Replace snappy.Writer/io.Pipe with snappyCompressReader. (#7316)

Prevents deferred close functions from being called while still
  attempting to copy reader to snappyWriter.
 Reduces code duplication when compressing objects.

cmd/object-api-utils.go

@@ -675,3 +675,54 @@ func CleanMinioInternalMetadataKeys(metadata map[string]string) map[string]strin
 	}
 	return newMeta
 }
+
+// snappyCompressReader compresses data as it reads
+// from the underlying io.Reader.
+type snappyCompressReader struct {
+	r      io.Reader
+	w      *snappy.Writer
+	closed bool
+	buf    bytes.Buffer
+}
+
+func newSnappyCompressReader(r io.Reader) *snappyCompressReader {
+	cr := &snappyCompressReader{r: r}
+	cr.w = snappy.NewBufferedWriter(&cr.buf)
+	return cr
+}
+
+func (cr *snappyCompressReader) Read(p []byte) (int, error) {
+	if cr.closed {
+		// if snappy writer is closed r has been completely read,
+		// return any remaining data in buf.
+		return cr.buf.Read(p)
+	}
+
+	// read from original using p as buffer
+	nr, readErr := cr.r.Read(p)
+
+	// write read bytes to snappy writer
+	nw, err := cr.w.Write(p[:nr])
+	if err != nil {
+		return 0, err
+	}
+	if nw != nr {
+		return 0, io.ErrShortWrite
+	}
+
+	// if last of data from reader, close snappy writer to flush
+	if readErr == io.EOF {
+		err := cr.w.Close()
+		cr.closed = true
+		if err != nil {
+			return 0, err
+		}
+	}
+
+	// read compressed bytes out of buf
+	n, err := cr.buf.Read(p)
+	if readErr != io.EOF && (err == nil || err == io.EOF) {
+		err = readErr
+	}
+	return n, err
+}