Downloading large number of submissions

Problem/Motivation

When exporting submissions from a webform with a large number of entries (20,000+), the resulting CSV file is incomplete — not all submissions are included.

The root cause is that batchProcess() in WebformResultsExportController loads submissions with loadMultiple() on every batch iteration, but the entity static cache is never cleared between iterations. Memory keeps growing — roughly 5 MB per 100 submissions — until PHP hits its memory_limit and the process dies silently mid-export.

Steps to reproduce

1. Create a webform with a few fields (textfield, email, textarea).
2. Have 1,000+ submissions (the issue becomes critical around 20,000+).
3. Go to Results → Download and export as CSV.
4. The downloaded CSV will be missing submissions from the end.

Fix

Added resetCache() on the webform_submission storage after writing each batch to free the loaded entities from the static cache.

Testing

Tested locally with 1,100 submissions and batch_limit of 500:

- Before fix: memory grows from 50 MB to 96 MB across 11 iterations, never stops growing.
- After fix: memory stays around 66–74 MB, stable after the first iteration.

All 1,100 rows present in the CSV, no duplicates, no gaps in serial numbers.

This looks similar to two existing bug reports. Can you verify whether this is a duplicate of those?

Thanks for the references. I reviewed both issues:

- #3369136 is about incomplete ZIP archives due to ZipArchive not being closed between batches — a different root cause.
- #3570311 is about Chrome blocking CSV downloads triggered via meta-refresh — a browser security issue.

This issue is specifically about entity static cache not being cleared during batch CSV export, causing memory to grow until PHP silently fails. The fix is a single resetCache() call after each batch write. Different root cause and different fix from both referenced issues.

Thanks for working on this. Bug reports are always against the development branch.

Based #7, it sounds like the other issues are not duplicates.

1. Added \Drupal::service('entity.memory_cache')->deleteAll() after each batch iteration to also clear cached non-submission entities (users, files, source entities) that accumulate during export processing. This follows the same pattern used in Drupal core's SqlFieldableEntityTypeListenerTrait.

2. Added a kernel test (WebformSubmissionBatchExportTest) that verifies both entity memory cache and persistent cache are properly cleared after a batch export iteration. The test fails without the patch and passes with it.

@larisse Could you test this updated patch?

CI results: 301/302 tests passed. The single exception (WebformAccessSubmissionPermissionsTest) is a pre-existing infrastructure issue (RecursiveDirectoryIterator failure on twig template directory), unrelated to this change.

Thanks!