msd-core/docs/adr/452-eslint-lint-harness.md

ADR 452: Adopt standard ESLint flat-config lint harness

• Status: Accepted
• Date: 2026-05-28

This codebase adopts ESLint flat config (eslint ≥ 9) with typescript-eslint, eslint-plugin-n, eslint-plugin-no-only-tests, and a local AST-rule plugin as the canonical lint harness, replacing the homegrown regex-based scripts/lint-*.cjs scripts. The ESLint harness becomes the single enforcement point for import-graph, Node API, and test-rigor rules. The three test-rigor rules (local/no-source-grep, local/no-magic-sleep-in-tests, local/no-elapsed-assertion) initially ship at warn; a follow-up issue (tracked at #453) promotes them to error after the cleanup phases merge.

Context

Existing homegrown regex harness

scripts/lint-no-source-grep.cjs, scripts/lint-no-magic-sleep.cjs, and related scripts implement test-rigor guards as regular-expression line scanners over raw source text. This approach has several structural weaknesses:

• False positives — regex on raw text fires on string literals, comments, and doc blocks that are not code paths.
• No AST context — the regex scanners cannot distinguish a banned call inside a helper wrapper from a banned call inside a live test assertion.
• No incremental mode — the scripts always scan the entire tree; they have no ESLint-style --cache, --fix, or --changed modes.
• No editor integration — IDEs speak LSP/ESLint, not project-local shell scripts; contributors see violations only in CI.
• Maintenance cost — each new rule requires a new bespoke script with its own exit-code wiring.

Type-aware linting stopgap

tsconfig.lint.json was introduced to give tsc --noEmit access to the hand-written .cjs files alongside the generated ones. It is explicitly described in the codebase as a stopgap pending a proper type-aware ESLint setup.

Generated vs hand-written split

Approximately 59 hand-written and 13 generated .cjs files currently coexist in msd-core/bin/lib/. The hand-written files are not checked by typescript-eslint type-aware rules because tsconfig.lint.json is not wired into an ESLint project. ADR 457 (457-generated-cjs-single-source.md) proposes collapsing this split; the present ADR is a prerequisite: the ESLint harness must exist before the collapse can surface type errors.

Decision

1. Adopt ESLint flat config (eslint.config.mjs at repo root) with:

   • typescript-eslint (type-aware rules enabled via tsconfig.lint.json project reference)
   • eslint-plugin-n for Node API and require() graph enforcement
   • eslint-plugin-no-only-tests (no-only-tests/no-only-tests) to prevent test.only / describe.only leaking into CI
   • A local plugin at scripts/eslint-rules/ that exposes the AST-rewrite of the three homegrown test-rigor rules:
     • local/no-source-grep — bans readFileSync on source files + .includes()/.match()/.startsWith() on the bound variable; also bans assert.match/doesNotMatch on .stdout/.stderr without JSON round-trip
     • local/no-magic-sleep-in-tests — bans setTimeout/sleep/delay calls inside test() / it() / describe() bodies
     • local/no-elapsed-assertion — bans assert on elapsed-time values (e.g., Date.now() - start > N, process.hrtime, performance.now() comparisons in assertions)

2. Phase in at warn. All three local/* rules ship at severity warn from the initial merge. The CI lint gate (npm run lint) does not fail on warnings; it does emit them as annotation. A dedicated follow-up (tracked at #453) flips all three to error after the cleanup phases that eliminate existing violations have merged.

3. Retire homegrown scripts. scripts/lint-no-source-grep.cjs and all sibling regex-scanner scripts are deleted in the same PR that introduces the ESLint config. The CI step that called them is replaced by a single npm run lint invocation.

4. no-restricted-syntax bans (in eslint.config.mjs, severity error from day one):

   • CallExpression[callee.name='setTimeout'] inside Program > ExpressionStatement (top-level sleeps — catches a different shape than local/no-magic-sleep-in-tests)
   • MemberExpression[property.name='only'][object.name=/^(test|it|describe)$/] as a belt-and-suspenders backstop alongside eslint-plugin-no-only-tests

5. eslint-plugin-n enforces:

   • n/no-missing-require — catches import-graph drift for hand-written CJS files
   • n/no-unsupported-features/es-syntax against engines.node (>=22.0.0)

6. Editor integration. Commit the recommended .vscode/extensions.json entry for dbaeumer.vscode-eslint and an .editorconfig fallback so contributors see inline violations without running CI.

Consequences

For contributors

• ESLint runs in CI (npm run lint) alongside the test suite. A clean lint is required before a PR is mergeable.
• The three test-rigor rules fire as warnings initially; they become errors after #453 merges. Violations added after the initial cleanup phase will block CI.
• Existing // allow-test-rule: <reason> comments in .cjs files translate to ESLint // eslint-disable-next-line local/no-source-grep -- <reason> comments. The old exemption syntax is no longer recognized.
• test.only / describe.only committed to any non-scratch file fail CI immediately (error from day one).

For the test-rigor audit

• Violations of local/no-source-grep, local/no-magic-sleep-in-tests, and local/no-elapsed-assertion are now surfaced in the IDE and in CI annotations before a PR is opened, removing the current pattern of discovering violations only in PR review.
• The no-elapsed-assertion rule enforces the clock-seam pattern codified in ADR 456 (456-test-rigor-architecture.md): tests that assert on elapsed time must use the injectable clock seam instead of wall-clock assertions.

For ADR 457

• The ESLint harness with typescript-eslint type-aware rules is a prerequisite for collapsing the hand-written/generated .cjs split. Once tsconfig.lint.json is wired into ESLint's project references, typescript-eslint will surface type drift between the hand-written CJS surface and the TS source.

Rejected Alternatives

(a) Keep the homegrown regex harness. Rejected. False positives, no AST context, no editor integration, and per-rule maintenance cost all compound over time. The homegrown scripts solved the immediate gap but are not a sustainable lint surface.

(b) Legacy .eslintrc format. Rejected. ESLint 9 deprecated .eslintrc; flat config is the supported path for new plugins and type-aware rules. Starting on a deprecated format incurs migration debt immediately.

(c) Per-file ts-check only (no ESLint). Rejected. @ts-check in .cjs files gives type feedback inside the file but does not enforce import-graph, test-rigor, or no-only-tests rules. It is a supplementary aid, not a lint harness.

References

• Tracking issue: #452
• Follow-up (warn → error): #453
• Test-rigor architecture: 456-test-rigor-architecture.md
• Generated CJS collapse (future): 457-generated-cjs-single-source.md
• Homegrown scripts retired: scripts/lint-no-source-grep.cjs, scripts/lint-no-magic-sleep.cjs
• Stopgap: tsconfig.lint.json