pemfile: Make test happy with Go1.16 (#4164)

Go1.16 adds a new unexported field to x509.CertPool which causes our
tests to fail because cmp.Equal() isn't happy. This change introduces a
helper function which compares certprovider.KeyMaterial in a way that
makes the test happy with the new Go version.

credentials/tls/certprovider/pemfile/watcher_test.go

@@ -20,7 +20,7 @@ package pemfile
 
 import (
 	"context"
-	"crypto/x509"
+	"fmt"
 	"io/ioutil"
 	"math/big"
 	"os"
@@ -29,6 +29,7 @@ import (
 	"time"
 
 	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
 
 	"google.golang.org/grpc/credentials/tls/certprovider"
 	"google.golang.org/grpc/internal/grpctest"
@@ -55,6 +56,30 @@ func Test(t *testing.T) {
 	grpctest.RunSubTests(t, s{})
 }
 
+func compareKeyMaterial(got, want *certprovider.KeyMaterial) error {
+	// x509.Certificate type defines an Equal() method, but does not check for
+	// nil. This has been fixed in
+	// https://github.com/golang/go/commit/89865f8ba64ccb27f439cce6daaa37c9aa38f351,
+	// but this is only available starting go1.14.
+	// TODO(easwars): Remove this check once we remove support for go1.13.
+	if (got.Certs == nil && want.Certs != nil) || (want.Certs == nil && got.Certs != nil) {
+		return fmt.Errorf("keyMaterial certs = %+v, want %+v", got, want)
+	}
+	if !cmp.Equal(got.Certs, want.Certs, cmp.AllowUnexported(big.Int{})) {
+		return fmt.Errorf("keyMaterial certs = %+v, want %+v", got, want)
+	}
+	// x509.CertPool contains only unexported fields some of which contain other
+	// unexported fields. So usage of cmp.AllowUnexported() or
+	// cmpopts.IgnoreUnexported() does not help us much here. Also, the standard
+	// library does not provide a way to compare CertPool values. Comparing the
+	// subjects field of the certs in the CertPool seems like a reasonable
+	// approach.
+	if gotR, wantR := got.Roots.Subjects(), want.Roots.Subjects(); !cmp.Equal(gotR, wantR, cmpopts.EquateEmpty()) {
+		return fmt.Errorf("keyMaterial roots = %v, want %v", gotR, wantR)
+	}
+	return nil
+}
+
 // TestNewProvider tests the NewProvider() function with different inputs.
 func (s) TestNewProvider(t *testing.T) {
 	tests := []struct {
@@ -263,7 +288,7 @@ func (s) TestProvider_UpdateSuccess(t *testing.T) {
 	if err != nil {
 		t.Fatalf("provider.KeyMaterial() failed: %v", err)
 	}
-	if cmp.Equal(km1, km2, cmp.AllowUnexported(big.Int{}, x509.CertPool{})) {
+	if err := compareKeyMaterial(km1, km2); err == nil {
 		t.Fatal("expected provider to return new key material after update to underlying file")
 	}
 
@@ -279,7 +304,7 @@ func (s) TestProvider_UpdateSuccess(t *testing.T) {
 	if err != nil {
 		t.Fatalf("provider.KeyMaterial() failed: %v", err)
 	}
-	if cmp.Equal(km2, km3, cmp.AllowUnexported(big.Int{}, x509.CertPool{})) {
+	if err := compareKeyMaterial(km2, km3); err == nil {
 		t.Fatal("expected provider to return new key material after update to underlying file")
 	}
 }
@@ -363,7 +388,7 @@ func (s) TestProvider_UpdateSuccessWithSymlink(t *testing.T) {
 		t.Fatalf("provider.KeyMaterial() failed: %v", err)
 	}
 
-	if cmp.Equal(km1, km2, cmp.AllowUnexported(big.Int{}, x509.CertPool{})) {
+	if err := compareKeyMaterial(km1, km2); err == nil {
 		t.Fatal("expected provider to return new key material after symlink update")
 	}
 }
@@ -403,8 +428,8 @@ func (s) TestProvider_UpdateFailure_ThenSuccess(t *testing.T) {
 	if err != nil {
 		t.Fatalf("provider.KeyMaterial() failed: %v", err)
 	}
-	if !cmp.Equal(km1, km2, cmp.AllowUnexported(big.Int{}, x509.CertPool{})) {
-		t.Fatal("expected provider to not update key material")
+	if err := compareKeyMaterial(km1, km2); err != nil {
+		t.Fatalf("expected provider to not update key material: %v", err)
 	}
 
 	// Update the key file to match the cert file.
@@ -418,7 +443,7 @@ func (s) TestProvider_UpdateFailure_ThenSuccess(t *testing.T) {
 	if err != nil {
 		t.Fatalf("provider.KeyMaterial() failed: %v", err)
 	}
-	if cmp.Equal(km2, km3, cmp.AllowUnexported(big.Int{}, x509.CertPool{})) {
+	if err := compareKeyMaterial(km2, km3); err == nil {
 		t.Fatal("expected provider to return new key material after update to underlying file")
 	}
 }