Check for and report bad protocol in TLSClientConfig.NextProtos

client.go

@@ -9,6 +9,7 @@ import (
 	"context"
 	"crypto/tls"
 	"errors"
+	"fmt"
 	"io"
 	"io/ioutil"
 	"net"
@@ -370,6 +371,17 @@ func (d *Dialer) DialContext(ctx context.Context, urlStr string, requestHeader h
 
 	resp, err := http.ReadResponse(conn.br, req)
 	if err != nil {
+		if d.TLSClientConfig != nil {
+			for _, proto := range d.TLSClientConfig.NextProtos {
+				if proto != "http/1.1" {
+					return nil, nil, fmt.Errorf(
+						"websocket: protocol %q was given but is not supported;"+
+							"sharing tls.Config with net/http Transport can cause this error: %w",
+						proto, err,
+					)
+				}
+			}
+		}
 		return nil, nil, err
 	}
 

client_server_test.go

@@ -1098,3 +1098,38 @@ func TestNetDialConnect(t *testing.T) {
 		}
 	}
 }
+func TestNextProtos(t *testing.T) {
+	ts := httptest.NewUnstartedServer(
+		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {}),
+	)
+	ts.EnableHTTP2 = true
+	ts.StartTLS()
+	defer ts.Close()
+
+	d := Dialer{
+		TLSClientConfig: ts.Client().Transport.(*http.Transport).TLSClientConfig,
+	}
+
+	r, err := ts.Client().Get(ts.URL)
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+	r.Body.Close()
+
+	// Asserts that Dialer.TLSClientConfig.NextProtos contains "h2"
+	// after the Client.Get call from net/http above.
+	var containsHTTP2 bool = false
+	for _, proto := range d.TLSClientConfig.NextProtos {
+		if proto == "h2" {
+			containsHTTP2 = true
+		}
+	}
+	if !containsHTTP2 {
+		t.Fatalf("Dialer.TLSClientConfig.NextProtos does not contain \"h2\"")
+	}
+
+	_, _, err = d.Dial(makeWsProto(ts.URL), nil)
+	if err == nil {
+		t.Fatalf("Dial succeeded, expect fail ")
+	}
+}