Don't use GetConfigForClient on go < 1.8

crypto/cert_chain.go

@@ -57,14 +57,9 @@ func (c *certChain) GetLeafCert(sni string) ([]byte, error) {
 
 func (cc *certChain) getCertForSNI(sni string) (*tls.Certificate, error) {
 	c := cc.config
-	if c.GetConfigForClient != nil {
-		var err error
-		c, err = c.GetConfigForClient(&tls.ClientHelloInfo{
-			ServerName: sni,
-		})
-		if err != nil {
-			return nil, err
-		}
+	c, err := maybeGetConfigForClient(c, sni)
+	if err != nil {
+		return nil, err
 	}
 	// The rest of this function is mostly copied from crypto/tls.getCertificate
 

crypto/cert_chain_test.go

@@ -5,6 +5,7 @@ import (
 	"compress/flate"
 	"compress/zlib"
 	"crypto/tls"
+	"reflect"
 
 	"github.com/lucas-clemente/quic-go/testdata"
 
@@ -129,11 +130,16 @@ var _ = Describe("Proof", func() {
 		})
 
 		It("respects GetConfigForClient", func() {
+			if !reflect.ValueOf(tls.Config{}).FieldByName("GetConfigForClient").IsValid() {
+				// Pre 1.8, we don't have to do anything
+				return
+			}
 			nestedConfig := &tls.Config{Certificates: []tls.Certificate{cert}}
-			config.GetConfigForClient = func(chi *tls.ClientHelloInfo) (*tls.Config, error) {
+			l := func(chi *tls.ClientHelloInfo) (*tls.Config, error) {
 				Expect(chi.ServerName).To(Equal("quic.clemente.io"))
 				return nestedConfig, nil
 			}
+			reflect.ValueOf(config).Elem().FieldByName("GetConfigForClient").Set(reflect.ValueOf(l))
 			resultCert, err := cc.getCertForSNI("quic.clemente.io")
 			Expect(err).NotTo(HaveOccurred())
 			Expect(*resultCert).To(Equal(cert))

crypto/config_for_client_1.8.go

@@ -0,0 +1,14 @@
+// +build go1.8
+
+package crypto
+
+import "crypto/tls"
+
+func maybeGetConfigForClient(c *tls.Config, sni string) (*tls.Config, error) {
+	if c.GetConfigForClient == nil {
+		return c, nil
+	}
+	return c.GetConfigForClient(&tls.ClientHelloInfo{
+		ServerName: sni,
+	})
+}

crypto/config_for_client_pre1.8.go

@@ -0,0 +1,9 @@
+// +build !go1.8
+
+package crypto
+
+import "crypto/tls"
+
+func maybeGetConfigForClient(c *tls.Config, sni string) (*tls.Config, error) {
+	return c, nil
+}