remotes/docker: implement seekable http requests

To support resumable download, the fetcher for a remote must implement
`io.Seeker`. If implemented the `content.Copy` function will detect the
seeker and begin from where the download was terminated by a previous
attempt.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

remotes/docker/httpreadseeker.go

@@ -0,0 +1,125 @@
+package docker
+
+import (
+	"bytes"
+	"io"
+	"io/ioutil"
+
+	"github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/log"
+	"github.com/pkg/errors"
+)
+
+type httpReadSeeker struct {
+	size   int64
+	offset int64
+	rc     io.ReadCloser
+	open   func(offset int64) (io.ReadCloser, error)
+	closed bool
+}
+
+func newHTTPReadSeeker(size int64, open func(offset int64) (io.ReadCloser, error)) (io.ReadCloser, error) {
+	return &httpReadSeeker{
+		size: size,
+		open: open,
+	}, nil
+}
+
+func (hrs *httpReadSeeker) Read(p []byte) (n int, err error) {
+	if hrs.closed {
+		return 0, io.EOF
+	}
+
+	rd, err := hrs.reader()
+	if err != nil {
+		return 0, err
+	}
+
+	n, err = rd.Read(p)
+	hrs.offset += int64(n)
+	return
+}
+
+func (hrs *httpReadSeeker) Close() error {
+	if hrs.closed {
+		return nil
+	}
+	hrs.closed = true
+	if hrs.rc != nil {
+		return hrs.rc.Close()
+	}
+
+	return nil
+}
+
+func (hrs *httpReadSeeker) Seek(offset int64, whence int) (int64, error) {
+	if hrs.closed {
+		return 0, errors.Wrap(errdefs.ErrUnavailable, "Fetcher.Seek: closed")
+	}
+
+	abs := hrs.offset
+	switch whence {
+	case io.SeekStart:
+		abs = offset
+	case io.SeekCurrent:
+		abs += offset
+	case io.SeekEnd:
+		abs = hrs.size + offset
+	default:
+		return 0, errors.Wrap(errdefs.ErrInvalidArgument, "Fetcher.Seek: invalid whence")
+	}
+
+	if abs < 0 {
+		return 0, errors.Wrapf(errdefs.ErrInvalidArgument, "Fetcher.Seek: negative offset")
+	}
+
+	if abs != hrs.offset {
+		if hrs.rc != nil {
+			if err := hrs.rc.Close(); err != nil {
+				log.L.WithError(err).Errorf("Fetcher.Seek: failed to close ReadCloser")
+			}
+
+			hrs.rc = nil
+		}
+
+		hrs.offset = abs
+	}
+
+	return hrs.offset, nil
+}
+
+func (hrs *httpReadSeeker) reader() (io.Reader, error) {
+	if hrs.rc != nil {
+		return hrs.rc, nil
+	}
+
+	if hrs.offset < hrs.size {
+		// only try to reopen the body request if we are seeking to a value
+		// less than the actual size.
+		if hrs.open == nil {
+			return nil, errors.Wrapf(errdefs.ErrNotImplemented, "cannot open")
+		}
+
+		rc, err := hrs.open(hrs.offset)
+		if err != nil {
+			return nil, errors.Wrapf(err, "httpReaderSeeker: failed open")
+		}
+
+		if hrs.rc != nil {
+			if err := hrs.rc.Close(); err != nil {
+				log.L.WithError(err).Errorf("httpReadSeeker: failed to close ReadCloser")
+			}
+		}
+		hrs.rc = rc
+	} else {
+		// There is an edge case here where offset == size of the content. If
+		// we seek, we will probably get an error for content that cannot be
+		// sought (?). In that case, we should err on committing the content,
+		// as the length is already satisified but we just return the empty
+		// reader instead.
+
+		hrs.rc = ioutil.NopCloser(bytes.NewReader([]byte{}))
+	}
+
+	return hrs.rc, nil
+}