Add `--nul-output|-0` flag to separate element with NUL character (#1550)

This is to ensure solid parsing of complex data (with any binary
content except NUL chars) by separating the `yq` root collection
member's output with NUL char. As a safe-guard, an error will be cast
if trying to use NUL character with content that contains itself NUL
characters inside.

pkg/yqlib/printer_test.go

@@ -340,3 +340,53 @@ func TestPrinterMultipleDocsJson(t *testing.T) {
 	writer.Flush()
 	test.AssertResult(t, expected, output.String())
 }
+
+func TestPrinterNulSeparator(t *testing.T) {
+	var output bytes.Buffer
+	var writer = bufio.NewWriter(&output)
+	printer := NewSimpleYamlPrinter(writer, YamlOutputFormat, true, false, 2, false)
+	printer.SetNulSepOutput(true)
+	node, err := getExpressionParser().ParseExpression(".a")
+	if err != nil {
+		panic(err)
+	}
+	streamEvaluator := NewStreamEvaluator()
+	_, err = streamEvaluator.Evaluate("sample", strings.NewReader(multiDocSample), node, printer, NewYamlDecoder(ConfiguredYamlPreferences))
+	if err != nil {
+		panic(err)
+	}
+
+	writer.Flush()
+	expected := "banana\x00apple\x00coconut\x00"
+	test.AssertResult(t, expected, output.String())
+}
+
+func TestPrinterNulSeparatorWithJson(t *testing.T) {
+	var output bytes.Buffer
+	var writer = bufio.NewWriter(&output)
+	// note printDocSeparators is true, it should still not print document separators
+	// when outputing JSON.
+	encoder := NewJSONEncoder(0, false, false)
+	if encoder == nil {
+		t.Skipf("no support for %s output format", "json")
+	}
+	printer := NewPrinter(encoder, NewSinglePrinterWriter(writer))
+	printer.SetNulSepOutput(true)
+
+	inputs, err := readDocuments(strings.NewReader(multiDocSample), "sample.yml", 0, NewYamlDecoder(ConfiguredYamlPreferences))
+	if err != nil {
+		panic(err)
+	}
+
+	inputs.Front().Value.(*CandidateNode).LeadingContent = "# ignore this\n"
+
+	err = printer.PrintResults(inputs)
+	if err != nil {
+		panic(err)
+	}
+
+	expected := `{"a":"banana"}` + "\x00" + `{"a":"apple"}` + "\x00" + `{"a":"coconut"}` + "\x00"
+
+	writer.Flush()
+	test.AssertResult(t, expected, output.String())
+}