Goal

Add a deterministic unit test that exercises the writeConflictRetry block at src/mongo/db/index_builds/primary_driven/util.cpp:368, inside deleteSorterEntriesOutsideRanges. This function runs on PDIB resume (step-up) and prunes sorter-table entries that fall outside the persisted ranges so the resumed build starts from a clean state.

Phase 1 invariant: cursor reset is safe because the loop terminates on entry->first < firstStart; even if the cursor restarts from the beginning, it re-traverses the same pre-range keys and the loop's bound prevents over-delete.

Phase 2 invariant: cursor reset would surface as a returned key < lastEnd after a flush; the inline re-seek to lastEnd restores the position before continuing.
Neither invariant has a unit test today.

What the test must do

Cover both phases. Recommended as two test variants in src/mongo/db/index_builds/primary_driven/util_test.cpp (file exists – use the established fixture there):

1. DeleteSorterEntriesOutsideRangesSurvivesWCEInPrePhase: WCE fires during phase 1 (pre-firstStart delete).
2. DeleteSorterEntriesOutsideRangesSurvivesWCEInPostPhase: WCE fires during phase 2 (post-lastEnd delete).
   For each:
3. Populate the sorter container (IntegerKeyedContainer via a LazyRecordStore) with a fixed key layout that has data in all three regions:
   • Keys < firstStart (phase 1 victims, e.g. 1..3).
   • Keys in [firstStart, lastEnd) (persisted – must survive, e.g. 4..6).
   • Keys >= lastEnd (phase 2 victims plus lastEnd anchor, e.g. 7..9).
4. Build an IndexStateInfo whose ranges cover only [firstStart, lastEnd). Wrap it in a std::vector<IndexStateInfo> of size 1.
5. Arm the engine-agnostic WCE failpoint with nTimes=1 immediately before calling deleteSorterEntriesOutsideRanges. Choose primaryDrivenIndexBuildSorterInsertionBatchSize so the batched flush lands the WCE in the targeted phase (e.g. batch size = 3 + 3 victims pre-range = one flush in phase 1; tune the post-phase variant similarly).

   auto wce = enableStorageEngineWriteConflictForWrites(
       FailPoint::ModeOptions{.mode = FailPoint::Mode::nTimes, .val = 1});
   const auto before = wce->initialTimesEntered();
   

1. Assert all of:
   • The call returned without exception.
   • Exactly one WCE fired (setMode returns before + 1).
   • The persisted range [firstStart, lastEnd) is intact – every key in that range still present.
   • All keys < firstStart are gone.
   • All keys >= lastEnd (including lastEnd itself) are gone.
2. Bonus assertions for the variants:
   • Phase 1 variant: verify by direct cursor scan that no key >= firstStart was touched even though the cursor presumably restarted from the beginning post-WCE.
   • Phase 2 variant: verify by direct cursor scan that no key < lastEnd was deleted even after the WCE-reset triggered the re-seek branch.

     Where to put it

src/mongo/db/index_builds/primary_driven/util_test.cpp (already exists). Use whatever fixture the existing tests use; add a new TEST_F (or two) at the end. If a BUILD.bazel change is needed to gate on use_wiredtiger_enabled (mirroring container_based_spiller_test), include it.

Constraints

• Use enableStorageEngineWriteConflictForWrites (SERVER-126328), not the WT-specific failpoint by name.
• FailPoint::Mode::nTimes only – no Mode::random.
• Test-only PR. Do not modify deleteSorterEntriesOutsideRanges or surrounding code. If the test surfaces a real bug (e.g. persisted-range keys get deleted, or the re-seek workaround fails), stop, file a follow-up SERVER ticket, and link from this ticket.
• Depends on SERVER-126385 landing first (engine-agnostic helper + CatalogTestFixture compatibility).
• Sibling but distinct from SERVER-126451: that ticket covers mergeSpills_remove during a live merge; this one covers resume-time cleanup. Cross-link both tickets so the relationship is visible.

  Reference patterns

• MergeSpillsSurvivesCursorResetUnderWCE in src/mongo/db/sorter/container_based_spiller_test.cpp – canonical "exactly one WCE fired" assertion idiom using initialTimesEntered() + setMode.
• PersistResumeStateSurvivesWriteConflict_* in multi_index_block_test.cpp (SERVER-126385) – canonical CatalogTestFixture + engine-agnostic-failpoint pattern.
• Existing tests in primary_driven/util_test.cpp – for fixture setup (resume-state read paths, etc.).

  Acceptance

• Both {{DeleteSorterEntriesOutsideRangesSurvivesWCEIn {Pre,Post}

  Phase}} tests pass.

• "Exactly one WCE fired" assertion present in each.
• Persisted range [firstStart, lastEnd) verified intact in each.
• Phase-specific bonus assertion present (no over-delete from cursor reset).
• No production code changed.
• No Mode::random, no jstest changes.