First version of streaming transfers.

usb/transfer_stream.go

@@ -0,0 +1,134 @@
+// Copyright 2017 the gousb Authors.  All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package usb
+
+import "io"
+
+type transferIntf interface {
+	submit() error
+	cancel() error
+	wait() (int, error)
+	free() error
+	data() []byte
+}
+
+type stream struct {
+	// a fifo of USB transfers.
+	transfers chan transferIntf
+	// current holds the last transfer to return.
+	current transferIntf
+	// total/used are the number of all/used bytes in the current transfer.
+	total, used int
+	// err is the first error encountered, returned to the user as soon
+	// as all remaining data was read.
+	err error
+}
+
+func (s *stream) cleanup() {
+	close(s.transfers)
+	for t := range s.transfers {
+		t.cancel()
+		t.wait()
+		t.free()
+	}
+}
+
+type ReadStream struct {
+	s *stream
+}
+
+// Read reads data from the transfer stream.
+// The data will come from at most a single transfer, so the returned number
+// might be smaller than the length of p.
+// After a non-nil error is returned, all subsequent attempts to read will
+// return io.ErrClosedPipe.
+func (r ReadStream) Read(p []byte) (int, error) {
+	s := r.s
+	if s.current == nil {
+		t, ok := <-s.transfers
+		if !ok {
+			// no more transfers in flight
+			retErr := io.ErrClosedPipe
+			if s.err != nil {
+				retErr = s.err
+				s.err = nil
+			}
+			return 0, retErr
+		}
+		n, err := t.wait()
+		if err != nil {
+			s.err = err
+		}
+		s.current = t
+		s.total = n
+		s.used = 0
+	}
+	use := s.total - s.used
+	if use > len(p) {
+		use = len(p)
+	}
+	copy(p, s.current.data()[s.used:s.used+use])
+	s.used += use
+	if s.used == s.total {
+		if s.err == nil {
+			if err := s.current.submit(); err == nil {
+				// guaranteed to not block, len(transfers) == number of allocated transfers
+				s.transfers <- s.current
+			} else {
+				s.err = err
+			}
+		}
+		if s.err != nil {
+			s.current.free()
+		}
+		s.current = nil
+	}
+	var retErr error
+	if s.current == nil && s.err != nil {
+		s.cleanup()
+		retErr = s.err
+		s.err = nil
+	}
+	return use, retErr
+}
+
+// Close signals that the transfer should stop. After Close is called,
+// subsequent Read()s will return data from all transfers that were already
+// in progress before returning an io.EOF error, unless another error
+// was encountered earlier.
+func (r ReadStream) Close() {
+	s := r.s
+	if s.err != nil {
+		s.err = io.EOF
+	}
+}
+
+func newStream(tt []transferIntf, submit bool) *stream {
+	s := &stream{
+		transfers: make(chan transferIntf, len(tt)),
+	}
+	for _, t := range tt {
+		s.transfers <- t
+	}
+	if submit {
+		for _, t := range tt {
+			if err := t.submit(); err != nil {
+				s.err = err
+				break
+			}
+		}
+	}
+	return s
+}