Fix certificate error detection on Go 1.20 and 1.21.

cert_error_go119.go

@@ -0,0 +1,14 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+//go:build !go1.20
+// +build !go1.20
+
+package retryablehttp
+
+import "crypto/x509"
+
+func isCertError(err error) bool {
+	_, ok := err.(x509.UnknownAuthorityError)
+	return ok
+}

cert_error_go120.go

@@ -0,0 +1,14 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+//go:build go1.20
+// +build go1.20
+
+package retryablehttp
+
+import "crypto/tls"
+
+func isCertError(err error) bool {
+	_, ok := err.(*tls.CertificateVerificationError)
+	return ok
+}

client.go

@@ -27,7 +27,6 @@ package retryablehttp
 import (
 	"bytes"
 	"context"
-	"crypto/x509"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -508,7 +507,7 @@ func baseRetryPolicy(resp *http.Response, err error) (bool, error) {
 			if notTrustedErrorRe.MatchString(v.Error()) {
 				return false, v
 			}
-			if _, ok := v.Err.(x509.UnknownAuthorityError); ok {
+			if isCertError(v.Err) {
 				return false, v
 			}
 		}