Avoid stat round-trips when fetching a blob

Without this commit, three round-trips are required to fetch a blob with
a progress bar. The first is a call to Stat (HEAD request), to determine
the size. Then Open is called, which also calls Stat, and finally
performs a GET request.

Only the GET request is actually needed. The size of the blob can be
sniffed from Content-Length in the GET response.

This commit changes HTTPReadSeeker to automatically detect the size from
Content-Length instead of requiring it to be passed in. The Stat call is
removed from Open because it is no longer necessary.

HTTPReadSeeker now takes an additional errorHandler callback argument which
translates an unsuccessful HTTP response into an appropriate API-level
error. Using a callback for this makes it possible to avoid leaking the
repsonse body to Read's caller, which would make lifecycle management
problematic.

Fixes #1223

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

docs/client/repository.go

@@ -391,17 +391,18 @@ func (bs *blobs) Get(ctx context.Context, dgst digest.Digest) ([]byte, error) {
 }
 
 func (bs *blobs) Open(ctx context.Context, dgst digest.Digest) (distribution.ReadSeekCloser, error) {
-	stat, err := bs.statter.Stat(ctx, dgst)
+	blobURL, err := bs.ub.BuildBlobURL(bs.name, dgst)
 	if err != nil {
 		return nil, err
 	}
 
-	blobURL, err := bs.ub.BuildBlobURL(bs.name, stat.Digest)
-	if err != nil {
-		return nil, err
-	}
-
-	return transport.NewHTTPReadSeeker(bs.client, blobURL, stat.Size), nil
+	return transport.NewHTTPReadSeeker(bs.client, blobURL,
+		func(resp *http.Response) error {
+			if resp.StatusCode == http.StatusNotFound {
+				return distribution.ErrBlobUnknown
+			}
+			return handleErrorResponse(resp)
+		}), nil
 }
 
 func (bs *blobs) ServeBlob(ctx context.Context, w http.ResponseWriter, r *http.Request, dgst digest.Digest) error {

docs/client/transport/http_reader.go

@@ -2,11 +2,9 @@ package transport
 
 import (
 	"bufio"
-	"bytes"
 	"errors"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"net/http"
 	"os"
 )
@@ -21,11 +19,11 @@ type ReadSeekCloser interface {
 // request. When seeking and starting a read from a non-zero offset
 // the a "Range" header will be added which sets the offset.
 // TODO(dmcgowan): Move this into a separate utility package
-func NewHTTPReadSeeker(client *http.Client, url string, size int64) ReadSeekCloser {
+func NewHTTPReadSeeker(client *http.Client, url string, errorHandler func(*http.Response) error) ReadSeekCloser {
 	return &httpReadSeeker{
-		client: client,
-		url:    url,
-		size:   size,
+		client:       client,
+		url:          url,
+		errorHandler: errorHandler,
 	}
 }
 
@@ -33,12 +31,26 @@ type httpReadSeeker struct {
 	client *http.Client
 	url    string
 
+	// errorHandler creates an error from an unsuccessful HTTP response.
+	// This allows the error to be created with the HTTP response body
+	// without leaking the body through a returned error.
+	errorHandler func(*http.Response) error
+
 	size int64
 
-	rc     io.ReadCloser // remote read closer
-	brd    *bufio.Reader // internal buffered io
-	offset int64
-	err    error
+	// rc is the remote read closer.
+	rc io.ReadCloser
+	// brd is a buffer for internal buffered io.
+	brd *bufio.Reader
+	// readerOffset tracks the offset as of the last read.
+	readerOffset int64
+	// seekOffset allows Seek to override the offset. Seek changes
+	// seekOffset instead of changing readOffset directly so that
+	// connection resets can be delayed and possibly avoided if the
+	// seek is undone (i.e. seeking to the end and then back to the
+	// beginning).
+	seekOffset int64
+	err        error
 }
 
 func (hrs *httpReadSeeker) Read(p []byte) (n int, err error) {
@@ -46,16 +58,29 @@ func (hrs *httpReadSeeker) Read(p []byte) (n int, err error) {
 		return 0, hrs.err
 	}
 
+	// If we seeked to a different position, we need to reset the
+	// connection. This logic is here instead of Seek so that if
+	// a seek is undone before the next read, the connection doesn't
+	// need to be closed and reopened. A common example of this is
+	// seeking to the end to determine the length, and then seeking
+	// back to the original position.
+	if hrs.readerOffset != hrs.seekOffset {
+		hrs.reset()
+	}
+
+	hrs.readerOffset = hrs.seekOffset
+
 	rd, err := hrs.reader()
 	if err != nil {
 		return 0, err
 	}
 
 	n, err = rd.Read(p)
-	hrs.offset += int64(n)
+	hrs.seekOffset += int64(n)
+	hrs.readerOffset += int64(n)
 
 	// Simulate io.EOF error if we reach filesize.
-	if err == nil && hrs.offset >= hrs.size {
+	if err == nil && hrs.size >= 0 && hrs.readerOffset >= hrs.size {
 		err = io.EOF
 	}
 
@@ -67,13 +92,20 @@ func (hrs *httpReadSeeker) Seek(offset int64, whence int) (int64, error) {
 		return 0, hrs.err
 	}
 
-	var err error
-	newOffset := hrs.offset
+	_, err := hrs.reader()
+	if err != nil {
+		return 0, err
+	}
+
+	newOffset := hrs.seekOffset
 
 	switch whence {
 	case os.SEEK_CUR:
 		newOffset += int64(offset)
 	case os.SEEK_END:
+		if hrs.size < 0 {
+			return 0, errors.New("content length not known")
+		}
 		newOffset = hrs.size + int64(offset)
 	case os.SEEK_SET:
 		newOffset = int64(offset)
@@ -82,15 +114,10 @@ func (hrs *httpReadSeeker) Seek(offset int64, whence int) (int64, error) {
 	if newOffset < 0 {
 		err = errors.New("cannot seek to negative position")
 	} else {
-		if hrs.offset != newOffset {
-			hrs.reset()
-		}
-
-		// No problems, set the offset.
-		hrs.offset = newOffset
+		hrs.seekOffset = newOffset
 	}
 
-	return hrs.offset, err
+	return hrs.seekOffset, err
 }
 
 func (hrs *httpReadSeeker) Close() error {
@@ -130,17 +157,12 @@ func (hrs *httpReadSeeker) reader() (io.Reader, error) {
 		return hrs.brd, nil
 	}
 
-	// If the offset is great than or equal to size, return a empty, noop reader.
-	if hrs.offset >= hrs.size {
-		return ioutil.NopCloser(bytes.NewReader([]byte{})), nil
-	}
-
 	req, err := http.NewRequest("GET", hrs.url, nil)
 	if err != nil {
 		return nil, err
 	}
 
-	if hrs.offset > 0 {
+	if hrs.readerOffset > 0 {
 		// TODO(stevvooe): Get this working correctly.
 
 		// If we are at different offset, issue a range request from there.
@@ -158,8 +180,16 @@ func (hrs *httpReadSeeker) reader() (io.Reader, error) {
 	// import
 	if resp.StatusCode >= 200 && resp.StatusCode <= 399 {
 		hrs.rc = resp.Body
+		if resp.StatusCode == http.StatusOK {
+			hrs.size = resp.ContentLength
+		} else {
+			hrs.size = -1
+		}
 	} else {
 		defer resp.Body.Close()
+		if hrs.errorHandler != nil {
+			return nil, hrs.errorHandler(resp)
+		}
 		return nil, fmt.Errorf("unexpected status resolving reader: %v", resp.Status)
 	}