Refactor checksum calculation to implement transparent checksum writer.

utils/checksum.go

@@ -20,41 +20,73 @@ type ChecksumInfo struct {
 
 // ChecksumsForFile generates size, MD5, SHA1 & SHA256 checksums for given file
 func ChecksumsForFile(path string) (*ChecksumInfo, error) {
-	result := &ChecksumInfo{}
-
-	st, err := os.Stat(path)
-	if err != nil {
-		return nil, err
-	}
-
-	result.Size = st.Size()
-
-	hashes := []hash.Hash{md5.New(), sha1.New(), sha256.New()}
-
 	file, err := os.Open(path)
 	if err != nil {
 		return nil, err
 	}
 	defer file.Close()
 
-	buf := make([]byte, 8192)
-	for {
-		n, err := file.Read(buf)
-		if err == io.EOF {
-			break
-		}
-		if err != nil {
-			return nil, err
-		}
+	w := NewChecksumWriter(&DevNull{})
 
-		for _, h := range hashes {
-			h.Write(buf[:n])
-		}
+	_, err = io.Copy(w, file)
+	if err != nil {
+		return nil, err
 	}
 
-	result.MD5 = fmt.Sprintf("%x", hashes[0].Sum(nil))
-	result.SHA1 = fmt.Sprintf("%x", hashes[1].Sum(nil))
-	result.SHA256 = fmt.Sprintf("%x", hashes[2].Sum(nil))
-
-	return result, nil
+	return w.Sum(), nil
+}
+
+// DevNull just accepts anything
+type DevNull struct {
+}
+
+// Write to DevNull always succeeds
+func (d *DevNull) Write(p []byte) (n int, err error) {
+	return len(p), nil
+}
+
+// ChecksumWriter is a writer that does checksum calculation on the fly passing data
+// to real writer
+type ChecksumWriter struct {
+	w      io.Writer
+	sum    ChecksumInfo
+	hashes []hash.Hash
+}
+
+// Interface check
+var (
+	_ io.Writer = &ChecksumWriter{}
+)
+
+// NewChecksumWriter creates checksum calculator for given writer w
+func NewChecksumWriter(w io.Writer) *ChecksumWriter {
+	return &ChecksumWriter{
+		w:      w,
+		hashes: []hash.Hash{md5.New(), sha1.New(), sha256.New()},
+	}
+}
+
+// Write implememnts pass-through writing with checksum calculation on the fly
+func (c *ChecksumWriter) Write(p []byte) (n int, err error) {
+	n, err = c.w.Write(p)
+	if err != nil {
+		return n, err
+	}
+
+	c.sum.Size += int64(n)
+
+	for _, h := range c.hashes {
+		h.Write(p[:n])
+	}
+
+	return
+}
+
+// Sum returns caculated ChecksumInfo
+func (c *ChecksumWriter) Sum() *ChecksumInfo {
+	c.sum.MD5 = fmt.Sprintf("%x", c.hashes[0].Sum(nil))
+	c.sum.SHA1 = fmt.Sprintf("%x", c.hashes[1].Sum(nil))
+	c.sum.SHA256 = fmt.Sprintf("%x", c.hashes[2].Sum(nil))
+
+	return &c.sum
 }

utils/checksum_test.go

@@ -21,7 +21,7 @@ func (s *ChecksumSuite) TearDownTest(c *C) {
 	s.tempfile.Close()
 }
 
-func (s *ChecksumSuite) TestChecksum(c *C) {
+func (s *ChecksumSuite) TestChecksumsForFile(c *C) {
 	info, err := ChecksumsForFile(s.tempfile.Name())
 
 	c.Assert(err, IsNil)