alist/pkg/utils/io.go

package utils

import (
	"context"
	"io"
)

// here is some syntaxic sugar inspired by the Tomas Senart's video,
// it allows me to inline the Reader interface
type readerFunc func(p []byte) (n int, err error)

func (rf readerFunc) Read(p []byte) (n int, err error) { return rf(p) }

// CopyWithCtx slightly modified function signature:
// - context has been added in order to propagate cancelation
// - I do not return the number of bytes written, has it is not useful in my use case
func CopyWithCtx(ctx context.Context, out io.Writer, in io.Reader, size int64, progress func(percentage int)) error {
	// Copy will call the Reader and Writer interface multiple time, in order
	// to copy by chunk (avoiding loading the whole file in memory).
	// I insert the ability to cancel before read time as it is the earliest
	// possible in the call process.
	var finish int64 = 0
	s := size / 100
	_, err := io.Copy(out, readerFunc(func(p []byte) (int, error) {
		// golang non-blocking channel: https://gobyexample.com/non-blocking-channel-operations
		select {
		// if context has been canceled
		case <-ctx.Done():
			// stop process and propagate "context canceled" error
			return 0, ctx.Err()
		default:
			// otherwise just run default io.Reader implementation
			n, err := in.Read(p)
			if s > 0 && (err == nil || err == io.EOF) {
				finish += int64(n)
				progress(int(finish / s))
			}
			return n, err
		}
	}))
	return err
}

type limitWriter struct {
	w     io.Writer
	count int64
	limit int64
}

func (l limitWriter) Write(p []byte) (n int, err error) {
	wn := int(l.limit - l.count)
	if wn > len(p) {
		wn = len(p)
	}
	if wn > 0 {
		if n, err = l.w.Write(p[:wn]); err != nil {
			return
		}
		if n < wn {
			err = io.ErrShortWrite
		}
	}
	if err == nil {
		n = len(p)
	}
	return
}

func LimitWriter(w io.Writer, size int64) io.Writer {
	return &limitWriter{w: w, limit: size}
}