fix(netxlite): ensure HTTP errors are always wrapped (#584)

1. introduce implementations of HTTPTransport and HTTPClient
that apply an error wrapping policy using the constructor
for a generic top-level error wrapper

2. make sure we use the implementations in point 1 when we
are constructing HTTPTransport and HTTPClient

3. make sure we apply error wrapping using the constructor for
a generic top-level error wrapper when reading bodies

4. acknowledge that error wrapping would be broken if we do
not return the same classification _and_ operation when we wrap
an already wrapped error, so fix the to code to do that

5. acknowledge that the classifiers already deal with preserving
the error string and explain why this is a quirk and why we
cannot remove it right now and what needs to happen to safely
remove this quirk from the codebase

Closes https://github.com/ooni/probe/issues/1860

internal/netxlite/http.go

@@ -19,6 +19,19 @@ type HTTPTransport interface {
 	CloseIdleConnections()
 }
 
+// httpTransportErrWrapper is an HTTPTransport with error wrapping.
+type httpTransportErrWrapper struct {
+	HTTPTransport
+}
+
+func (txp *httpTransportErrWrapper) RoundTrip(req *http.Request) (*http.Response, error) {
+	resp, err := txp.HTTPTransport.RoundTrip(req)
+	if err != nil {
+		return nil, NewTopLevelGenericErrWrapper(err)
+	}
+	return resp, nil
+}
+
 // httpTransportLogger is an HTTPTransport with logging.
 type httpTransportLogger struct {
 	// HTTPTransport is the underlying HTTP transport.
@@ -141,12 +154,13 @@ func NewOOHTTPBaseTransport(dialer Dialer, tlsDialer TLSDialer) HTTPTransport {
 	}
 }
 
-// WrapHTTPTransport creates an HTTPTransport using the given logger.
+// WrapHTTPTransport creates an HTTPTransport using the given logger
+// and guarantees that returned errors are wrapped.
 //
 // This is a low level factory. Consider not using it directly.
 func WrapHTTPTransport(logger Logger, txp HTTPTransport) HTTPTransport {
 	return &httpTransportLogger{
-		HTTPTransport: txp,
+		HTTPTransport: &httpTransportErrWrapper{txp},
 		Logger:        logger,
 	}
 }
@@ -250,3 +264,26 @@ func NewHTTPTransportStdlib(logger Logger) HTTPTransport {
 	tlsDialer := NewTLSDialer(dialer, NewTLSHandshakerStdlib(logger))
 	return NewHTTPTransport(logger, dialer, tlsDialer)
 }
+
+// HTTPClient is an http.Client-like interface.
+type HTTPClient interface {
+	Do(req *http.Request) (*http.Response, error)
+	CloseIdleConnections()
+}
+
+// WrapHTTPClient wraps an HTTP client to add error wrapping capabilities.
+func WrapHTTPClient(clnt HTTPClient) HTTPClient {
+	return &httpClientErrWrapper{clnt}
+}
+
+type httpClientErrWrapper struct {
+	HTTPClient
+}
+
+func (c *httpClientErrWrapper) Do(req *http.Request) (*http.Response, error) {
+	resp, err := c.HTTPClient.Do(req)
+	if err != nil {
+		return nil, NewTopLevelGenericErrWrapper(err)
+	}
+	return resp, nil
+}