[AGiorgetti]

Scope

Addresses #85 independently of #87 and #86, now rebased onto develop bc30a39 (including #101).

• 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 current first commit, 13bf041, contains only permanent regression tests. The historical red-test validation below used the original test commit 75ddea0.
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.

Historical local validation (before the current rebase)

Previously validated 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 at that time used MongoDB 8. net472 was compiled, not executed on Linux. No timing benchmarks. No package, workflow or unrelated fix changes.

Historical 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.

Do not merge or publish packages.

Current rebase (2026-11-01)

Head: 2f74d41cddd80d40d4e16f0b75e23637df4b25eb; tree: a57996e4be797e13259ff50947da5afb111398e0.

• Preserved the three tests-first, implementation, and changelog commits without a merge commit or squash. All three patches are unchanged by git range-diff; git diff --check passed.
• Inherits ci: update stable action tags and MongoDB service image #101's updated GitHub Actions and mongo:9.0.4 image. No driver or runner changes were added by this rebase.
• Current local tests were not run: this executor has no dotnet or Docker. Historical results above are not current-head validation.
• Exact-head GitHub CI succeeded on the first attempt: push and pull request. Both runs passed Windows and Ubuntu builds and all Linux net8.0/net9.0/net10.0 test jobs using mongo:9.0.4; each test job reported 285 passed, 0 failed, 0 skipped.
• MongoDB 9 remains a user-approved test trial, not a claim of official driver 3.0 compatibility.

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