net: add initial MPTCP support

This currently defines an internal function supportsMultipathTCP which
reports whether MPTCP[1] is supported on the current platform.

Only Linux is supported here.

The check on Linux is performed once by attemting to create an MPTCP
socket and look at the returned error:

- If the protocol is not supported, EINVAL (kernel < 5.6) or
  EPROTONOSUPPORT (kernel >= 5.6) is returned and there is no point to
  try again.

- Other errors can be returned:
  - ENOPROTOOPT: the sysctl knob net.mptcp.enabled is set to 0
  - Unpredictable ones: if MPTCP is blocked using SELinux, eBPF, etc.

These other errors are due to modifications that can be reverted during
the session: MPTCP can be available again later. In this case, it is
fine to always try to create an MPTCP socket and fallback to TCP in case
of error.

This work has been co-developped by Gregory Detal
<gregory.detal@tessares.net>.

[1] https://www.rfc-editor.org/rfc/rfc8684.html

Updates #56539

Change-Id: Ic84fe85aad887a2be4556a898e649bf6b6f12f03
Reviewed-on: https://go-review.googlesource.com/c/go/+/471135
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/net/mptcpsock_linux.go

@@ -0,0 +1,43 @@
+// Copyright 2023 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.
+
+package net
+
+import (
+	"errors"
+	"internal/poll"
+	"sync"
+	"syscall"
+)
+
+var (
+	mptcpOnce      sync.Once
+	mptcpAvailable bool
+)
+
+// These constants aren't in the syscall package, which is frozen
+const (
+	_IPPROTO_MPTCP = 0x106
+)
+
+func supportsMultipathTCP() bool {
+	mptcpOnce.Do(initMPTCPavailable)
+	return mptcpAvailable
+}
+
+// Check that MPTCP is supported by attemting to create an MPTCP socket and by
+// looking at the returned error if any.
+func initMPTCPavailable() {
+	s, err := sysSocket(syscall.AF_INET, syscall.SOCK_STREAM, _IPPROTO_MPTCP)
+	switch {
+	case errors.Is(err, syscall.EPROTONOSUPPORT): // Not supported: >= v5.6
+	case errors.Is(err, syscall.EINVAL): // Not supported: < v5.6
+	case err == nil: // Supported and no error
+		poll.CloseFunc(s)
+		fallthrough
+	default:
+		// another error: MPTCP was not available but it might be later
+		mptcpAvailable = true
+	}
+}

src/net/mptcpsock_stub.go

@@ -0,0 +1,7 @@
+// Copyright 2023 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 !linux
+
+package net