Add a failing test for golang/go#5128.

101/main_test.go

@@ -0,0 +1,70 @@
+package main
+
+import (
+	"go/format"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"testing"
+)
+
+func Test(t *testing.T) {
+	in := []byte(`package main
+
+import "fmt"
+
+// An example of gofmt not being idempotent.
+// Repeatedly invoking gofmt will add a level of indent to the b string line each time.
+
+type Foo struct {
+	/*a string
+
+	b string*/
+	c string
+}
+
+func main() {
+	fmt.Println("Hello, playground")
+}
+`)
+
+	out, err := format.Source(in)
+	if err != nil {
+		panic(err)
+	}
+
+	if string(in) != string(out) {
+		diff, err := diff(in, out)
+		if err != nil {
+			panic(err)
+		}
+		t.Errorf("diff:\n%s\n", diff)
+	}
+}
+
+func diff(b1, b2 []byte) (data []byte, err error) {
+	f1, err := ioutil.TempFile("", "")
+	if err != nil {
+		return
+	}
+	defer os.Remove(f1.Name())
+	defer f1.Close()
+
+	f2, err := ioutil.TempFile("", "")
+	if err != nil {
+		return
+	}
+	defer os.Remove(f2.Name())
+	defer f2.Close()
+
+	f1.Write(b1)
+	f2.Write(b2)
+
+	data, err = exec.Command("diff", "-u", f1.Name(), f2.Name()).CombinedOutput()
+	if len(data) > 0 {
+		// diff exits with a non-zero status when the files don't match.
+		// Ignore that failure as long as we get output.
+		err = nil
+	}
+	return
+}