Split plugin framework into its own package

Signed-off-by: Carlisia <carlisiac@vmware.com>

pkg/plugin/framework/stream_reader.go

@@ -0,0 +1,75 @@
+/*
+Copyright 2017, 2019 the Velero contributors.
+
+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 framework
+
+import (
+	"bytes"
+	"io"
+)
+
+// ReceiveFunc is a function that either returns a slice
+// of an arbitrary number of bytes OR an error. Returning
+// an io.EOF means there is no more data to be read; any
+// other error is considered an actual error.
+type ReceiveFunc func() ([]byte, error)
+
+// CloseFunc is used to signal to the source of data that
+// the StreamReadCloser has been closed.
+type CloseFunc func() error
+
+// StreamReadCloser wraps a ReceiveFunc and a CloseSendFunc
+// to implement io.ReadCloser.
+type StreamReadCloser struct {
+	buf     *bytes.Buffer
+	receive ReceiveFunc
+	close   CloseFunc
+}
+
+func (s *StreamReadCloser) Read(p []byte) (n int, err error) {
+	for {
+		// if buf exists and holds at least as much as we're trying to read,
+		// read from the buffer
+		if s.buf != nil && s.buf.Len() >= len(p) {
+			return s.buf.Read(p)
+		}
+
+		// if buf is nil, create it
+		if s.buf == nil {
+			s.buf = new(bytes.Buffer)
+		}
+
+		// buf exists but doesn't hold enough data to fill p, so
+		// receive again. If we get an EOF, return what's in the
+		// buffer; else, write the new data to the buffer and
+		// try another read.
+		data, err := s.receive()
+		if err == io.EOF {
+			return s.buf.Read(p)
+		}
+		if err != nil {
+			return 0, err
+		}
+
+		if _, err := s.buf.Write(data); err != nil {
+			return 0, err
+		}
+	}
+}
+
+func (s *StreamReadCloser) Close() error {
+	return s.close()
+}