Add panic on double Close() method call

credentials/tls/certprovider/pemfile/watcher.go

@@ -34,6 +34,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"sync/atomic"
 	"time"
 
 	"google.golang.org/grpc/credentials/tls/certprovider"
@@ -130,6 +131,7 @@ type watcher struct {
 	certFileContents    []byte
 	keyFileContents     []byte
 	rootFileContents    []byte
+	closeMark           atomic.Bool
 	cancel              context.CancelFunc
 }
 
@@ -256,5 +258,8 @@ func (w *watcher) KeyMaterial(ctx context.Context) (*certprovider.KeyMaterial, e
 
 // Close cleans up resources allocated by the watcher.
 func (w *watcher) Close() {
+	if w.closeMark.Swap(true) {
+		panic("Close called more than once on certificate provider")
+	}
 	w.cancel()
 }

credentials/tls/certprovider/provider.go

@@ -85,6 +85,7 @@ type Provider interface {
 	KeyMaterial(ctx context.Context) (*KeyMaterial, error)
 
 	// Close cleans up resources allocated by the Provider.
+	// This method panic when called more than once.
 	Close()
 }
 

credentials/tls/certprovider/store.go

@@ -62,6 +62,7 @@ func (c closedProvider) KeyMaterial(ctx context.Context) (*KeyMaterial, error) {
 }
 
 func (c closedProvider) Close() {
+	panic("Close called more than once on certificate provider")
 }
 
 // singleCloseWrappedProvider wraps a provider instance with a reference count to avoid double

credentials/tls/certprovider/store_test.go

@@ -303,7 +303,11 @@ func (s) TestStoreSingleProviderDifferentConfigs(t *testing.T) {
 	cfg2 := fakeConfig + "2222"
 
 	prov1 := createProvider(t, fakeProvider1Name, cfg1, opts)
-	defer prov1.Close()
+	defer func() {
+		if prov1 != nil {
+			prov1.Close()
+		}
+	}()
 	// Our fakeProviderBuilder pushes newly created providers on a channel. Grab
 	// the fake provider from that channel.
 	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
@@ -349,6 +353,7 @@ func (s) TestStoreSingleProviderDifferentConfigs(t *testing.T) {
 
 	// Close one of the providers and verify that the other one is not affected.
 	prov1.Close()
+	prov1 = nil
 	if err := readAndVerifyKeyMaterial(ctx, prov2, km2); err != nil {
 		t.Fatal(err)
 	}
@@ -359,7 +364,11 @@ func (s) TestStoreSingleProviderDifferentConfigs(t *testing.T) {
 func (s) TestStoreMultipleProviders(t *testing.T) {
 	opts := BuildOptions{CertName: "foo"}
 	prov1 := createProvider(t, fakeProvider1Name, fakeConfig, opts)
-	defer prov1.Close()
+	defer func() {
+		if prov1 != nil {
+			prov1.Close()
+		}
+	}()
 	// Our fakeProviderBuilder pushes newly created providers on a channel. Grab
 	// the fake provider from that channel.
 	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
@@ -394,6 +403,7 @@ func (s) TestStoreMultipleProviders(t *testing.T) {
 
 	// Close one of the providers and verify that the other one is not affected.
 	prov1.Close()
+	prov1 = nil
 	if err := readAndVerifyKeyMaterial(ctx, prov2, km2); err != nil {
 		t.Fatal(err)
 	}