refactor: No error return for print (oras-project#1375)

Signed-off-by: Terry Howe <[email protected]>

cmd/oras/internal/display/status/print.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"io"
 	"oras.land/oras/internal/descriptor"
+	"os"
 	"sync"
 
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
@@ -45,7 +46,12 @@ func (p *Printer) Println(a ...any) error {
 	p.lock.Lock()
 	defer p.lock.Unlock()
 	_, err := fmt.Fprintln(p.out, a...)
-	return err
+	if err != nil {
+		err = fmt.Errorf("display output error: %w", err)
+		_, _ = fmt.Fprint(os.Stderr, err)
+	}
+	// Errors are handled above, so return nil
+	return nil
 }
 
 // PrintStatus prints transfer status.

cmd/oras/internal/display/status/print_test.go

@@ -0,0 +1,65 @@
+/*
+Copyright The ORAS Authors.
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package status
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+type mockWriter struct {
+	errorCount int
+	written    string
+}
+
+func (mw *mockWriter) Write(p []byte) (n int, err error) {
+	mw.written += string(p)
+	if strings.TrimSpace(string(p)) != "boom" {
+		return len(string(p)), nil
+	}
+	mw.errorCount++
+	return 0, fmt.Errorf("Boom: " + string(p))
+}
+
+func (mw *mockWriter) String() string {
+	return mw.written
+}
+
+func TestPrint_Error(t *testing.T) {
+	mockWriter := &mockWriter{}
+	printer := NewPrinter(mockWriter)
+	printer.Println("boom")
+	if mockWriter.errorCount != 1 {
+		t.Error("Expected one errors actual <" + strconv.Itoa(mockWriter.errorCount) + ">")
+	}
+}
+
+func TestPrint_NoError(t *testing.T) {
+	mockWriter := &mockWriter{}
+	printer := NewPrinter(mockWriter)
+
+	expected := "blah blah"
+	printer.Println(expected)
+	actual := strings.TrimSpace(mockWriter.String())
+	if expected != actual {
+		t.Error("Expected <" + expected + "> not equal to actual <" + actual + ">")
+	}
+	if mockWriter.errorCount != 0 {
+		t.Error("Expected no errors actual <" + strconv.Itoa(mockWriter.errorCount) + ">")
+	}
+}