tdas opened a new pull request #29696:
URL: https://github.com/apache/spark/pull/29696
### What changes were proposed in this pull request?
Make MicroBatchExecution explicitly call `getBatch` when the start and end
offsets are the same.
### Why are the changes needed?
Structured Streaming micro-batch engine has the contract with V1 data
sources that, after a restart, it will call `source.getBatch()` on the last
batch attempted before the restart. However, a very rare combination of
sequences violates this contract. It occurs only when
- The streaming query has specific types of stateful operations with
watermarks (e.g., aggregation in append, mapGroupsWithState with timeouts).
- These queries can execute a batch even without new data when the
previous updates the watermark and the stateful ops are such that the new
watermark can cause new output/cleanup. Such batches are called no-data-batches.
- The last batch before termination was an incomplete no-data-batch. Upon
restart, the micro-batch engine fails to call `source.getBatch` when attempting
to re-execute the incomplete no-data-batch.
This occurs because no-data-batches has the same and end offsets, and when a
batch is executed, if the start and end offset is same then calling
`source.getBatch` is skipped as it is assumed the generated plan will be empty.
This only affects V1 data sources like Delta and Autoloader which rely on this
invariant to detect in the source whether the query is being started from
scratch or restarted.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
New unit test with a mock v1 source that fails without the fix.
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