package request

import (

	"io"

	"sync"

	"github.com/aws/aws-sdk-go/internal/sdkio"

)

// offsetReader is a thread-safe io.ReadCloser to prevent racing

// with retrying requests

type offsetReader struct {

	buf    io.ReadSeeker

	lock   sync.Mutex

	closed bool

}

func newOffsetReader(buf io.ReadSeeker, offset int64) (*offsetReader, error) {

	reader := &offsetReader{}

	_, err := buf.Seek(offset, sdkio.SeekStart)

	if err != nil {

		return nil, err

	}

	reader.buf = buf

	return reader, nil

}

// Close will close the instance of the offset reader's access to

// the underlying io.ReadSeeker.

func (o *offsetReader) Close() error {

	o.lock.Lock()

	defer o.lock.Unlock()

	o.closed = true

	return nil

}

// Read is a thread-safe read of the underlying io.ReadSeeker

func (o *offsetReader) Read(p []byte) (int, error) {

	o.lock.Lock()

	defer o.lock.Unlock()

	if o.closed {

		return 0, io.EOF

	}

	return o.buf.Read(p)

}

// Seek is a thread-safe seeking operation.

func (o *offsetReader) Seek(offset int64, whence int) (int64, error) {

	o.lock.Lock()

	defer o.lock.Unlock()

	return o.buf.Seek(offset, whence)

}

// CloseAndCopy will return a new offsetReader with a copy of the old buffer

// and close the old buffer.

func (o *offsetReader) CloseAndCopy(offset int64) (*offsetReader, error) {

	if err := o.Close(); err != nil {

		return nil, err

	}

	return newOffsetReader(o.buf, offset)

}