mvcc/backend: add unit test for ConcurrentReadTx

Add TestConcurrentReadTx to ensure concurrentReadTx can see all the
prior writes which are stored on the read buffer.

mvcc/backend/backend_test.go

@@ -250,6 +250,35 @@ func TestBackendWriteback(t *testing.T) {
 	}
 }
 
+// TestConcurrentReadTx ensures that current read transaction can see all prior writes stored in read buffer
+func TestConcurrentReadTx(t *testing.T) {
+	b, tmpPath := NewTmpBackend(time.Hour, 10000)
+	defer cleanup(b, tmpPath)
+
+	wtx1 := b.BatchTx()
+	wtx1.Lock()
+	wtx1.UnsafeCreateBucket([]byte("key"))
+	wtx1.UnsafePut([]byte("key"), []byte("abc"), []byte("ABC"))
+	wtx1.UnsafePut([]byte("key"), []byte("overwrite"), []byte("1"))
+	wtx1.Unlock()
+
+	wtx2 := b.BatchTx()
+	wtx2.Lock()
+	wtx2.UnsafePut([]byte("key"), []byte("def"), []byte("DEF"))
+	wtx2.UnsafePut([]byte("key"), []byte("overwrite"), []byte("2"))
+	wtx2.Unlock()
+
+	rtx := b.ConcurrentReadTx()
+	rtx.RLock() // no-op
+	k, v := rtx.UnsafeRange([]byte("key"), []byte("abc"), []byte("\xff"), 0)
+	rtx.RUnlock()
+	wKey := [][]byte{[]byte("abc"), []byte("def"), []byte("overwrite")}
+	wVal := [][]byte{[]byte("ABC"), []byte("DEF"), []byte("2")}
+	if !reflect.DeepEqual(wKey, k) || !reflect.DeepEqual(wVal, v) {
+		t.Errorf("want k=%+v, v=%+v; got k=%+v, v=%+v", wKey, wVal, k, v)
+	}
+}
+
 // TestBackendWritebackForEach checks that partially written / buffered
 // data is visited in the same order as fully committed data.
 func TestBackendWritebackForEach(t *testing.T) {
@@ -300,8 +329,6 @@ func TestBackendWritebackForEach(t *testing.T) {
 	}
 }
 
-// TODO: add a unit test for concurrentReadTx
-
 func cleanup(b Backend, path string) {
 	b.Close()
 	os.Remove(path)