Made resolveIndices() in TransportSingleShardAction exception-safe

[nibix]

Description

This is a minor fix in the "explicit index resolution API" feature recently introduced in #18523. Tests in the security plugin showed that actions extending TransportSingleShardAction could throw exceptions when the resolveIndices() method was called.

Originally, the resolveIndices() method of TransportSingleShardAction was implemented with concreteSingleIndex(), which however can throw exceptions for validation issues.

As throwing exceptions is not suitable for the intended meta-data use, this was moved to concreteResolvedIndices(). This MIGHT return a super-set of the indices of concreteSingleIndex() when the latter would throw an exception. This is for the intended purposes of resolveIndices() however acceptable.

Check List

• Functionality includes testing. (via security plugin)

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[github-actions]

✅ Gradle check result for acfe562: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.20%. Comparing base (8dd1286) to head (acfe562).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...pport/single/shard/TransportSingleShardAction.java	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19804      +/-   ##
============================================
+ Coverage     73.12%   73.20%   +0.07%     
- Complexity    71099    71147      +48     
============================================
  Files          5754     5754              
  Lines        325296   325298       +2     
  Branches      47052    47052              
============================================
+ Hits         237868   238120     +252     
+ Misses        68282    68044     -238     
+ Partials      19146    19134      -12     

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[cwperks]

We do not use resolveToConcreteSingleIndex() here because it might throw exceptions for validation errors.

What are these validation errors?

Is it the one from here? In what test instances is that hit?

[nibix]

Is it the one from here? In what test instances is that hit?

Yes, it is. It actually makes multi tenancy tests in the security plugin fail, as the special handling for MT sometimes makes requests refer to "virtual", non-existant indices or aliases.