feat: tlsmiddlebox experiment (#817)

See https://github.com/ooni/probe/issues/2124

internal/engine/experiment/tlsmiddlebox/conn.go

@@ -0,0 +1,62 @@
+package tlsmiddlebox
+
+//
+// Wrapped TTL conn
+//
+
+import (
+	"errors"
+	"net"
+	"syscall"
+
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+)
+
+var errInvalidConnWrapper = errors.New("invalid conn wrapper")
+
+// setConnTTL calls SetTTL to set the TTL for a dialerTTLWrapperConn
+func setConnTTL(conn net.Conn, ttl int) error {
+	ttlWrapper, ok := conn.(*dialerTTLWrapperConn)
+	if !ok {
+		return errInvalidConnWrapper
+	}
+	return ttlWrapper.SetTTL(ttl)
+}
+
+// getSoErr calls GetSoErr to fetch the SO_ERROR value
+func getSoErr(conn net.Conn) (soErr error, err error) {
+	ttlWrapper, ok := conn.(*dialerTTLWrapperConn)
+	if !ok {
+		return nil, errInvalidConnWrapper
+	}
+	errno, err := ttlWrapper.GetSoErr()
+	if err != nil {
+		return nil, err
+	}
+	return syscall.Errno(errno), nil
+}
+
+// dialerTTLWrapperConn wraps errors as well as allows us to set the TTL
+type dialerTTLWrapperConn struct {
+	net.Conn
+}
+
+var _ net.Conn = &dialerTTLWrapperConn{}
+
+// Read implements net.Conn.Read
+func (c *dialerTTLWrapperConn) Read(b []byte) (int, error) {
+	count, err := c.Conn.Read(b)
+	if err != nil {
+		return 0, netxlite.NewErrWrapper(netxlite.ClassifyGenericError, netxlite.ReadOperation, err)
+	}
+	return count, nil
+}
+
+// Write implements net.Conn.Write
+func (c *dialerTTLWrapperConn) Write(b []byte) (int, error) {
+	count, err := c.Conn.Write(b)
+	if err != nil {
+		return 0, netxlite.NewErrWrapper(netxlite.ClassifyGenericError, netxlite.WriteOperation, err)
+	}
+	return count, nil
+}