nmon/internal/cliout/cliout.go

// Package cliout writes command output and remembers the first failure, so a
// run of prints needs one error check at the end instead of one per line.
package cliout

import (
	"fmt"
	"io"
)

// Writer wraps an io.Writer. After the first write error every later write is
// skipped, and Err reports that error. A CLI returns Err so a closed pipe or a
// full disk becomes exit code 1 instead of silently truncated output.
type Writer struct {
	w   io.Writer
	err error
}

// New returns a Writer that prints to w.
func New(w io.Writer) *Writer { return &Writer{w: w} }

// Printf formats like fmt.Fprintf.
func (w *Writer) Printf(format string, a ...any) {
	if w.err == nil {
		_, w.err = fmt.Fprintf(w.w, format, a...)
	}
}

// Println formats like fmt.Fprintln.
func (w *Writer) Println(a ...any) {
	if w.err == nil {
		_, w.err = fmt.Fprintln(w.w, a...)
	}
}

// Write makes Writer an io.Writer, for example for a tabwriter. It reports
// success after a failure so callers keep going; check Err once at the end.
func (w *Writer) Write(p []byte) (int, error) {
	if w.err == nil {
		_, w.err = w.w.Write(p)
	}
	return len(p), nil
}

// Err returns the first write error, or nil.
func (w *Writer) Err() error { return w.err }