add option to continue pipeline when the input is empty

cmd/config/internal/commands/source.go

@@ -77,6 +77,6 @@ func (r *SourceRunner) runE(c *cobra.Command, args []string) error {
 		inputs = []kio.Reader{&kio.ByteReader{Reader: c.InOrStdin()}}
 	}
 
-	err := kio.Pipeline{Inputs: inputs, Outputs: outputs}.Execute()
+	err := kio.Pipeline{Inputs: inputs, Outputs: outputs, ContinueIfInputEmpty: true}.Execute()
 	return handleError(c, err)
 }

kyaml/kio/kio.go

@@ -75,6 +75,11 @@ type Pipeline struct {
 
 	// Outputs are where the transformed Resource Configuration is written.
 	Outputs []Writer `yaml:"outputs,omitempty"`
+
+	// ContinueIfInputEmpty indicates should pipeline continue when the
+	// the input result is empty. This is useful when we want to run outputs
+	// even the input is empty.
+	ContinueIfInputEmpty bool
 }
 
 // Execute executes each step in the sequence, returning immediately after encountering
@@ -99,7 +104,7 @@ func (p Pipeline) ExecuteWithCallback(callback PipelineExecuteCallbackFunc) erro
 		}
 		result = append(result, nodes...)
 	}
-	if len(result) == 0 {
+	if len(result) == 0 && !p.ContinueIfInputEmpty {
 		// no inputs to operate on
 		return nil
 	}

kyaml/kio/kio_test.go

@@ -4,7 +4,9 @@
 package kio_test
 
 import (
+	"bytes"
 	"reflect"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -75,3 +77,34 @@ func TestPipelineWithCallback(t *testing.T) {
 		)
 	}
 }
+
+func TestContinueIfInputEmpty(t *testing.T) {
+	actual := &bytes.Buffer{}
+	output := ByteWriter{
+		Sort:               true,
+		WrappingKind:       ResourceListKind,
+		WrappingAPIVersion: ResourceListAPIVersion,
+	}
+	output.Writer = actual
+
+	p := Pipeline{
+		Outputs:              []Writer{output},
+		ContinueIfInputEmpty: true,
+	}
+
+	err := p.Execute()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	expected := `
+apiVersion: config.kubernetes.io/v1alpha1
+kind: ResourceList
+items: []
+`
+
+	if !assert.Equal(t,
+		strings.TrimSpace(expected), strings.TrimSpace(actual.String())) {
+		t.FailNow()
+	}
+}