly.
 func (ip *Informers) Start(ctx context.Context) error {
+	select {
+	case <-ip.startWait:
+		return errors.New("Informer already started") //nolint:sylecheck
+	default:
+		// continue
+	}
 	func() {
 		ip.mu.Lock()
 		defer ip.mu.Unlock()

pkg/cache/multi_namespace_cache.go

@@ -163,12 +163,13 @@ func (c *multiNamespaceCache) GetInformerForKind(ctx context.Context, gvk schema
 }
 
 func (c *multiNamespaceCache) Start(ctx context.Context) error {
+	errs := make(chan error, 0)
 	// start global cache
 	if c.clusterCache != nil {
 		go func() {
 			err := c.clusterCache.Start(ctx)
 			if err != nil {
-				log.Error(err, "cluster scoped cache failed to start")
+				errs <- fmt.Errorf("failed to start cluster-scoped cache: %w", err)
 			}
 		}()
 	}
@@ -177,13 +178,16 @@ func (c *multiNamespaceCache) Start(ctx context.Context) error {
 	for ns, cache := range c.namespaceToCache {
 		go func(ns string, cache Cache) {
 			if err := cache.Start(ctx); err != nil {
-				log.Error(err, "multi-namespace cache failed to start namespaced informer", "namespace", ns)
+				errs <- fmt.Errorf("failed to start cache for namespace %s: %w", ns, err)
 			}
 		}(ns, cache)
 	}
-
-	<-ctx.Done()
-	return nil
+	select {
+	case <-ctx.Done():
+		return nil
+	case err := <-errs:
+		return err
+	}
 }
 
 func (c *multiNamespaceCache) WaitForCacheSync(ctx context.Context) bool {