[IMPROVEMENT] Zstd: Contract and test coverage for ZstdBlobStoreDAO atop file and memory implementations

[HesandaLiyanage]

Context & Motivation

Following the discussion on #3193 regarding ZstdBlobStoreDAO running atop non-S3 storage layers, @chibenwa noted:

"ZstdBlobStoreDAO should work out of the box atop file implem. If this is not the case a separated PR is welcomed."

Prior to this change:

1. ZstdBlobStoreDAOTest in server/blob/blob-zstd was hardcoded to S3BlobStoreDAO and required spinning up a Docker AWS S3 container (DockerAwsS3Extension).
2. There was zero test coverage proving that ZstdBlobStoreDAO operates correctly atop FileBlobStoreDAO (or MemoryBlobStoreDAO).
3. Running Zstd compression tests locally always incurred Docker container startup overhead.
4. BlobStoreModulesChooserTest in james-server-guice-distributed tested S3 and Cassandra module resolution, but lacked test coverage for file() and postgres() implementations.

---

Solution Overview

1. Extracted ZstdBlobStoreDAOContract:

   • Created org.apache.james.blob.zstd.ZstdBlobStoreDAOContract extending BlobStoreDAOContract and MetadataAwareBlobStoreDAOContract.
   • Encapsulates all Zstd-specific contract assertions (compression threshold, min ratio, custom caller metadata preservation, decompression, corrupt payload handling, metrics verification, and multi-file roundtrips).

2. Added ZstdFileBlobStoreDAOTest:

   • Tests ZstdBlobStoreDAO directly atop FileBlobStoreDAO backed by FileSystemImpl.forTesting().
   • Confirms that FileBlobStoreDAO's user-defined extended attributes (UserDefinedFileAttributeView) correctly store and propagate ContentEncoding.ZSTD and content-original-size metadata out of the box.

3. Added ZstdMemoryBlobStoreDAOTest:

   • Provides ultra-fast, containerless contract verification of ZstdBlobStoreDAO atop MemoryBlobStoreDAO.

4. Refactored ZstdBlobStoreDAOTest:

   • ZstdBlobStoreDAOTest now implements ZstdBlobStoreDAOContract for S3BlobStoreDAO (retaining full Docker S3 integration coverage while reducing boilerplate from 454 lines down to 88 lines).

5. Enhanced BlobStoreModulesChooserTest:

   • Added test assertions verifying BlobStoreModulesChooser correctly selects FileBlobStoreDAODeclarationModule and PostgresBlobStoreDAODeclarationModule.

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Verification & Tests

• server/blob/blob-zstd:
  • ZstdFileBlobStoreDAOTest: 109 / 109 tests passed (100%)
  • ZstdMemoryBlobStoreDAOTest: 109 / 109 tests passed (100%)
  • ZstdBlobStoreDAOTest (S3 + Testcontainers): 110 / 110 tests passed (100%)
  • Total: 328 tests passed, 0 failures.
• server/container/guice/distributed:
  • BlobStoreModulesChooserTest: 8 / 8 tests passed (100%)
• Checkstyle: 0 violations across all modified modules.
• Sortpom: Clean pom ordering verified.

[chibenwa]

(Just need ICLA reception)

[chibenwa]

"ZstdBlobStoreDAO should work out of the box atop file implem. If this is not the case a separated PR is welcomed."

My concern was not necessarily about the DAO compatibility (which is possible through the BlobStoreDAOMetadataContract) but more about guice wiring.

But those tests do not hurt.

[HesandaLiyanage]

(Just need ICLA reception)

So i have to sign this https://www.apache.org/licenses/icla.pdf and then send to secretary@apache.org right ?

[chibenwa]

So i have to sign this https://www.apache.org/licenses/icla.pdf and then send to secretary@apache.org right ?

Yes

Fill with PDF for automate reading and sign a separated paper.

notify project james add this in the email body so that I get notified

[HesandaLiyanage]

notify project james add this in the email body so that I get notified

Done

[chibenwa]

Thanks a lot for this first contribution.

Wishing you to be able to contribute many more!

Best regard,

Benoit