From 44619d5ca27b442e03299e3c54c0a86e8138664d Mon Sep 17 00:00:00 2001
From: Katrina Verey <kn.verey@gmail.com>
Date: Fri, 16 Oct 2020 11:28:07 -0700
Subject: [PATCH] Option to continue pipeline processing when filter returns
 empty result

---
 kyaml/kio/kio.go      | 12 +++++++++++-
 kyaml/kio/kio_test.go | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/kyaml/kio/kio.go b/kyaml/kio/kio.go
index 91efde1d9..12271aa81 100644
--- a/kyaml/kio/kio.go
+++ b/kyaml/kio/kio.go
@@ -75,6 +75,16 @@ type Pipeline struct {
 
 	// Outputs are where the transformed Resource Configuration is written.
 	Outputs []Writer `yaml:"outputs,omitempty"`
+
+	// ContinueOnEmptyResult configures what happens when a filter in the pipeline
+	// returns an empty result.
+	// If it is false (default), subsequent filters will be skipped and the result
+	// will be returned immediately. This is useful as an optimization when you
+	// know that subsequent filters will not alter the empty result.
+	// If it is true, the empty result will be provided as input to the next
+	// filter in the list. This is useful when subsequent functions in the
+	// pipeline may generate new resources.
+	ContinueOnEmptyResult bool `yaml:"continueOnEmptyResult,omitempty"`
 }
 
 // Execute executes each step in the sequence, returning immediately after encountering
@@ -111,7 +121,7 @@ func (p Pipeline) ExecuteWithCallback(callback PipelineExecuteCallbackFunc) erro
 		// TODO (issue 2872): This len(result) == 0 should be removed and empty result list should be
 		// handled by outputs. However currently some writer like LocalPackageReadWriter
 		// will clear the output directory and which will cause unpredictable results
-		if len(result) == 0 || err != nil {
+		if len(result) == 0 && !p.ContinueOnEmptyResult || err != nil {
 			return errors.Wrap(err)
 		}
 	}
diff --git a/kyaml/kio/kio_test.go b/kyaml/kio/kio_test.go
index 9a3a9ea27..0529c97d7 100644
--- a/kyaml/kio/kio_test.go
+++ b/kyaml/kio/kio_test.go
@@ -149,3 +149,41 @@ items:
 		t.FailNow()
 	}
 }
+
+func TestContinueOnEmptyBehavior(t *testing.T) {
+	cases := map[string]struct {
+		continueOnEmptyResult bool
+		expected              string
+	}{
+		"quit on empty":     {continueOnEmptyResult: false, expected: ""},
+		"continue on empty": {continueOnEmptyResult: true, expected: "foo: bar"},
+	}
+	for _, tc := range cases {
+		actual := &bytes.Buffer{}
+		output := ByteWriter{Writer: actual}
+
+		generatorFunc := FilterFunc(func(nodes []*yaml.RNode) ([]*yaml.RNode, error) {
+			nodes = append(nodes, yaml.NewMapRNode(&map[string]string{
+				"foo": "bar",
+			}))
+			return nodes, nil
+		})
+		emptyFunc := FilterFunc(func(nodes []*yaml.RNode) ([]*yaml.RNode, error) { return nodes, nil })
+
+		p := Pipeline{
+			Outputs:               []Writer{output},
+			Filters:               []Filter{emptyFunc, generatorFunc},
+			ContinueOnEmptyResult: tc.continueOnEmptyResult,
+		}
+
+		err := p.Execute()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if !assert.Equal(t,
+			tc.expected, strings.TrimSpace(actual.String())) {
+			t.Fail()
+		}
+	}
+}