msd-core/docs/contributing/adding-a-portability-rule.md

Adding a cross-platform portability lint rule

MSD must run correctly on Windows as well as macOS/Linux. The DEFECT.WINDOWS-* taxonomy in CONTEXT.md names the recurring failure shapes; a family of AST-based ESLint rules (the local/* plugin) enforces them at write-time (in your editor) and in CI, so a Windows-only defect is caught before it ships — not after it reaches the windows-latest CI lane.

This page is the forward recipe: how to add a new rule when you identify a portability defect class that isn't yet mechanically enforced. The architecture and rationale live in ADR-1703; the per-rule reference and fix how-tos live in cross-platform-portability-rules.md.

Diátaxis note: this is an Explanation — it describes the architecture and the reasoning behind the seams, so the recipe at the end makes sense. For "how do I fix a violation I got", see the per-rule how-tos in the reference page.

Why AST rules, not regex

The original enforcement was a regex scanner with a hand-rolled balanced-paren parser, a frozen KNOWN_OFFENDERS ratchet, and a bespoke // windows-portability-ok: comment opt-out. Adversarial review of an attempt to extend the regex found it silently could not match deepStrictEqual, had loose normalizer recognition, and hand-rolled paren-splitting fragility (Kernighan's Law / Greenspun's Tenth — parsing a language with regex). The rip-and-replace decision: one mechanism, AST-based ESLint rules using the parsers already in the stack, hard-fail with zero escape hatches, no ratchet/grandfathering. Full rationale: ADR-1703 "Alternatives considered".

The five seams (and where each lives)

Every portability rule composes the same five seams. Adding a rule means touching each one.

1. The rule — eslint-rules/<rule-name>.cjs

One file per rule, exporting { meta, create }. Matches real syntax nodes (CallExpression, MemberExpression, Literal, TemplateLiteral, BinaryExpression, TryStatement, …), not text. Each rule runs in-editor and in CI via the existing eslint . (invoked by lint:ci).

The two shapes that recur:

• Test-side rules (surface tests/**/*.test.cjs) — flag a non-portable assertion or test fixture shape (path-literal-in-assert, posix-mode-bit-assert, unguarded exec, CRLF split, hardcoded /tmp, bare npm, HOME-without-USERPROFILE).
• Production rules (surface src/**/*.cts, bin/install.js, scripts/build-hooks.js) — flag a non-portable production shape (path-leak-in-content, unguarded fs-rename).

2. The shared vocabulary — eslint-rules/lib/portability-vocab.cjs

The single source of truth for path-related portability: PATH_RETURNING_FNS (Node builtins + project resolvers), the POSIX-normalizer recognizers (.replace(/\\/g,'/'), toPosixPath, …), and string-unwrap helpers. A new path resolver added to src/runtime-homes.cts MUST be registered here — the drift-guard test (tests/portability-vocab-drift.test.cjs) parses that source and fails CI if a path-returning export is missing from PATH_RETURNING_FNS.

3. The platform guard — eslint-rules/lib/platform-guard.cjs

The precision backbone. isWindowsExcludedNode(node, sourceCode) answers "is this node control-dependent on a Windows platform condition?" via a dominator check, not a textual mention: if (process.platform !== 'win32') { … }, early-return guards (if (process.platform === 'win32') return;), os.platform(), and hoisted binding-aware booleans (const isWindows = … consumed by if (!isWindows), with reassignment detection). This is what makes zero escape hatches viable: legitimately POSIX-only code is structured behind a recognized guard, never annotated around (Postel's Law mitigation). If a legitimate shape isn't recognized, teach the helper — never add an opt-out.

4. The disable ban — tests/portability-rule-disable-ban.test.cjs

Because there is no opt-out, an eslint-disable of a portability rule would silently bypass it. This test runs outside ESLint (so it cannot itself be eslint-disabled) and fails the build on any eslint-disable[-next-line|-line] that names a protected portability rule, or any blanket disable. Every new rule MUST be appended to PROTECTED_RULES here, and if the rule covers a new surface (e.g. bin/install.js), that surface MUST be added to collectTestFiles().

5. CI test selection — scripts/ci-test-scope.cjs

The portability lint rules (ADR-1703) rule selects the rule suites + disable-ban when eslint-rules/, eslint.config.mjs, or a covered production surface changes. Add new test files to its tests: list.

The zero-escape-hatch contract

Two things must hold, and they are the epic's primary risk:

1. Rules must be precise. Every rule recognizes legitimate platform-gating via platform-guard.cjs and the canonical compliant shapes, so correctly-written platform-specific code is never flagged. A false positive is a rule bug, fixed in the rule — never by adding an opt-out.
2. Recognition must mean the cure, not just the symptom. When a rule's compliance shape is "the catch handles the transient errno", the handler must actually retry/fallback (a loop continue backedge or a return <call> delegation), not merely reference the errno and rethrow. The require-fs-op-fallback rule encodes this (codex-review-tightened): the defect's cure is retry, not just recognition.

