fix race condition on Close

Makefile

@@ -1,2 +1,2 @@
 test:
-	@go run github.com/onsi/ginkgo/ginkgo -keepGoing -progress -timeout 1m -race
+	@go run github.com/onsi/ginkgo/ginkgo -keepGoing -progress -timeout 1m -race --randomizeAllSpecs --randomizeSuites

buffer.go

@@ -45,22 +45,25 @@ func (buffer *Buffer) Flush() error {
 	}
 }
 
-// Close flushes the buffer and prevents it from being further used. The buffer
-// cannot be used after it has been closed as all further operations will panic.
+// Close flushes the buffer and prevents it from being further used. If it succeeds,
+// the buffer cannot be used after it has been closed as all further operations will panic.
 func (buffer *Buffer) Close() error {
-	close(buffer.closeCh)
+	select {
+	case buffer.closeCh <- struct{}{}:
+		// noop
+	case <-time.After(buffer.options.CloseTimeout):
+		return ErrTimeout
+	}
 
-	var err error
 	select {
 	case <-buffer.doneCh:
-		err = nil
+		close(buffer.dataCh)
+		close(buffer.flushCh)
+		close(buffer.closeCh)
+		return nil
 	case <-time.After(buffer.options.CloseTimeout):
-		err = ErrTimeout
+		return ErrTimeout
 	}
-
-	close(buffer.dataCh)
-	close(buffer.flushCh)
-	return err
 }
 
 func (buffer *Buffer) consume() {

buffer_test.go

@@ -243,6 +243,33 @@ var _ = Describe("Buffer", func() {
 			// assert
 			Expect(err).To(MatchError(buffer.ErrTimeout))
 		})
+
+		It("allow Close to be called again if it fails", func() {
+			// arrange
+			flusher.Func = func() { time.Sleep(2 * time.Second) }
+
+			sut := buffer.New(
+				buffer.WithSize(1),
+				buffer.WithFlusher(flusher),
+				buffer.WithCloseTimeout(time.Second),
+			)
+			_ = sut.Push(1)
+
+			// act
+			err := sut.Close()
+
+			// assert
+			Expect(err).To(MatchError(buffer.ErrTimeout))
+
+			// arrange
+			time.Sleep(time.Second)
+
+			// act
+			err = sut.Close()
+
+			// assert
+			Expect(err).To(BeNil())
+		})
 	})
 })