fix(reduceErrors): return error when given an empty list (#675)

See https://github.com/ooni/probe/issues/1985 for context.

While there, ensure nextlite has 100% of coverage.

internal/netxlite/dnsovertcp_test.go

@@ -208,6 +208,7 @@ func TestDNSOverTCP(t *testing.T) {
 		if txp.Address() != address {
 			t.Fatal("invalid Address")
 		}
+		txp.CloseIdleConnections()
 	})
 
 	t.Run("other functions okay with TLS", func(t *testing.T) {
@@ -222,5 +223,6 @@ func TestDNSOverTCP(t *testing.T) {
 		if txp.Address() != address {
 			t.Fatal("invalid Address")
 		}
+		txp.CloseIdleConnections()
 	})
 }

internal/netxlite/http3_test.go

@@ -59,6 +59,13 @@ func TestHTTP3Transport(t *testing.T) {
 		}
 	})
 
+	t.Run("Network", func(t *testing.T) {
+		txp := &http3Transport{}
+		if txp.Network() != "quic" {
+			t.Fatal("unexpected .Network return value")
+		}
+	})
+
 	t.Run("RoundTrip", func(t *testing.T) {
 		expected := errors.New("mocked error")
 		txp := &http3Transport{

internal/netxlite/http_test.go

@@ -439,7 +439,6 @@ func TestHTTPTLSDialerWithReadTimeout(t *testing.T) {
 }
 
 func TestNewHTTPTransportStdlib(t *testing.T) {
-	// What to test about this factory?
 	txp := NewHTTPTransportStdlib(log.Log)
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel() // immediately!
@@ -454,6 +453,9 @@ func TestNewHTTPTransportStdlib(t *testing.T) {
 	if resp != nil {
 		t.Fatal("unexpected resp")
 	}
+	if txp.Network() != "tcp" {
+		t.Fatal("unexpected .Network return value")
+	}
 	txp.CloseIdleConnections()
 }
 

internal/netxlite/quirks.go

@@ -9,6 +9,9 @@ import (
 // the original netx implementation and that we cannot remove
 // or change without thinking about the consequences.
 
+// See https://github.com/ooni/probe/issues/1985
+var errReduceErrorsEmptyList = errors.New("bug: reduceErrors given an empty list")
+
 // quirkReduceErrors finds a known error in a list of errors since
 // it's probably most relevant. If this error is not found, just
 // return the first error according to this reasoning:
@@ -31,7 +34,8 @@ import (
 // See TODO(https://github.com/ooni/probe/issues/1779).
 func quirkReduceErrors(errorslist []error) error {
 	if len(errorslist) == 0 {
-		return nil
+		// See https://github.com/ooni/probe/issues/1985
+		return errReduceErrorsEmptyList
 	}
 	for _, err := range errorslist {
 		var wrapper *ErrWrapper

internal/netxlite/quirks_test.go

@@ -10,7 +10,7 @@ import (
 func TestQuirkReduceErrors(t *testing.T) {
 	t.Run("no errors", func(t *testing.T) {
 		result := quirkReduceErrors(nil)
-		if result != nil {
+		if !errors.Is(result, errReduceErrorsEmptyList) {
 			t.Fatal("wrong result")
 		}
 	})