[CELEBORN-2423] Fix rack-aware replica storage selection

[Kalvin2077]

What changes were proposed in this pull request?

This follow-up to PR #3781:

• Validates that rack-aware replica candidates support the requested storage type.
• Builds replica StorageInfo from the selected replica worker.
• Adds regression tests for diskless replicas and different mount points.

Why are the changes needed?

During best-effort fallback, the allocator could select a diskless worker for a local-disk replica or incorrectly reuse the primary worker's mount point.

Does this PR resolve a correctness bug?

• Yes

Does this PR introduce any user-facing change?

• Yes

How was this patch tested?

build/mvn -pl master -am -Dtest=SlotsAllocatorRackAwareSuiteJ test

[Kalvin2077]

@zaynt4606 @SteNicholas
PTAL, thx.

[yew1eb]

@Kalvin2077 LGTM overall. One nit on test coverage: offerSlotsRackAwareFallbackRequiresReplicaDisk only covers the case where no eligible replica exists. Could we add a case where the replica rack has both a diskless and a local-disk worker, and assert the allocator skips the diskless on and still allocates? That's the path the new canAssign predicate actually guards — currently it would regress unnoticed.

[Kalvin2077]

@Kalvin2077 LGTM overall. One nit on test coverage: offerSlotsRackAwareFallbackRequiresReplicaDisk only covers the case where no eligible replica exists. Could we add a case where the replica rack has both a diskless and a local-disk worker, and assert the allocator skips the diskless on and still allocates? That's the path the new canAssign predicate actually guards — currently it would regress unnoticed.

@yew1eb
Thx.

I've added offerSlotsRackAwareFallbackSkipsDisklessReplica to cover this path.

It uses two partitions so the diskless candidate is scanned regardless of the randomized initial index, and verifies both replicas are allocated to the eligible disk-backed worker. Removing the canAssign guard causes the test to fail.

PTAL.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.59%. Comparing base (90954f7) to head (55ecb31).
⚠️ Report is 24 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3803      +/-   ##
============================================
- Coverage     58.74%   58.59%   -0.14%     
  Complexity      231      231              
============================================
  Files           399      399              
  Lines         28058    28065       +7     
  Branches       2740     2745       +5     
============================================
- Hits          16479    16443      -36     
- Misses        10380    10418      +38     
- Partials       1199     1204       +5     

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