Use string builder to avoid copies on notes rendering

cmd/release-notes/main.go

@@ -294,12 +294,15 @@ func WriteReleaseNotes(releaseNotes notes.ReleaseNotes, history notes.ReleaseNot
 			return errors.Wrapf(err, "creating release note document")
 		}
 
-		if err := notes.RenderMarkdown(
-			output, doc, opts.ReleaseBucket,
-			opts.ReleaseTars, opts.StartRev, opts.EndRev,
-		); err != nil {
+		markdown, err := notes.RenderMarkdown(
+			doc, opts.ReleaseBucket, opts.ReleaseTars, opts.StartRev, opts.EndRev,
+		)
+		if err != nil {
 			return errors.Wrapf(err, "rendering release note document to markdown")
 		}
+		if _, err := output.WriteString(markdown); err != nil {
+			return errors.Wrapf(err, "writing output file")
+		}
 
 	default:
 		return errors.Errorf("%q is an unsupported format", opts.Format)

pkg/notes/document.go

@@ -111,9 +111,10 @@ func CreateDocument(notes ReleaseNotes, history ReleaseNotesHistory) (*Document,
 
 // RenderMarkdown accepts a Document and writes a version of that document to
 // supplied io.Writer in markdown format.
-func RenderMarkdown(w io.Writer, doc *Document, bucket, tars, prevTag, newTag string) error {
-	if err := createDownloadsTable(w, bucket, tars, prevTag, newTag); err != nil {
-		return err
+func RenderMarkdown(doc *Document, bucket, tars, prevTag, newTag string) (string, error) {
+	o := &strings.Builder{}
+	if err := createDownloadsTable(o, bucket, tars, prevTag, newTag); err != nil {
+		return "", err
 	}
 
 	// we always want to render the document with SIGs in alphabetical order
@@ -123,103 +124,109 @@ func RenderMarkdown(w io.Writer, doc *Document, bucket, tars, prevTag, newTag st
 	}
 	sort.Strings(sortedSIGs)
 
-	// this is a helper so that we don't have to check err != nil on every write
-
-	// first, we create a long-lived err that we can re-use
-	var err error
-
-	// write is a helper that writes a string to the in-scope io.Writer w
-	write := func(s string) {
-		// if write has already failed, just return and don't do anything
-		if err != nil {
-			return
-		}
-		// perform the write
-		_, err = w.Write([]byte(s))
+	nl := func() {
+		o.WriteRune('\n')
+	}
+	nlnl := func() {
+		nl()
+		nl()
 	}
 
 	// writeNote encapsulates the pre-processing that might happen on a note text
 	// before it gets bulleted and written to the io.Writer
 	writeNote := func(s string) {
-		if !strings.HasPrefix(s, "- ") {
-			s = "- " + s
+		const prefix = "- "
+		if !strings.HasPrefix(s, prefix) {
+			o.WriteString(prefix)
 		}
-		write(s + "\n")
+		o.WriteString(s)
+		nl()
 	}
 
 	// the "Action Required" section
 	if len(doc.ActionRequired) > 0 {
-		write("## Action Required\n\n")
+		o.WriteString("## Action Required")
+		nlnl()
 		for _, note := range doc.ActionRequired {
 			writeNote(note)
 		}
-		write("\n\n")
+		nlnl()
 	}
 
 	// the "New Feautres" section
 	if len(doc.NewFeatures) > 0 {
-		write("## New Features\n\n")
+		o.WriteString("## New Features")
+		nlnl()
 		for _, note := range doc.NewFeatures {
 			writeNote(note)
 		}
-		write("\n\n")
+		nlnl()
 	}
 
 	// the "API Changes" section
 	if len(doc.APIChanges) > 0 {
-		write("### API Changes\n\n")
+		o.WriteString("### API Changes")
+		nlnl()
 		for _, note := range doc.APIChanges {
 			writeNote(note)
 		}
-		write("\n\n")
+		nlnl()
 	}
 
 	// the "Duplicate Notes" section
 	if len(doc.Duplicates) > 0 {
-		write("### Notes from Multiple SIGs\n\n")
+		o.WriteString("### Notes from Multiple SIGs")
+		nlnl()
 		for header, notes := range doc.Duplicates {
-			write(fmt.Sprintf("#### %s\n\n", header))
+			o.WriteString("#### ")
+			o.WriteString(header)
+			nlnl()
 			for _, note := range notes {
 				writeNote(note)
 			}
-			write("\n")
+			nl()
 		}
-		write("\n")
+		nl()
 	}
 
 	// each SIG gets a section (in alphabetical order)
 	if len(sortedSIGs) > 0 {
-		write("### Notes from Individual SIGs\n\n")
+		o.WriteString("### Notes from Individual SIGs")
+		nlnl()
 		for _, sig := range sortedSIGs {
-			write("#### SIG " + prettySIG(sig) + "\n\n")
+			o.WriteString("#### SIG ")
+			o.WriteString(prettySIG(sig))
+			nlnl()
 			for _, note := range doc.SIGs[sig] {
 				writeNote(note)
 			}
-			write("\n")
+			nl()
 		}
-		write("\n\n")
+		nlnl()
 	}
 
 	// the "Bug Fixes" section
 	if len(doc.BugFixes) > 0 {
-		write("### Bug Fixes\n\n")
+		o.WriteString("### Bug Fixes")
+		nlnl()
 		for _, note := range doc.BugFixes {
 			writeNote(note)
 		}
-		write("\n\n")
+		nlnl()
 	}
 
 	// we call the uncategorized notes "Other Notable Changes". ideally these
 	// notes would at least have a SIG label.
 	if len(doc.Uncategorized) > 0 {
-		write("### Other Notable Changes\n\n")
+		o.WriteString("### Other Notable Changes")
+		nlnl()
 		for _, note := range doc.Uncategorized {
 			writeNote(note)
 		}
-		write("\n\n")
+		nlnl()
 	}
 
-	return err
+	return o.String(), nil
 }
 
 // prettySIG takes a sig name as parsed by the `sig-foo` label and returns a

pkg/notes/document_test.go

@@ -17,10 +17,10 @@ limitations under the License.
 package notes
 
 import (
-	"bytes"
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -44,7 +44,6 @@ func TestPrettySIG(t *testing.T) {
 
 func TestCreateDownloadsTable(t *testing.T) {
 	// Given
-	output := &bytes.Buffer{}
 	dir, err := ioutil.TempDir("", "")
 	require.Nil(t, err)
 	defer os.RemoveAll(dir)
@@ -80,8 +79,10 @@ func TestCreateDownloadsTable(t *testing.T) {
 	}
 
 	// When
-	require.Nil(t, createDownloadsTable(output, "kubernetes-release", dir,
-		"v1.16.0", "v1.16.1"))
+	output := &strings.Builder{}
+	require.Nil(t, createDownloadsTable(
+		output, "kubernetes-release", dir, "v1.16.0", "v1.16.1",
+	))
 
 	// Then
 	require.Equal(t, output.String(), `# v1.16.1