An unrecognized legitimate shape is fixed by teaching the helper/rule, never by annotation. This is the discipline that keeps the rules honest as the codebase grows.

Recipe — add a new local/* portability rule

Run the full engineering directive (rubber-duck → software laws → architecture → qa-test-architect → strict TDD via RuleTester → adversarial review → Diátaxis → rebase+PR). Concretely:

1. Classify the defect. Confirm it's a real DEFECT.WINDOWS-* shape (or a new class worth a predicate in CONTEXT.md). Decide the sound statically-detectable scope — narrow or document rather than ship FP-prone (Phase 5/6 each narrowed scope and documented the boundary).
2. Write the rule — eslint-rules/<rule-name>.cjs ({ meta, create }, type: 'problem', message cites the DEFECT.* predicate). Reuse portability-vocab.cjs / platform-guard.cjs.
3. TDD via RuleTester — tests/<rule-name>.rule.test.cjs. Cover: the violation shape(s), every recognized compliant shape (platform guard, normalizer, retry signal, …), and the anti-patterns that must NOT satisfy compliance (silent-swallow catch, rethrow-only, unrelated errno). Use both espree (.cjs) and @typescript-eslint/parser (.cts) where the rule spans both. Tests are written FIRST and must fail before the rule exists, then pass.
4. Register + scope — in eslint.config.mjs: add the rule to the local plugin's rules map and enable at 'error' in the matching file-glob block. If the rule covers a new surface (e.g. bin/install.js), add a config block for it — apply ONLY the portability rules to generated code, not the full recommended set.
5. Disable ban — append the rule name to PROTECTED_RULES in tests/portability-rule-disable-ban.test.cjs; add any new surface to collectTestFiles().
6. CI selection — add the new test file to the portability lint rules (ADR-1703) rule in scripts/ci-test-scope.cjs.
7. Fix every violation — no ratchet, no grandfathering. Every existing + grandfathered offender is fixed in the same phase (route through a shared helper, add a guard, or normalize).
8. Docs — add the rule to the reference table + a fix how-to in cross-platform-portability-rules.md; rewrite the DEFECT.* predicate's detect=/fix-forward= in CONTEXT.md to point at the rule; record known boundaries honestly.
9. Verify — npm run lint:ci green; the touched modules' tests green; the windows-latest CI lane is the only true Windows signal.

Catalog (shipped)

Rule	DEFECT	Surface	Phase
no-path-literal-in-assert	WINDOWS-PATH-LITERAL-IN-ASSERT	tests	1
no-posix-mode-bit-assert	WINDOWS-POSIX-MODE-BIT-ASSERT	tests	2
no-unguarded-nonportable-exec	WINDOWS-TEST-PORTABILITY (chmod+sh -c)	tests	3
no-crlf-fragile-split	WINDOWS-TEST-PORTABILITY (G1–G3)	tests	4
no-hardcoded-tmp	WINDOWS-TEST-PORTABILITY (G4)	tests	4
no-bare-npm-exec	WINDOWS-TEST-PORTABILITY (G5)	tests	4
require-userprofile-with-home	WINDOWS-TEST-PORTABILITY (G6)	tests	4
normalize-path-in-content	WINDOWS-PATH-LEAK-IN-MARKDOWN-CONTENT	src/**/*.cts	5
require-fs-op-fallback	WINDOWS-FS-OPS	src/**/*.cts, bin/install.js, scripts/build-hooks.js	6
require-full-tmpdir-triad	WINDOWS-TEST-PORTABILITY (#4220)	tests	#4244
no-unbounded-dirname-walk	WINDOWS-TEST-PORTABILITY (#4020/#4220)	tests, scripts/**/*.cjs	#4244

DEFECT.WINDOWS-ARGV-OVERFLOW is deliberately not in this catalog: argv length is a runtime property (the args-array size is not statically knowable), so no AST rule can soundly detect it. It is addressed at the source (run-tests.cjs chunking under RUN_TESTS_MAX_CMDLINE_CHARS).

Teardown (complete)

The legacy machinery this architecture replaced is fully retired: the regex scanner scripts/lint-windows-test-portability.cjs (Phase 3), the tests/windows-test-parity-guard.test.cjs ratchet (Phase 4), allowlist-ratchet.cjs usage for portability classes (the module remains for unrelated size-budget lints), and the // windows-portability-ok: comment convention (swept — zero remain). Every DEFECT.WINDOWS-* predicate in CONTEXT.md now points at its enforcing rule.