Dsh CI Test Reliability

Reliable DSH CI tests

Build tests that remain correct under the repository's real CI topology, not only when run alone on a quiet workstation. This skill owns isolation and reliability decisions; it does not replace the repository's test-tier policy or select every command for a push.

Read the owning rules

• Use the testing policy to select unit, coverage, expected-output, snapshot, browser, or real-API evidence.
• Use the defensive patterns for lifecycle, subprocess, cancellation, and teardown behavior.
• Read the active Vitest config and GitHub workflow when their worker or job topology affects the test.
• For recorded-session scenarios, also follow the snapshot instructions.
• Use dsh-pre-push-checks after the test design is sound to select outgoing validation.

Model the execution topology

Assume these layers can overlap unless the active configuration proves otherwise:

1. Tests in one Vitest file.
2. Separate Vitest files or worker processes.
3. Independent Vitest or repository-gate processes in one job.
4. Different Actions jobs whose runners share one host.

Process isolation does not isolate host ports, predictable filesystem paths, external services, databases, sockets, or inherited child processes. For every acquired resource, identify its owner, atomic allocation mechanism, observable readiness signal, registered cleanup, and quiescent completion signal.

Do not serialize an entire suite merely because one fixture lacks isolation. Narrow the exclusive scope or change the resource allocation first. A sequential Vitest block cannot protect a host resource from another file, process, job, or runner.

Allocate resources atomically

Use the resource owner's allocator instead of checking availability and claiming it later.

• Network fixtures bind loopback with listen(0) and read the assigned address only after the server reports that it is listening. Never scan for a free port and bind it later.
• Create private per-test temporary roots with mkdtemp; do not acquire predictable shared paths.
• Give shared databases, sockets, sessions, and output locations unique per-test namespaces.
• Use exclusive creation where a path must not already exist.
• Keep stable recorded identifiers separate from ephemeral transport addresses. Translate inside the fixture instead of forcing the live resource to use the recorded value.

Literal paths and URLs used only as parser inputs or expected values are not acquired resources. Do not rewrite them merely because they look fixed.

Contain process-global state

Treat process.env, cwd, fake timers, locale and timezone, module mocks, registries, console hooks, globalThis, and global fetch interception as exclusive mutable resources.

Prefer an injected dependency or instance-local adapter. When mutation is required:

• capture whether the original value was absent or present;
• restore that exact state;
• register restoration immediately;
• use try/finally around the smallest mutation scope;
• keep an afterEach fallback when failure before the local finally is plausible;
• intercept the narrowest exact request or call that the fixture owns.

Respect platform-owned semantics

CI runs the same suite on Windows and on POSIX hosts, and a value the operating system owns does not always come back the way a test wrote it.

• Writing a value back is safe only when the assertion tolerates the write-back failing. Restoring a file's mtime to prove that a fingerprint invalidates anyway holds everywhere; restoring it to prove that a record stays valid assumes a lossless round trip, which NTFS's 100-nanosecond ticks do not give a fractional millisecond. When the assertion depends on the restoration, take the expected value from a fresh read rather than from the remembered one.
• Windows matches environment variable names case-insensitively, so a fixture seeding http_proxy and HTTP_PROXY as separate keys holds one entry there.
• Windows releases file handles asynchronously, so a rename or removal that completes at once on a POSIX host needs a bounded retry sized to the observed contention.
• Windows has no POSIX permission or signal semantics. A case that depends on them takes an explicit platform skip naming the reason, rather than an assertion weakened everywhere.

Prefer an observation that holds on every platform. When a case genuinely cannot, exclude it on that platform explicitly.

Budget timeouts against the lane

A describe or case timeout overrides the runner's --testTimeout instead of yielding to it, so a value below the lane's budget lowers what CI already granted — and the same literal reads as a widening on a host whose default is smaller. A suite bound by process creation takes the lane budget; a tighter value carries the reason it is tighter.

Raise the hook budget with the test budget. Setup and teardown pay the same contention, so lifting only the case budget moves a contended failure into afterEach.

Where a timeout is the subject, keep the outer wait far larger than the timeout under test. A case proving that a 20 ms deadline fires must not race the harness's own wait, or load decides which deadline reports first.