[AGiorgetti]

Scope

Addresses #85 independently of #87 and #86, based directly on develop 286c0ae.

• Share one initialization attempt so concurrent callers wait for the same success or exception.
• Retain successful initialization only after all existing index and checkpoint-generator setup completes.
• Allow a later call to retry a failed attempt.
• Reject same-thread recursive initialization before it can poison the shared attempt.
• Keep setup logic, exception mapping, index definitions and dependencies unchanged. Initialize is synchronous; no async initialization API is added.

Tests first

The first commit, 75ddea0, contains only permanent regression tests.
Against unchanged product code on MongoDB 7.0.45 / net8.0: 5 failed, 1 passed, 0 skipped.

Six scenarios cover real index-conflict retry, checkpoint-constructor failure after indexes succeed, successful idempotence, gated concurrent success/failure, and caught recursive entry. Concurrent waiters must observe the same failed attempt rather than silently starting a retry. Tests use gates and thread state, not a timing benchmark, and follow Context/Because/Fact conventions.

The recursion regression also caught and rejected an intermediate implementation that could poison Lazy state; the final guard passes it.

Final local validation

Exact published tree: 399ca0fd95914fdfb87c57970e9223001555e90e.

• Whole solution Release build, including net472: 0 errors, 296 warnings (including existing dependency audit warnings).
• Each of net8.0/net9.0/net10.0: 6 focused tests passed; 536 core tests passed; 225 adapter tests passed, 1 unchanged known baseline [P2] Exact-ID explain assertion rejects MongoDB 7 IDHACK despite using the _id index #94 failure; 0 skipped.
• The only adapter failure is Stream_head_delete_by_id_should_use_the_builtin_id_index, the existing IDHACK query-plan portability assertion. No test was suppressed or weakened.
• Focused command: dotnet test src/NEventStore.Persistence.MongoDB.Tests/NEventStore.Persistence.MongoDB.Core.Tests.csproj -c Release -f net8.0 --filter FullyQualifiedName~Issue_85
• Independent review found no remaining findings in the final initialization guard and tests.

Local database: MongoDB 7.0.45. MongoDB 8 cannot start on this cloud host because required /sys CPU metadata is unavailable; PR CI uses MongoDB 8. net472 was compiled, not executed on Linux. No timing benchmarks. No package, workflow or unrelated fix changes.

Exact-head GitHub CI

CI run 39969206709 succeeded on head 3fb5929: Ubuntu and Windows builds and all MongoDB 8 Linux net8.0/net9.0/net10.0 test jobs passed. No reruns were needed.

Draft for user review; do not merge or publish packages.

Co-authored by Alessandro Giorgetti and Codex (OpenAI).