correctly treat nils and empty slices in stream.Write

stream.go

@@ -127,6 +127,15 @@ func (s *stream) ReadByte() (byte, error) {
 
 func (s *stream) Write(p []byte) (int, error) {
 	s.mutex.Lock()
+	defer s.mutex.Unlock()
+
+	if s.err != nil {
+		return 0, s.err
+	}
+
+	if len(p) == 0 {
+		return 0, nil
+	}
 
 	s.dataForWriting = make([]byte, len(p))
 	copy(s.dataForWriting, p)
@@ -137,10 +146,10 @@ func (s *stream) Write(p []byte) (int, error) {
 		s.doneWritingOrErrCond.Wait()
 	}
 
-	defer s.mutex.Unlock()
 	if s.err != nil {
 		return 0, s.err
 	}
+
 	return len(p), nil
 }
 

stream_test.go

@@ -371,6 +371,18 @@ var _ = Describe("Stream", func() {
 			s[0] = 'v'
 			Expect(str.getDataForWriting(3)).To(Equal([]byte("foo")))
 		})
+
+		It("returns when given a nil input", func() {
+			n, err := str.Write(nil)
+			Expect(n).To(BeZero())
+			Expect(err).ToNot(HaveOccurred())
+		})
+
+		It("returns when given an empty slice", func() {
+			n, err := str.Write([]byte(""))
+			Expect(n).To(BeZero())
+			Expect(err).ToNot(HaveOccurred())
+		})
 	})
 
 	Context("closing", func() {