fix(oohelperd): reduce errors to what the old TH would emit (#543)

Reducing the errors is not done in a perfect way.

We have documented the most striking differences inside
https://github.com/ooni/probe/issues/1707#issuecomment-942283746 and
some attempts to improve the situation further inside
https://github.com/ooni/probe/issues/1707#issuecomment-942341255.

A better strategy for the future would be to introduce more
specific timeout errors, such as dns_timeout_error, etc.

More testing may be needed to further validate and compare the
old and the new TH, but this requires Jafar improvements to
more precisely simulate more complex censorship.

internal/cmd/oohelperd/internal/webconnectivity/http.go

@@ -8,6 +8,7 @@ import (
 	"sync"
 
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/archival"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
@@ -32,7 +33,7 @@ func HTTPDo(ctx context.Context, config *HTTPConfig) {
 	if err != nil {
 		config.Out <- CtrlHTTPResponse{ // fix: emit -1 like the old test helper does
 			BodyLength: -1,
-			Failure:    newfailure(err),
+			Failure:    httpMapFailure(err),
 			StatusCode: -1,
 			Headers:    map[string]string{},
 		}
@@ -52,7 +53,7 @@ func HTTPDo(ctx context.Context, config *HTTPConfig) {
 	if err != nil {
 		config.Out <- CtrlHTTPResponse{ // fix: emit -1 like old test helper does
 			BodyLength: -1,
-			Failure:    newfailure(err),
+			Failure:    httpMapFailure(err),
 			StatusCode: -1,
 			Headers:    map[string]string{},
 		}
@@ -67,9 +68,56 @@ func HTTPDo(ctx context.Context, config *HTTPConfig) {
 	data, err := netxlite.ReadAllContext(ctx, reader)
 	config.Out <- CtrlHTTPResponse{
 		BodyLength: int64(len(data)),
-		Failure:    newfailure(err),
+		Failure:    httpMapFailure(err),
 		StatusCode: int64(resp.StatusCode),
 		Headers:    headers,
 		Title:      webconnectivity.GetTitle(string(data)),
 	}
 }
+
+// httpMapFailure attempts to map netxlite failures to the strings
+// used by the original OONI test helper.
+//
+// See https://github.com/ooni/backend/blob/6ec4fda5b18/oonib/testhelpers/http_helpers.py#L361
+func httpMapFailure(err error) *string {
+	failure := newfailure(err)
+	failedOperation := archival.NewFailedOperation(err)
+	switch failure {
+	case nil:
+		return nil
+	default:
+		switch *failure {
+		case netxlite.FailureDNSNXDOMAINError,
+			netxlite.FailureDNSNoAnswer,
+			netxlite.FailureDNSNonRecoverableFailure,
+			netxlite.FailureDNSRefusedError,
+			netxlite.FailureDNSServerMisbehaving,
+			netxlite.FailureDNSTemporaryFailure:
+			// Strangely the HTTP code uses the more broad
+			// dns_lookup_error and does not check for
+			// the NXDOMAIN-equivalent-error dns_name_error
+			s := "dns_lookup_error"
+			return &s
+		case netxlite.FailureGenericTimeoutError:
+			// The old TH would return "dns_lookup_error" when
+			// there is a timeout error during the DNS phase of HTTP.
+			switch failedOperation {
+			case nil:
+				// nothing
+			default:
+				switch *failedOperation {
+				case netxlite.ResolveOperation:
+					s := "dns_lookup_error"
+					return &s
+				}
+			}
+			return failure // already using the same name
+		case netxlite.FailureConnectionRefused:
+			s := "connection_refused_error"
+			return &s
+		default:
+			s := "unknown_error"
+			return &s
+		}
+	}
+}