net: mptcp: implement dialMPTCP

This function is called when the user has requested MPTCP via
SetMultipathTCP in the Dialer.

This new function falls back to dialTCP on operating systems that do not
support MPTCP or if MPTCP is not supported.

On Dialer side, MultipathTCP function can be used to know if the package
will try to use MPTCP or not when Dial is called.

Note that this new dialMPTCP function returns a TCPConn object, like
dialTCP. A new MPTCPConn object using the following composition could
have been returned:

    type MPTCPConn struct {
        *TCPConn
    }

But the drawback is that if MPTCP is used by default one day (see #56539
issue on GitHub), Dial will return a different object: this new
MPTCPConn type instead of the previously expected TCPConn. This can
cause issues for apps checking the returned object.

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

Updates #56539

Change-Id: I0f9b5b81f630b39142bdd553d4f1b4c775f1dff0
Reviewed-on: https://go-review.googlesource.com/c/go/+/471136
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

api/next/56539.txt

@@ -0,0 +1,2 @@
+pkg net, method (*Dialer) MultipathTCP() bool #56539
+pkg net, method (*Dialer) SetMultipathTCP(bool) #56539

src/net/dial.go

@@ -141,6 +141,11 @@ type Dialer struct {
 	//
 	// If ControlContext is not nil, Control is ignored.
 	ControlContext func(ctx context.Context, network, address string, c syscall.RawConn) error
+
+	// If mptcpStatus is set to a value allowing Multipath TCP (MPTCP) to be
+	// used, any call to Dial with "tcp(4|6)" as network will use MPTCP if
+	// supported by the operating system.
+	mptcpStatus mptcpStatus
 }
 
 func (d *Dialer) dualStack() bool { return d.FallbackDelay >= 0 }
@@ -314,6 +319,24 @@ func (r *Resolver) resolveAddrList(ctx context.Context, op, network, addr string
 	return naddrs, nil
 }
 
+// MultipathTCP reports whether MPTCP will be used.
+//
+// This method doesn't check if MPTCP is supported by the operating
+// system or not.
+func (d *Dialer) MultipathTCP() bool {
+	return d.mptcpStatus.get()
+}
+
+// SetMultipathTCP directs the Dial methods to use, or not use, MPTCP,
+// if supported by the operating system. This method overrides the
+// system default.
+//
+// If MPTCP is not available on the host or not supported by the server,
+// the Dial methods will fall back to TCP.
+func (d *Dialer) SetMultipathTCP(use bool) {
+	d.mptcpStatus.set(use)
+}
+
 // Dial connects to the address on the named network.
 //
 // Known networks are "tcp", "tcp4" (IPv4-only), "tcp6" (IPv6-only),
@@ -610,7 +633,11 @@ func (sd *sysDialer) dialSingle(ctx context.Context, ra Addr) (c Conn, err error
 	switch ra := ra.(type) {
 	case *TCPAddr:
 		la, _ := la.(*TCPAddr)
+		if sd.MultipathTCP() {
+			c, err = sd.dialMPTCP(ctx, la, ra)
+		} else {
 			c, err = sd.dialTCP(ctx, la, ra)
+		}
 	case *UDPAddr:
 		la, _ := la.(*UDPAddr)
 		c, err = sd.dialUDP(ctx, la, ra)

src/net/mptcpsock_linux.go

@@ -5,6 +5,7 @@
 package net
 
 import (
+	"context"
 	"errors"
 	"internal/poll"
 	"sync"
@@ -41,3 +42,12 @@ func initMPTCPavailable() {
 		mptcpAvailable = true
 	}
 }
+
+func (sd *sysDialer) dialMPTCP(ctx context.Context, laddr, raddr *TCPAddr) (*TCPConn, error) {
+	// Fallback to dialTCP if Multipath TCP isn't supported on this operating system.
+	if !supportsMultipathTCP() {
+		return sd.dialTCP(ctx, laddr, raddr)
+	}
+
+	return sd.doDialTCPProto(ctx, laddr, raddr, _IPPROTO_MPTCP)
+}

src/net/mptcpsock_stub.go

@@ -5,3 +5,11 @@
 //go:build !linux
 
 package net
+
+import (
+	"context"
+)
+
+func (sd *sysDialer) dialMPTCP(ctx context.Context, laddr, raddr *TCPAddr) (*TCPConn, error) {
+	return sd.dialTCP(ctx, laddr, raddr)
+}

src/net/tcpsock_posix.go

@@ -65,13 +65,17 @@ func (sd *sysDialer) dialTCP(ctx context.Context, laddr, raddr *TCPAddr) (*TCPCo
 }
 
 func (sd *sysDialer) doDialTCP(ctx context.Context, laddr, raddr *TCPAddr) (*TCPConn, error) {
+	return sd.doDialTCPProto(ctx, laddr, raddr, 0)
+}
+
+func (sd *sysDialer) doDialTCPProto(ctx context.Context, laddr, raddr *TCPAddr, proto int) (*TCPConn, error) {
 	ctrlCtxFn := sd.Dialer.ControlContext
 	if ctrlCtxFn == nil && sd.Dialer.Control != nil {
 		ctrlCtxFn = func(cxt context.Context, network, address string, c syscall.RawConn) error {
 			return sd.Dialer.Control(network, address, c)
 		}
 	}
-	fd, err := internetSocket(ctx, sd.network, laddr, raddr, syscall.SOCK_STREAM, 0, "dial", ctrlCtxFn)
+	fd, err := internetSocket(ctx, sd.network, laddr, raddr, syscall.SOCK_STREAM, proto, "dial", ctrlCtxFn)
 
 	// TCP has a rarely used mechanism called a 'simultaneous connection' in
 	// which Dial("tcp", addr1, addr2) run on the machine at addr1 can
@@ -101,7 +105,7 @@ func (sd *sysDialer) doDialTCP(ctx context.Context, laddr, raddr *TCPAddr) (*TCP
 		if err == nil {
 			fd.Close()
 		}
-		fd, err = internetSocket(ctx, sd.network, laddr, raddr, syscall.SOCK_STREAM, 0, "dial", ctrlCtxFn)
+		fd, err = internetSocket(ctx, sd.network, laddr, raddr, syscall.SOCK_STREAM, proto, "dial", ctrlCtxFn)
 	}
 
 	if err != nil {