perf: use io.WriteString in writeString to avoid heap allocation (#2809)

writeString converted every string to []byte before calling
io.Writer.Write, which Go escape analysis reports as escaping to the
heap. io.WriteString uses the io.StringWriter fast path when available
(the standard printer writer is a *bufio.Writer) and falls back to
Write([]byte(txt)) otherwise.

Fixes #2807

Co-authored-by: Pablo Garcia <pablito@MacBook-Neo-de-Pablo.local>

pkg/yqlib/utils.go

@@ -50,7 +50,7 @@ func readStream(filename string) (io.Reader, error) {
 }
 
 func writeString(writer io.Writer, txt string) error {
-	_, errorWriting := writer.Write([]byte(txt))
+	_, errorWriting := io.WriteString(writer, txt)
 	return errorWriting
 }
 

pkg/yqlib/utils_test.go

@@ -0,0 +1,41 @@
+package yqlib
+
+import (
+	"bufio"
+	"bytes"
+	"io"
+	"testing"
+
+	"github.com/mikefarah/yq/v4/test"
+)
+
+// plainWriter only implements io.Writer, so io.WriteString must fall back to Write.
+type plainWriter struct {
+	buf bytes.Buffer
+}
+
+func (w *plainWriter) Write(p []byte) (int, error) {
+	return w.buf.Write(p)
+}
+
+func TestWriteStringToStringWriter(t *testing.T) {
+	var buf bytes.Buffer
+	writer := bufio.NewWriter(&buf)
+	test.AssertResult(t, nil, writeString(writer, "hello world"))
+	test.AssertResult(t, nil, writer.Flush())
+	test.AssertResult(t, "hello world", buf.String())
+}
+
+func TestWriteStringToPlainWriter(t *testing.T) {
+	writer := &plainWriter{}
+	test.AssertResult(t, nil, writeString(writer, "hello world"))
+	test.AssertResult(t, "hello world", writer.buf.String())
+}
+
+func TestWriteStringDoesNotAllocate(t *testing.T) {
+	writer := bufio.NewWriter(io.Discard)
+	allocations := testing.AllocsPerRun(100, func() {
+		_ = writeString(writer, "hello world")
+	})
+	test.AssertResult(t, 0.0, allocations)
+}