Image encryption/decryption support in skopeo

Signed-off-by: Harshal Patil <harshal.patil@in.ibm.com>
Signed-off-by: Brandon Lum <lumjjb@gmail.com>

vendor/github.com/containers/ocicrypt/utils/delayedreader.go

@@ -0,0 +1,109 @@
+/*
+   Copyright The ocicrypt Authors.
+
+   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 utils
+
+import (
+	"io"
+)
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+// DelayedReader wraps a io.Reader and allows a client to use the Reader
+// interface. The DelayedReader holds back some buffer to the client
+// so that it can report any error that occurred on the Reader it wraps
+// early to the client while it may still have held some data back.
+type DelayedReader struct {
+	reader   io.Reader // Reader to Read() bytes from and delay them
+	err      error     // error that occurred on the reader
+	buffer   []byte    // delay buffer
+	bufbytes int       // number of bytes in the delay buffer to give to Read(); on '0' we return 'EOF' to caller
+	bufoff   int       // offset in the delay buffer to give to Read()
+}
+
+// NewDelayedReader wraps a io.Reader and allocates a delay buffer of bufsize bytes
+func NewDelayedReader(reader io.Reader, bufsize uint) io.Reader {
+	return &DelayedReader{
+		reader: reader,
+		buffer: make([]byte, bufsize),
+	}
+}
+
+// Read implements the io.Reader interface
+func (dr *DelayedReader) Read(p []byte) (int, error) {
+	if dr.err != nil && dr.err != io.EOF {
+		return 0, dr.err
+	}
+
+	// if we are completely drained, return io.EOF
+	if dr.err == io.EOF && dr.bufbytes == 0 {
+		return 0, io.EOF
+	}
+
+	// only at the beginning we fill our delay buffer in an extra step
+	if dr.bufbytes < len(dr.buffer) && dr.err == nil {
+		dr.bufbytes, dr.err = FillBuffer(dr.reader, dr.buffer)
+		if dr.err != nil && dr.err != io.EOF {
+			return 0, dr.err
+		}
+	}
+	// dr.err != nil means we have EOF and can drain the delay buffer
+	// otherwise we need to still read from the reader
+
+	var tmpbuf []byte
+	tmpbufbytes := 0
+	if dr.err == nil {
+		tmpbuf = make([]byte, len(p))
+		tmpbufbytes, dr.err = FillBuffer(dr.reader, tmpbuf)
+		if dr.err != nil && dr.err != io.EOF {
+			return 0, dr.err
+		}
+	}
+
+	// copy out of the delay buffer into 'p'
+	tocopy1 := min(len(p), dr.bufbytes)
+	c1 := copy(p[:tocopy1], dr.buffer[dr.bufoff:])
+	dr.bufoff += c1
+	dr.bufbytes -= c1
+
+	c2 := 0
+	// can p still hold more data?
+	if c1 < len(p) {
+		// copy out of the tmpbuf into 'p'
+		c2 = copy(p[tocopy1:], tmpbuf[:tmpbufbytes])
+	}
+
+	// if tmpbuf holds data we need to hold onto, copy them
+	// into the delay buffer
+	if tmpbufbytes-c2 > 0 {
+		// left-shift the delay buffer and append the tmpbuf's remaining data
+		dr.buffer = dr.buffer[dr.bufoff : dr.bufoff+dr.bufbytes]
+		dr.buffer = append(dr.buffer, tmpbuf[c2:tmpbufbytes]...)
+		dr.bufoff = 0
+		dr.bufbytes = len(dr.buffer)
+	}
+
+	var err error
+	if dr.bufbytes == 0 {
+		err = io.EOF
+	}
+	return c1 + c2, err
+}