docs(selfhost): verify + document the shared backend's concurrency model

[davion-knight]

What & why

The AMS local-store concurrency guarantees were originally designed for two local processes sharing one SQLite file. #7175 migrated that layer onto the shared SelfHostD1Database seam (src/selfhost/backend-contracts.ts, #4010), which now has two interchangeable adapters — SQLite (d1-adapter.ts) and Postgres (pg-adapter.ts). This PR verifies and documents what concurrency those adapters actually guarantee, so the hosted service's assumptions are stated explicitly instead of inherited implicitly from the old local-file design.

Deliverables

1. Documented concurrency model — src/selfhost/backend-concurrency-model.md. Records, per adapter, what is guaranteed (batch() all-or-nothing atomicity; no lost updates for atomic single-statement writes; committed batches apply in order; reads never see an uncommitted intermediate) and what is not (non-atomic read-modify-write across awaits; cross-process SQLite sharing — out of scope for the single-process-per-deployment topology the admission system already assumes). Every claim cites the exact code it describes.

2. A deterministic verification test — test/unit/selfhost-d1-concurrency.test.ts. Exercises concurrent read/write against the real createD1Adapter(nodeSqliteDriver(...)) seam (opened with the production PRAGMAs) and asserts: N concurrent atomic increments lose no updates; batch() rolls back fully on a failing statement; a committed batch applies in order; a read concurrent with a batch never observes a rolled-back intermediate; and — as the documented hazard — a non-atomic read-modify-write does lose updates.

Why this test shape

The SQLite backend's real topology is single-process with a synchronous driver, so its guarantees are verified deterministically in-process — no external dependency, no flakiness. Real multi-connection Postgres concurrency needs a live server, so it stays behind the existing PG_TEST_URL-gated test/integration/selfhost-pg.test.ts rather than a scripted mock that cannot exhibit real races. No production source changes.

Closes #4942

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

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Caution

🛑 LoopOver review result - fixes required

_{Review updated: 2026-07-22 19:05:20 UTC}

2 files · 1 AI reviewer · no blockers · CI failing · blocked

🛑 Suggested Action - Fix Blockers

Review summary
This is a pure docs + test PR that documents the SQLite/Postgres D1 adapter's concurrency guarantees and backs them with a deterministic in-process test suite exercising the real createD1Adapter(nodeSqliteDriver(...)) seam. The reasoning in the tests is sound: since nodeSqliteDriver wraps synchronous work in already-resolved promises, the 'N concurrent atomic increments' test and the 'non-atomic read-modify-write loses updates' test are both deterministic (not flaky) given JS's microtask scheduling, and the batch()/rollback/read-isolation assertions match the BEGIN/COMMIT/ROLLBACK implementation shown in d1-adapter.ts. The PR explicitly closes issue #4942 and stays narrowly scoped to verification/documentation with no production code touched.

Nits — 5 non-blocking

• test/unit/selfhost-d1-concurrency.test.ts: makeDb() sets `PRAGMA journal_mode = WAL` on an in-memory (`:memory:`) database, which many SQLite builds silently ignore/coerce for in-memory files, so the comment claiming it 'matches the deployed configuration' is only partially true — worth a one-line caveat.
• backend-concurrency-model.md cites specific line numbers (e.g. `d1-adapter.ts:70`, `backend-contracts.ts:87-94`, `server.ts:266`) that will drift as the files change — consider a lighter-weight anchor (symbol name only) or a note that line numbers may go stale.
• The 4 FAILED CI checks (validate, validate-tests 1/2/3) have no detail provided and this branch is 1 commit behind default — worth rebasing to rule out that the failures are just inherited from what landed upstream after this branch diverged, rather than caused by this diff.
• Rebase onto the latest default branch to get a clean CI signal on the currently-undetailed validate/validate-tests failures.
• Consider asserting on the exact rollback error type/message in the batch-rollback test (currently just `.rejects.toThrow()`), to make sure it's failing for the expected PK-violation reason rather than any driver error.

CI checks failing

• validate
• validate-tests (3)
• validate-tests (1)
• validate-tests (2)

Decision drivers

• ✅ Code review — No blockers (1 reviewer)
• ✅ Gate result — Passing (No configured blocker found.)

Context & advisory signals — never blocks the verdict

Signal	Result	Evidence
Linked issue	✅ Linked	#4942
Related work	✅ No active overlap found	No same-issue or scoped active PR overlap found.
Change scope	✅ 20/20	Low review scope from cached public metadata (1 linked issue).
Validation posture	✅ 25/25	PR body includes validation/test evidence.
Contributor workload	✅ 10/10	Author activity: 35 registered-repo PR(s), 19 merged, 0 issue(s).
Contributor context	✅ Confirmed Gittensor contributor	davion-knight; Gittensor profile; 35 PR(s), 0 issue(s).
Improvement	ℹ️ Insufficient signal	risk: clean · value: insufficient-signal · LLM: moderate

Linked issue satisfaction

Addressed
The PR adds a dedicated concurrency-model doc (backend-concurrency-model.md) explicitly documenting guaranteed and non-guaranteed behavior for the SQLite and Postgres adapters, plus a deterministic test suite exercising concurrent reads/writes, batch atomicity, and lost-update scenarios, directly matching both deliverables in the issue.

Review context

• Author: davion-knight
• Role context: outside_contributor
• Public audience mode: oss maintainer
• Lane context: Repository is configured for direct PR review.
• Public profile languages: Python, Rust
• Official Gittensor activity: 35 PR(s), 0 issue(s).
• PR-specific overlap: none found.

Contributor next steps

• Keep the PR focused and include validation evidence before maintainer review.

Signal definitions

• Related work = same linked issue, overlapping active PRs, or title/path similarity.
• Change scope = cached public metadata such as size labels, draft state, and review-burden hints.
• Validation posture = whether the PR provides enough public validation/test evidence for maintainer review.
• Contributor workload = public contributor activity and cleanup pressure, not a repo-wide quality failure.
• Contributor context = public GitHub/Gittensor identity context; non-Gittensor status is not a blocker.

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_{🟩 Safe / merged · 🟦 Advisory · 🟨 Held for review · 🟥 Blocked / closed}

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LoopOver is closing this pull request on the maintainer's behalf (CI is failing (validate, validate-tests (3), validate-tests (1), validate-tests (2))). This is an automated maintenance action — to pursue this change, please open a new pull request with the issues resolved. Closed PRs may be analyzed later to improve review accuracy, but they are not automatically reopened or re-reviewed.