Fix GetSnapshots to not return non-existent snapshots with ignore_unavailable=true

[gaobinlong]

Description

Relates to #6820.

Currently, the Get Snapshot API returns all of the snapshots in-progress when the specified snapshot is non-existent and the parameter ignore_unavailable is set to true.

The main changes are:

1. Fix the bug for Get Snapshot API, return empty result when getting a non-existing snapshot with setting ignore_unavailable to true and there are just some snapshots in-progress.
2. Add some test code for the change.

Issues Resolved

#6820.

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/12940/
• CommitID: 8084a87

[codecov-commenter]

Codecov Report

Merging #6839 (f128d73) into main (8c9cfdc) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

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@@             Coverage Diff              @@
##               main    #6839      +/-   ##
============================================
- Coverage     70.73%   70.71%   -0.02%     
+ Complexity    59281    59235      -46     
============================================
  Files          4812     4812              
  Lines        283614   283614              
  Branches      40896    40897       +1     
============================================
- Hits         200606   200550      -56     
- Misses        66552    66628      +76     
+ Partials      16456    16436      -20     

Impacted Files	Coverage Δ
...ter/snapshots/get/TransportGetSnapshotsAction.java	41.58% <0.00%> (ø)

... and 482 files with indirect coverage changes

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[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/12941/
• CommitID: f128d73

[dblock]

Shouldn't specifying a non-existent snapshot be an error? (different from an unavailable snapshot)

[gaobinlong]

@dblock just like Get index API or Get alias API, I think the common parameter ignore_unavailable contains the meaning of ignoring missing, by going through the code of Get Snapshot API I found that's the same logic, when the specified snapshot is non-existent, the api will throw SnapshotMissingException if ignore_unavailable is false(by default), but return empty result if ignore_unavailable is true (has a bug when there are some snapshots in-progress, this is what this PR wants to fix).

[dblock]

Thanks for the clarification. Is there a unit test possible for this?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.indices.replication.SegmentReplicationRelocationIT.testRelocateWithQueuedOperationsDuringHandoff
      1 org.opensearch.indices.replication.SegmentReplicationIT.testScrollCreatedOnReplica

• URL: https://build.ci.opensearch.org/job/gradle-check/13639/
• CommitID: 518cce7
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[gaobinlong]

Thanks for the clarification. Is there a unit test possible for this?

Thanks for your review and suggestion, I've tried adding unit tests for the changed method but found repositoryService is referenced in the method and is hard to mock:

OpenSearch/server/src/main/java/org/opensearch/action/admin/cluster/snapshots/get/TransportGetSnapshotsAction.java

Line 247 in 53b128f

final Repository repository = repositoriesService.repository(repositoryName);

, so maybe we can only add integration test for this change and it seems that most snapshots related operations only have integration test.

[dblock]

Let's merge. Consider some refactoring to make unit tests possible.