gistfile1.txt

package main

import (
	"context"
	"io"
)

func main() {
}

type reader struct {
	ctx context.Context
	r   io.Reader
}

func (r reader) Read(b []byte) (i int, err error) {
	ch := make(chan struct{})
	go func() {
		i, err = r.r.Read(b)
		close(ch)
	}()
	select {
	case <-ch:
		return
	case <-r.ctx.Done():
		return 0, r.ctx.Err()
	}
}

func contextReader(ctx context.Context, r io.Reader) *reader {
	return &reader{
		ctx: ctx,
		r:   r,
	}
}

type writer struct {
	ctx context.Context
	w   io.Writer
}

func (w writer) Write(b []byte) (i int, err error) {
	ch := make(chan struct{})
	go func() {
		i, err = w.w.Write(b)
		close(ch)
	}()
	select {
	case <-ch:
		return
	case <-w.ctx.Done():
		return 0, w.ctx.Err()
	}
}

func contextWriter(ctx context.Context, w io.Writer) *writer {
	return &writer{
		ctx: ctx,
		w:   w,
	}
}

func CopyContext(ctx context.Context, w io.Writer, r io.Reader) (int64, error) {
	rc := contextReader(ctx, r)
	wc := contextWriter(ctx, w)
	return io.Copy(wc, rc)
}