+/**
+ * Peeks the first block in the FIFO.
+ * If necessary, wait until there is one block.
+ * This function is (always) a cancellation point.
+ *
+ * @warning This function leaves the block in the FIFO.
+ * You need to protect against concurrent threads who could dequeue the block.
+ * Preferrably, there should be only one thread reading from the FIFO.
+ *
+ * @return a valid block.
+ */