Add GetObjectNInfo to object layer (#6449)

The new call combines GetObjectInfo and GetObject, and returns an
object with a ReadCloser interface.

Also adds a number of end-to-end encryption tests at the handler
level.

pkg/ioutil/ioutil.go

@@ -126,3 +126,34 @@ func (nopCloser) Close() error { return nil }
 func NopCloser(w io.Writer) io.WriteCloser {
 	return nopCloser{w}
 }
+
+// SkipReader skips a given number of bytes and then returns all
+// remaining data.
+type SkipReader struct {
+	io.Reader
+
+	skipCount int64
+}
+
+func (s *SkipReader) Read(p []byte) (int, error) {
+	l := int64(len(p))
+	if l == 0 {
+		return 0, nil
+	}
+	for s.skipCount > 0 {
+		if l > s.skipCount {
+			l = s.skipCount
+		}
+		n, err := s.Reader.Read(p[:l])
+		if err != nil {
+			return 0, err
+		}
+		s.skipCount -= int64(n)
+	}
+	return s.Reader.Read(p)
+}
+
+// NewSkipReader - creates a SkipReader
+func NewSkipReader(r io.Reader, n int64) io.Reader {
+	return &SkipReader{r, n}
+}

pkg/ioutil/ioutil_test.go

@@ -17,6 +17,8 @@
 package ioutil
 
 import (
+	"bytes"
+	"io"
 	goioutil "io/ioutil"
 	"os"
 	"testing"
@@ -73,3 +75,29 @@ func TestAppendFile(t *testing.T) {
 		t.Errorf("AppendFile() failed, expected: %s, got %s", expected, string(b))
 	}
 }
+
+func TestSkipReader(t *testing.T) {
+	testCases := []struct {
+		src      io.Reader
+		skipLen  int64
+		expected string
+	}{
+		{bytes.NewBuffer([]byte("")), 0, ""},
+		{bytes.NewBuffer([]byte("")), 1, ""},
+		{bytes.NewBuffer([]byte("abc")), 0, "abc"},
+		{bytes.NewBuffer([]byte("abc")), 1, "bc"},
+		{bytes.NewBuffer([]byte("abc")), 2, "c"},
+		{bytes.NewBuffer([]byte("abc")), 3, ""},
+		{bytes.NewBuffer([]byte("abc")), 4, ""},
+	}
+	for i, testCase := range testCases {
+		r := NewSkipReader(testCase.src, testCase.skipLen)
+		b, err := goioutil.ReadAll(r)
+		if err != nil {
+			t.Errorf("Case %d: Unexpected err %v", i, err)
+		}
+		if string(b) != testCase.expected {
+			t.Errorf("Case %d: Got wrong result: %v", i, string(b))
+		}
+	}
+}