bytes: make Join return a new buffer on len(a) == 1

Fixes #3844.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6432054

src/pkg/bytes/bytes.go

@@ -333,14 +333,15 @@ func FieldsFunc(s []byte, f func(rune) bool) [][]byte {
 	return a[0:na]
 }
 
-// Join concatenates the elements of a to create a single byte array.   The separator
+// Join concatenates the elements of a to create a new byte array. The separator
 // sep is placed between elements in the resulting array.
 func Join(a [][]byte, sep []byte) []byte {
 	if len(a) == 0 {
 		return []byte{}
 	}
 	if len(a) == 1 {
-		return a[0]
+		// Just return a copy.
+		return append([]byte(nil), a[0]...)
 	}
 	n := len(sep) * (len(a) - 1)
 	for i := 0; i < len(a); i++ {
@@ -619,10 +620,8 @@ func Replace(s, old, new []byte, n int) []byte {
 		m = Count(s, old)
 	}
 	if m == 0 {
-		// Nothing to do. Just copy.
+		// Just return a copy.
-		t := make([]byte, len(s))
+		return append([]byte(nil), s...)
-		copy(t, s)
-		return t
 	}
 	if n < 0 || m < n {
 		n = m

src/pkg/bytes/bytes_test.go

@@ -490,6 +490,12 @@ func TestSplit(t *testing.T) {
 				t.Errorf("Split disagrees withSplitN(%q, %q, %d) = %v; want %v", tt.s, tt.sep, tt.n, b, a)
 			}
 		}
+		if len(a) > 0 {
+			in, out := a[0], s
+			if cap(in) == cap(out) && &in[:1][0] == &out[:1][0] {
+				t.Errorf("Join(%#v, %q) didn't copy", a, tt.sep)
+			}
+		}
 	}
 }