Fix not reading partial responses correctly

When dealing with a separate header and payload, make sure that we
do not expect to receive additional USB packet headers in the payload portion of
the transfer. Previously, we were assuming that this would only occur when the
device was sending more than a full packet's worth of data, but this is not correct -
we should continue the bulk transfer any time the device sends us less data than
the header says it will be sending.

mtp/mtp.go

@@ -304,21 +304,21 @@ func (d *Device) sendReq(req *Container) error {
 
 // Fetches one USB packet. The header is split off, and the remainder is returned.
 // dest should be at least 512bytes.
-func (d *Device) fetchPacket(dest []byte, header *usbBulkHeader) (rest []byte, err error) {
+func (d *Device) fetchPacket(dest []byte, header *usbBulkHeader) (rest []byte, bytesRead int, err error) {
 	n, err := d.h.BulkTransfer(d.fetchEP, dest[:d.fetchMaxPacketSize()], d.Timeout)
 	if n > 0 {
 		d.dataPrint(d.fetchEP, dest[:n])
 	}
 
 	if err != nil {
-		return nil, err
+		return nil, n, err
 	}
 
 	buf := bytes.NewBuffer(dest[:n])
 	if err = binary.Read(buf, binary.LittleEndian, header); err != nil {
-		return nil, err
+		return nil, n, err
 	}
-	return buf.Bytes(), nil
+	return buf.Bytes(), n, nil
 }
 
 func (d *Device) decodeRep(h *usbBulkHeader, rest []byte, rep *Container) error {
@@ -419,7 +419,7 @@ func (d *Device) runTransaction(req *Container, rep *Container,
 	fetchPacketSize := d.fetchMaxPacketSize()
 	data := make([]byte, fetchPacketSize)
 	h := &usbBulkHeader{}
-	rest, err := d.fetchPacket(data[:], h)
+	rest, n, err := d.fetchPacket(data[:], h)
 	if err != nil {
 		return err
 	}
@@ -438,9 +438,10 @@ func (d *Device) runTransaction(req *Container, rep *Container,
 
 		dest.Write(rest)
 
-		if len(rest)+usbHdrLen == fetchPacketSize {
-			// If this was a full packet, read until we
-			// have a short read.
+		if len(rest)+usbHdrLen == fetchPacketSize || uint32(n) < h.Length {
+			// If this was a full packet, or if the packet wasn't full but
+			// the device said it was sending more data than we received,
+			// continue reading until we have a read less than a full packet.
 			_, finalPacket, err = d.bulkRead(dest, progressCb)
 			if err != nil {
 				return err
@@ -456,7 +457,7 @@ func (d *Device) runTransaction(req *Container, rep *Container,
 			finalBuf := bytes.NewBuffer(finalPacket[:len(finalPacket)])
 			err = binary.Read(finalBuf, binary.LittleEndian, h)
 		} else {
-			rest, err = d.fetchPacket(data[:], h)
+			rest, _, err = d.fetchPacket(data[:], h)
 		}
 	}