Support range requests in the client's httpReadSeeker

Remove buffering on the reader, because it's not useful. Also remove
artificial io.EOF return.

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

docs/client/transport/http_reader.go

@@ -1,12 +1,22 @@
 package transport
 
 import (
-	"bufio"
 	"errors"
 	"fmt"
 	"io"
 	"net/http"
 	"os"
+	"regexp"
+	"strconv"
+)
+
+var (
+	contentRangeRegexp = regexp.MustCompile(`bytes ([0-9]+)-([0-9]+)/([0-9]+|\\*)`)
+
+	// ErrWrongCodeForByteRange is returned if the client sends a request
+	// with a Range header but the server returns a 2xx or 3xx code other
+	// than 206 Partial Content.
+	ErrWrongCodeForByteRange = errors.New("expected HTTP 206 from byte range request")
 )
 
 // ReadSeekCloser combines io.ReadSeeker with io.Closer.
@@ -40,8 +50,6 @@ type httpReadSeeker struct {
 
 	// 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
@@ -79,11 +87,6 @@ func (hrs *httpReadSeeker) Read(p []byte) (n int, err error) {
 	hrs.seekOffset += int64(n)
 	hrs.readerOffset += int64(n)
 
-	// Simulate io.EOF error if we reach filesize.
-	if err == nil && hrs.size >= 0 && hrs.readerOffset >= hrs.size {
-		err = io.EOF
-	}
-
 	return n, err
 }
 
@@ -92,8 +95,18 @@ func (hrs *httpReadSeeker) Seek(offset int64, whence int) (int64, error) {
 		return 0, hrs.err
 	}
 
+	lastReaderOffset := hrs.readerOffset
+
+	if whence == os.SEEK_SET && hrs.rc == nil {
+		// If no request has been made yet, and we are seeking to an
+		// absolute position, set the read offset as well to avoid an
+		// unnecessary request.
+		hrs.readerOffset = offset
+	}
+
 	_, err := hrs.reader()
 	if err != nil {
+		hrs.readerOffset = lastReaderOffset
 		return 0, err
 	}
 
@@ -101,14 +114,14 @@ func (hrs *httpReadSeeker) Seek(offset int64, whence int) (int64, error) {
 
 	switch whence {
 	case os.SEEK_CUR:
-		newOffset += int64(offset)
+		newOffset += offset
 	case os.SEEK_END:
 		if hrs.size < 0 {
 			return 0, errors.New("content length not known")
 		}
-		newOffset = hrs.size + int64(offset)
+		newOffset = hrs.size + offset
 	case os.SEEK_SET:
-		newOffset = int64(offset)
+		newOffset = offset
 	}
 
 	if newOffset < 0 {
@@ -131,7 +144,6 @@ func (hrs *httpReadSeeker) Close() error {
 	}
 
 	hrs.rc = nil
-	hrs.brd = nil
 
 	hrs.err = errors.New("httpLayer: closed")
 
@@ -154,7 +166,7 @@ func (hrs *httpReadSeeker) reader() (io.Reader, error) {
 	}
 
 	if hrs.rc != nil {
-		return hrs.brd, nil
+		return hrs.rc, nil
 	}
 
 	req, err := http.NewRequest("GET", hrs.url, nil)
@@ -163,10 +175,8 @@ func (hrs *httpReadSeeker) reader() (io.Reader, error) {
 	}
 
 	if hrs.readerOffset > 0 {
-		// TODO(stevvooe): Get this working correctly.
-
 		// If we are at different offset, issue a range request from there.
-		req.Header.Add("Range", "1-")
+		req.Header.Add("Range", fmt.Sprintf("bytes=%d-", hrs.readerOffset))
 		// TODO: get context in here
 		// context.GetLogger(hrs.context).Infof("Range: %s", req.Header.Get("Range"))
 	}
@@ -179,12 +189,55 @@ func (hrs *httpReadSeeker) reader() (io.Reader, error) {
 	// Normally would use client.SuccessStatus, but that would be a cyclic
 	// import
 	if resp.StatusCode >= 200 && resp.StatusCode <= 399 {
-		hrs.rc = resp.Body
-		if resp.StatusCode == http.StatusOK {
+		if hrs.readerOffset > 0 {
+			if resp.StatusCode != http.StatusPartialContent {
+				return nil, ErrWrongCodeForByteRange
+			}
+
+			contentRange := resp.Header.Get("Content-Range")
+			if contentRange == "" {
+				return nil, errors.New("no Content-Range header found in HTTP 206 response")
+			}
+
+			submatches := contentRangeRegexp.FindStringSubmatch(contentRange)
+			if len(submatches) < 4 {
+				return nil, fmt.Errorf("could not parse Content-Range header: %s", contentRange)
+			}
+
+			startByte, err := strconv.ParseUint(submatches[1], 10, 64)
+			if err != nil {
+				return nil, fmt.Errorf("could not parse start of range in Content-Range header: %s", contentRange)
+			}
+
+			if startByte != uint64(hrs.readerOffset) {
+				return nil, fmt.Errorf("received Content-Range starting at offset %d instead of requested %d", startByte, hrs.readerOffset)
+			}
+
+			endByte, err := strconv.ParseUint(submatches[2], 10, 64)
+			if err != nil {
+				return nil, fmt.Errorf("could not parse end of range in Content-Range header: %s", contentRange)
+			}
+
+			if submatches[3] == "*" {
+				hrs.size = -1
+			} else {
+				size, err := strconv.ParseUint(submatches[3], 10, 64)
+				if err != nil {
+					return nil, fmt.Errorf("could not parse total size in Content-Range header: %s", contentRange)
+				}
+
+				if endByte+1 != size {
+					return nil, fmt.Errorf("range in Content-Range stops before the end of the content: %s", contentRange)
+				}
+
+				hrs.size = int64(size)
+			}
+		} else if resp.StatusCode == http.StatusOK {
 			hrs.size = resp.ContentLength
 		} else {
 			hrs.size = -1
 		}
+		hrs.rc = resp.Body
 	} else {
 		defer resp.Body.Close()
 		if hrs.errorHandler != nil {
@@ -193,11 +246,5 @@ func (hrs *httpReadSeeker) reader() (io.Reader, error) {
 		return nil, fmt.Errorf("unexpected status resolving reader: %v", resp.Status)
 	}
 
-	if hrs.brd == nil {
-		hrs.brd = bufio.NewReader(hrs.rc)
-	} else {
-		hrs.brd.Reset(hrs.rc)
-	}
-
-	return hrs.brd, nil
+	return hrs.rc, nil
 }