Refactor markup/csv: don't read all to memory (#29760)

modules/markup/csv/csv_test.go

@@ -4,6 +4,8 @@
 package markup
 
 import (
+	"bufio"
+	"bytes"
 	"strings"
 	"testing"
 
@@ -29,4 +31,12 @@ func TestRenderCSV(t *testing.T) {
 		assert.NoError(t, err)
 		assert.EqualValues(t, v, buf.String())
 	}
+
+	t.Run("fallbackRender", func(t *testing.T) {
+		var buf bytes.Buffer
+		err := render.fallbackRender(strings.NewReader("1,<a>\n2,<b>"), bufio.NewWriter(&buf))
+		assert.NoError(t, err)
+		want := "<pre>1,&lt;a&gt;\n2,&lt;b&gt;</pre>"
+		assert.Equal(t, want, buf.String())
+	})
 }