Github user jrthe42 commented on the issue:
https://github.com/apache/flink/pull/6301
Hi @sihuazhou , I am not familiar with the checkpoint mechanism of Flink,
and I check the source code again.
Although the ```RichSinkFunction#invoke()``` and
```RichSinkFunction#snapshotState()``` are not executed in the same thread, but
there is already synchronization mechanism in ```StreamTask```.
```StreamTask``` use a **checkpoint lock object** to make sure they won't be
called concurrently. Check ```StreamTask#performCheckpoint()``` and
```StreamInputProcessor#processInput()``` if you want to know more.
Thanks for your comment, I removed synchronization here, and this PR is
updated. cc @yanghua
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