nmon/internal/cliout/cliout_test.go

package cliout

import (
	"bytes"
	"errors"
	"strings"
	"testing"
)

type failAfter struct {
	n   int
	buf bytes.Buffer
}

var errFull = errors.New("disk full")

func (f *failAfter) Write(p []byte) (int, error) {
	if f.n <= 0 {
		return 0, errFull
	}
	f.n--
	return f.buf.Write(p)
}

func TestWriterPrintsAndReportsNoError(t *testing.T) {
	var b bytes.Buffer
	w := New(&b)
	w.Printf("a=%d\n", 1)
	w.Println("b", 2)
	if _, err := w.Write([]byte("c\n")); err != nil {
		t.Fatal(err)
	}
	if w.Err() != nil || b.String() != "a=1\nb 2\nc\n" {
		t.Fatalf("err=%v out=%q", w.Err(), b.String())
	}
}

func TestWriterKeepsFirstErrorAndStopsWriting(t *testing.T) {
	f := &failAfter{n: 1}
	w := New(f)
	w.Println("kept")
	w.Printf("lost %s\n", "x")
	w.Println("lost")
	_, _ = w.Write([]byte("lost"))
	if !errors.Is(w.Err(), errFull) {
		t.Fatalf("err = %v", w.Err())
	}
	if got := f.buf.String(); !strings.HasPrefix(got, "kept") || strings.Contains(got, "lost") {
		t.Fatalf("out = %q", got)
	}
}