fix(dnscheck): log "ok" rather than "<nil>" on success (#695)

See https://github.com/ooni/probe/issues/2020

CONTRIBUTING.md

@@ -68,6 +68,9 @@ run `go mod tidy` to minimize such changes.
 - use `./internal/netxlite.ReadAllContext` instead of `io.ReadAll`
 and `./internal/netxlite.CopyContext` instead of `io.Copy`
 
+- use `./internal/model.ErrorToStringOrOK` when 
+an experiment logs intermediate results
+
 ## Code testing requirements
 
 Make sure all tests pass with `go test -race ./...` run from the

internal/engine/experiment/dnscheck/dnscheck.go

@@ -272,7 +272,7 @@ func Collect(ctx context.Context, multi urlgetter.Multi, inputs []urlgetter.Mult
 			count++
 			percentage := float64(count) / float64(expect)
 			callbacks.OnProgress(percentage, fmt.Sprintf(
-				"dnscheck: measure %s: %+v", entry.Input.Config.ResolverURL, entry.Err,
+				"dnscheck: measure %s: %+v", entry.Input.Config.ResolverURL, model.ErrorToStringOrOK(entry.Err),
 			))
 			outputch <- entry
 		}

internal/model/logger.go

@@ -61,3 +61,11 @@ func (logDiscarder) Warn(msg string) {}
 
 // Warnf implements Logger.Warnf
 func (logDiscarder) Warnf(format string, v ...interface{}) {}
+
+// ErrorToStringOrOK emits "ok" on "<nil>"" values for success.
+func ErrorToStringOrOK(err error) string {
+	if err != nil {
+		return err.Error()
+	}
+	return "ok"
+}

internal/model/logger_test.go

@@ -1,6 +1,9 @@
 package model
 
-import "testing"
+import (
+	"io"
+	"testing"
+)
 
 func TestDiscardLoggerWorksAsIntended(t *testing.T) {
 	logger := DiscardLogger
@@ -11,3 +14,20 @@ func TestDiscardLoggerWorksAsIntended(t *testing.T) {
 	logger.Warn("foo")
 	logger.Warnf("%s", "foo")
 }
+
+func TestErrorToStringOrOK(t *testing.T) {
+	t.Run("on success", func(t *testing.T) {
+		expectedResult := ErrorToStringOrOK(nil)
+		if expectedResult != "ok" {
+			t.Fatal("expected ok")
+		}
+	})
+
+	t.Run("on failure", func(t *testing.T) {
+		err := io.EOF
+		expectedResult := ErrorToStringOrOK(err)
+		if expectedResult != err.Error() {
+			t.Fatal("not the result we expected", expectedResult)
+		}
+	})
+}