add a failpoint for sleep (#460)

Signed-off-by: ekexium <ekexium@gmail.com>

Co-authored-by: Yilin Chen <sticnarf@gmail.com>
Co-authored-by: disksing <i@disksing.com>

txnkv/transaction/prewrite.go

@@ -190,6 +190,11 @@ func (action actionPrewrite) handleSingleBatch(c *twoPhaseCommitter, bo *retry.B
 				return errors.New("injected error on prewriting secondary batch")
 			}
 			util.EvalFailpoint("prewriteSecondary") // for other failures like sleep or pause
+			// concurrent failpoint sleep doesn't work as expected. So we need a separate fail point.
+			// `1*sleep()` can block multiple concurrent threads that meet the failpoint.
+			if val, err := util.EvalFailpoint("prewriteSecondarySleep"); err == nil {
+				time.Sleep(time.Millisecond * time.Duration(val.(int)))
+			}
 		}
 	}