hashing: support WriteTo in the reader

internal/hashing/reader.go

@@ -5,25 +5,47 @@ import (
 	"io"
 )
 
-// Reader hashes all data read from the underlying reader.
-type Reader struct {
-	r io.Reader
+// ReadSumer hashes all data read from the underlying reader.
+type ReadSumer interface {
+	io.Reader
+	// Sum returns the hash of the data read so far.
+	Sum(d []byte) []byte
+}
+
+type reader struct {
+	io.Reader
 	h hash.Hash
 }
 
-// NewReader returns a new Reader that uses the hash h.
-func NewReader(r io.Reader, h hash.Hash) *Reader {
-	return &Reader{
-		h: h,
-		r: io.TeeReader(r, h),
-	}
+type readWriterTo struct {
+	reader
+	writerTo io.WriterTo
 }
 
-func (h *Reader) Read(p []byte) (int, error) {
-	return h.r.Read(p)
+// NewReader returns a new ReadSummer that uses the hash h. If the underlying
+// reader supports WriteTo then the returned reader will do so too.
+func NewReader(r io.Reader, h hash.Hash) ReadSumer {
+	rs := reader{
+		Reader: io.TeeReader(r, h),
+		h:      h,
+	}
+
+	if _, ok := r.(io.WriterTo); ok {
+		return &readWriterTo{
+			reader:   rs,
+			writerTo: r.(io.WriterTo),
+		}
+	}
+
+	return &rs
 }
 
 // Sum returns the hash of the data read so far.
-func (h *Reader) Sum(d []byte) []byte {
+func (h *reader) Sum(d []byte) []byte {
 	return h.h.Sum(d)
 }
+
+// WriteTo reads all data into the passed writer
+func (h *readWriterTo) WriteTo(w io.Writer) (int64, error) {
+	return h.writerTo.WriteTo(NewWriter(w, h.h))
+}

internal/hashing/reader_test.go

@@ -7,8 +7,26 @@ import (
 	"io"
 	"io/ioutil"
 	"testing"
+
+	rtest "github.com/restic/restic/internal/test"
 )
 
+// only expose Read method
+type onlyReader struct {
+	io.Reader
+}
+
+type traceWriterTo struct {
+	io.Reader
+	writerTo io.WriterTo
+	Traced   bool
+}
+
+func (r *traceWriterTo) WriteTo(w io.Writer) (n int64, err error) {
+	r.Traced = true
+	return r.writerTo.WriteTo(w)
+}
+
 func TestReader(t *testing.T) {
 	tests := []int{5, 23, 2<<18 + 23, 1 << 20}
 
@@ -21,8 +39,25 @@ func TestReader(t *testing.T) {
 
 		expectedHash := sha256.Sum256(data)
 
-		rd := NewReader(bytes.NewReader(data), sha256.New())
-		n, err := io.Copy(ioutil.Discard, rd)
+		for _, test := range []struct {
+			innerWriteTo, outerWriteTo bool
+		}{{false, false}, {false, true}, {true, false}, {true, true}} {
+			// test both code paths in WriteTo
+			src := bytes.NewReader(data)
+			rawSrc := &traceWriterTo{Reader: src, writerTo: src}
+			innerSrc := io.Reader(rawSrc)
+			if !test.innerWriteTo {
+				innerSrc = &onlyReader{Reader: rawSrc}
+			}
+
+			rd := NewReader(innerSrc, sha256.New())
+			// test both Read and WriteTo
+			outerSrc := io.Reader(rd)
+			if !test.outerWriteTo {
+				outerSrc = &onlyReader{Reader: outerSrc}
+			}
+
+			n, err := io.Copy(ioutil.Discard, outerSrc)
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -38,6 +73,11 @@ func TestReader(t *testing.T) {
 				t.Errorf("Reader: hashes do not match: expected %02x, got %02x",
 					expectedHash, resultingHash)
 			}
+
+			rtest.Assert(t, rawSrc.Traced == (test.innerWriteTo && test.outerWriteTo),
+				"unexpected/missing writeTo call innerWriteTo %v outerWriteTo %v",
+				test.innerWriteTo, test.outerWriteTo)
+		}
 	}
 }