[#160] Add internal/net package with multinet dialer source

Signed-off-by: Alex Vanin <a.vanin@yadro.com>

internal/net/dialer.go

@@ -0,0 +1,36 @@
+package net
+
+import (
+	"net"
+	"syscall"
+	"time"
+
+	"golang.org/x/sys/unix"
+)
+
+func newDefaultDialer() net.Dialer {
+	// From `grpc.WithContextDialer` comment:
+	//
+	// Note: All supported releases of Go (as of December 2023) override the OS
+	// defaults for TCP keepalive time and interval to 15s. To enable TCP keepalive
+	// with OS defaults for keepalive time and interval, use a net.Dialer that sets
+	// the KeepAlive field to a negative value, and sets the SO_KEEPALIVE socket
+	// option to true from the Control field. For a concrete example of how to do
+	// this, see internal.NetDialerWithTCPKeepalive().
+	//
+	// https://github.com/grpc/grpc-go/blob/830135e6c5a351abf75f0c9cfdf978e5df8daeba/dialoptions.go#L432
+	//
+	// From `internal.NetDialerWithTCPKeepalive` comment:
+	//
+	// TODO: Once https://github.com/golang/go/issues/62254 lands, and the
+	// appropriate Go version becomes less than our least supported Go version, we
+	// should look into using the new API to make things more straightforward.
+	return net.Dialer{
+		KeepAlive: time.Duration(-1),
+		Control: func(_, _ string, c syscall.RawConn) error {
+			return c.Control(func(fd uintptr) {
+				_ = unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_KEEPALIVE, 1)
+			})
+		},
+	}
+}