Simple golden file test

golden_test.go

package main_test

import (
	"testing"	
)

/*
This is an example implementation of a golden file test that is easy to update.
*/

// Run test with "go test -update" to regenerate golden files.
var updateFlag = flag.Bool("update", false, "If set, golden files are updated.")

func TestGolden(t *testing.T) {
	const goldenFile = "testdata/golden.txt"
  
	// Capture test output in a buffer.
	buf := new(bytes.Buffer)
	buf.WriteString("output from test...")

	// Compare the output to a golden file.
	checkDiff(t, goldenFile, buf.Bytes(), *updateFlag)
}

// checkDiff checks the changes between the contents of file "from"
// and the contents of "to". If this fails, then the test is marked as
// having failed.
//
// If update is true, then the contents of the golden file is updated.
func checkDiff(tb testing.TB, from string, to []byte, update bool) {
	tb.Helper()
	if err := diff(from, to); err != nil {
		if update {
			if err := ioutil.WriteFile(from, to, 0644); err != nil {
				tb.Error(err)
			}
		}
		tb.Fatal(err)
	}
}

// diff checks the changes between the contents of file "from" and the
// contents of "to". If this fails, then the diff is included in the
// error.
func diff(from string, to []byte) (err error) {
	cmd := exec.Command("diff", "-u", "--from-file="+from, "-")
	cmd.Stdin = bytes.NewReader(to)

	var out bytes.Buffer
	cmd.Stdout = &out
	cmd.Stderr = &out

	if err = cmd.Run(); err != nil {
		err = fmt.Errorf("%v: %s", err, &out)
	}
	return
}