gaoyunhaii commented on pull request #14831:
URL: https://github.com/apache/flink/pull/14831#issuecomment-774941437


   Suppose a task `B` has two precessors `A1` and `A2`, `A1` and `A2` has sent 
`Barrier_5` and `Barrier_6` after `EndOfInputEvent` and then finish. The two 
barrier might just reached `B`'s input channels and are not processed yet. At 
the same time, the `CheckpointCoordinator` starts a new checkpoint and since 
`A1` and `A2` are finished, `B` becomes the new source and 
`CheckpointCoordinator` would trigger `Barrier_7` to `B`. In this case, 
`Barrier_7` could be viewed as a new checkpoint come after all the un-processed 
ones (namely `Barrier_5` and `Barrier_6`), thus we could trigger checkpoint 7 
after all the un-process ones, namely after we processed `Barrier_5` and 
`Barrier_6`.


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