### What changes were proposed in this pull request?
This patch fixes the bug regarding accessing `DStreamCheckpointData.currentCheckpointFiles` without guarding which makes the test `basic rdd checkpoints + dstream graph checkpoint recovery` being flaky.
There're two possible points to make test failing:
1. checkpoint logic is too slow so that checkpoint cannot be handled within real delay
2. There's multithreads-unsafe point in `DStreamCheckpointData.update`: it clears `currentCheckpointFiles` and adds new checkpointFiles. Race condition can happen between main thread for test and JobGenerator's event loop thread.
`lastProcessedBatch` guarantees that all events for given time are processed, as commented:
`// last batch whose completion,checkpointing and metadata cleanup has been completed`. That means, if we wait for time for exactly same amount as advanced the time in test (multiply of checkpoint interval as well as batch duration) we can expect nothing will happen in DStreamCheckpointData afterwards unless we advance the clock.
This patch applies the observation above.
### Why are the changes needed?
The test is reported as flaky as [SPARK-28214](https://issues.apache.org/jira/browse/SPARK-28214), and the test code seems unsafe.
### Does this PR introduce any user-facing change?
No
### How was this patch tested?
Modified UT. I've added some debug messages and confirmed no method in DStreamCheckpointData is being called between "after waiting lastProcessedBatch" and "advancing clock" even I added huge amount of sleep between twos, which avoids race-condition.
I was also able to make existing test artificially failing (not 100% consistently but high likely) via adding sleep between `currentCheckpointFiles.clear()` and `currentCheckpointFiles ++= checkpointFiles` in `DStreamCheckpointData.update`, and confirmed modified test doesn't fail the test multiple times.
Closes#25731 from HeartSaVioR/SPARK-28214.
Authored-by: Jungtaek Lim (HeartSaVioR) <kabhwan@gmail.com>
Signed-off-by: Marcelo Vanzin <vanzin@cloudera.com>
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