crypto/tls: don't use tls.Dialer on pre-Go1.15 systems

We have received many reports of build failures using Go 1.14,
enough to make it worth supporting that release even though it
is out of our support window.

For golang/go#45942
For golang/go#45943
For golang/go#45946
For golang/go#45950
For golang/go#45968

Change-Id: I9d6b1762ae622b3e9a5de5b1967264387166b1c7
Reviewed-on: https://go-review.googlesource.com/c/net/+/317069
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>

http2/go115.go

@@ -0,0 +1,27 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.15
+// +build go1.15
+
+package http2
+
+import (
+	"context"
+	"crypto/tls"
+)
+
+// dialTLSWithContext uses tls.Dialer, added in Go 1.15, to open a TLS
+// connection.
+func (t *Transport) dialTLSWithContext(ctx context.Context, network, addr string, cfg *tls.Config) (*tls.Conn, error) {
+	dialer := &tls.Dialer{
+		Config: cfg,
+	}
+	cn, err := dialer.DialContext(ctx, network, addr)
+	if err != nil {
+		return nil, err
+	}
+	tlsCn := cn.(*tls.Conn) // DialContext comment promises this will always succeed
+	return tlsCn, nil
+}

http2/not_go115.go

@@ -0,0 +1,31 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !go1.15
+// +build !go1.15
+
+package http2
+
+import (
+	"context"
+	"crypto/tls"
+)
+
+// dialTLSWithContext opens a TLS connection.
+func (t *Transport) dialTLSWithContext(ctx context.Context, network, addr string, cfg *tls.Config) (*tls.Conn, error) {
+	cn, err := tls.Dial(network, addr, cfg)
+	if err != nil {
+		return nil, err
+	}
+	if err := cn.Handshake(); err != nil {
+		return nil, err
+	}
+	if cfg.InsecureSkipVerify {
+		return cn, nil
+	}
+	if err := cn.VerifyHostname(cfg.ServerName); err != nil {
+		return nil, err
+	}
+	return cn, nil
+}

http2/transport.go

@@ -595,14 +595,10 @@ func (t *Transport) dialTLS(ctx context.Context) func(string, string, *tls.Confi
 		return t.DialTLS
 	}
 	return func(network, addr string, cfg *tls.Config) (net.Conn, error) {
-		dialer := &tls.Dialer{
-			Config: cfg,
-		}
-		cn, err := dialer.DialContext(ctx, network, addr)
+		tlsCn, err := t.dialTLSWithContext(ctx, network, addr, cfg)
 		if err != nil {
 			return nil, err
 		}
-		tlsCn := cn.(*tls.Conn) // DialContext comment promises this will always succeed
 		state := tlsCn.ConnectionState()
 		if p := state.NegotiatedProtocol; p != NextProtoTLS {
 			return nil, fmt.Errorf("http2: unexpected ALPN protocol %q; want %q", p, NextProtoTLS)
@@ -610,7 +606,7 @@ func (t *Transport) dialTLS(ctx context.Context) func(string, string, *tls.Confi
 		if !state.NegotiatedProtocolIsMutual {
 			return nil, errors.New("http2: could not negotiate protocol mutually")
 		}
-		return cn, nil
+		return tlsCn, nil
 	}
 }