sanderling/docs/development/ci.md

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CI

CI

ci.yml runs on every pull request: it builds, unit-tests, and drives three small web fixtures through headless Chrome (test/browser/testdata). It never runs sanderling against a real app.

Two other workflows do, and both are workflow_dispatch only. They boot devices, build apps and take minutes, which is not what you want on every push, and neither is a merge gate.

folio

Actions -> folio -> Run workflow. Inputs pick the legs (all, android, ios, web), and override the seed, the step budget and the wall-clock budget. Seed and step budget take 0 to mean "use the calibrated value in the workflow"; the wall-clock budget has no such sentinel and is passed through as written, so every leg gets whatever you type there.

Each leg builds examples/folio for its platform, builds the CLI with only the tags that platform needs (make sanderling-android and friends), and runs examples/folio/sanderling/spec.ts through .github/scripts/folio-run.sh. That script is plain bash so you can reproduce a job locally, with that leg's pinned numbers:

SEED=9 MAX_STEPS=200 .github/scripts/folio-run.sh android

web and ios expect the bug. Folio double-submits a transaction when the submit button is double-tapped, and two properties catch it: submitMovesBalanceByTypedAmount, which demands the total balance move by exactly the amount typed, and submitCommitsOneTransactionPerAction, which demands no more transactions committed over a window than there were submit actions in it. A double tap is one action committing two transactions, so it breaks both.

The runs pass --exit-on-violation, which exits 2 when the run recorded a violation and 1 when something went wrong. Telling those apart is the whole point of the exit code: a job that only knew "non-zero" could not tell a working fuzzer from a broken emulator.

Exit 2 on its own is not a conviction, though, so the script reads the trace before it decides:

what the trace says	job
a violation of one of the two properties above, with no is_error on its witness	green
a violation whose witness carries is_error	red: a predicate threw, and a thrown predicate is recorded as a violation like any other
a violation of any other property (newAccountBalanceIsZero fires on one android seed)	red: a real finding, but not the one this leg gates on
no violation and exit 0	red: the fuzzer stopped finding a bug that is still there
no trace at all, on exit 0 or 2	red: the run recorded nothing, so there is no verdict to read
exit 1, or any other code	red: the harness broke, and the code propagates

The first two rows are why the check is worth the code it takes. A TypeError in predicates.ts and a fuzzer that no longer reaches the bug both used to print "found the submit bug" and exit 0, and they need opposite responses: one is a spec to fix, the other is a seed to recalibrate. A thrown predicate fails android as well, where a conviction is otherwise only a bonus, because a spec that stopped running is not evidence about the app.

The balance property only judges a window holding exactly one submit action. Without that rule the recorded delta covers every transaction since the last Home visit, and the property convicts on arithmetic it cannot attribute: an early version of this gate went green on a witness whose delta was 3.16x the typed amount. Honest windows are rare, so the counting invariant carries most of the detection: it needs no amount and survives a wide window.

android is a health gate, not a conviction gate, because it convicts in four runs out of five rather than five. The leg asserts that the run stayed healthy and reached the transaction screen, and reports the conviction it usually gets as a bonus. A gate that fails one run in five would be useless here: its failure message, "the double-submit bug was NOT found", is indistinguishable from the regression the gate exists to catch.

It used to be two runs in five. The android backend now waits out a route cross-fade before it snapshots, the way the ios companion and the chrome driver do, because a dump holding two screens at once makes the runner refuse to act on it and a quarter of android steps therefore applied no action at all. The number of those wasted steps varied run to run, so the same seed never walked the same trajectory. With the wait, four runs of one seed produced byte-identical decision sequences over 179 steps.

The fifth diverged for the remaining reason: a route can settle before its content composes, so the tree holds one screen and almost nothing in it, and the fuzzer acts on a screen that is still filling in. That happened once in about a thousand steps. Catching it needs structural stability polling on every snapshot, which costs roughly 2.8s per mutating step and was removed for that reason, so it is the open item between android and a real conviction gate.

The wasmJs app is served with Cross-Origin-Opener-Policy and Cross-Origin-Embedder-Policy headers, because its sqlite worker needs cross-origin isolation. Served without them the app loads a blank canvas and every step observes an empty accessibility tree.

The seeds are calibrated, not guessed. On an M-series mac, web seed 3 convicts at step 185-187 and ios seed 7 at step 97-101, each 3 runs out of 3 and each with a delta of exactly twice the typed amount. Both run a 240-step budget. Keep them pinned: honest evidence is rare, and across 2261 ios steps only one submit tap landing on Home had a single-submit window.

Android runs seed 9 over 200 steps: its conviction lands at step 178, so a shorter budget would never see the bonus. A full run costs about five minutes.

Repeating the ios leg by hand is not the same as running it in CI: with --clear-data=false a second local run inherits the first one's accounts, so simctl uninstall before each repeat or the numbers drift.

The ios leg passes --clear-data=false, because the job installs a fresh build immediately before the run and a freshly installed app is already clear state. The in-run reinstall is worth avoiding: simctl uninstall + install followed straight away by the XCTest runner's own launch fails with app.folio is unknown to FrontBoard maybe half the time. That used to hang the run outright; the launch RPC is bounded now, so it fails in about 90 seconds with a real error instead, but a failing leg is still a failing leg. The job timeouts are the backstop if it happens anyway.

Only one sanderling run may drive a given simulator at a time. The driver takes an advisory lock on the target's UDID and a second run is refused with the lock path in the message, because two runs interleaving app lifecycle leave the first run's automation session bound to a bundle the simulator no longer knows.

replay-ui

Actions -> replay-ui -> Run workflow. This one is dogfooding: it records a trace from test/browser/testdata/throwing (violations and uncaught exceptions, so every panel has something to render), serves it with sanderling replay, and fuzzes that UI with replay-ui/sanderling/spec.ts.

Six of the seven properties there are cross-panel agreements - two panels deriving the same fact by different paths have to say the same thing - so they hold for any trace and need no recalibrating when the fixture changes. The seventh is the stock noUncaughtExceptions, which asks nothing of the panels and only fails if the UI throws. Any violation fails the job.

So does a run that judged nothing. Exit 0 says no property returned false, which is not the same as any property having been evaluated: each one declines to judge when the elements it reads are absent, so a run where the trace failed to serve, or where the fuzzer sat on the run list, renders nothing and passes. .github/scripts/replay-ui-summary.sh reads the trace and puts a per-property count of judged against declined steps in the job summary. Run it by hand with GITHUB_STEP_SUMMARY=/dev/stdout .github/scripts/replay-ui-summary.sh runs/dogfood.

It fails the job when any of the four properties that need nothing beyond the step page having rendered - selectedStepIsInRange, exactlyOneStepIsSelected, stepCountMatchesTheList, screenshotShowsTheSelectedStep - judged nothing at all. The other three are reported and not gated, because a zero on them is a seed getting unlucky rather than a broken leg: switchingTabsKeepsTheStep needs a tab switch between consecutive steps, and badgeCountMatchesThePanel needs the fuzzer to land on a violating step and open the violations tab in that same step. On the first run measured this way (seed 3, 80 steps) that last one judged nothing at all, so the fixture reaches it far too rarely to be worth gating on.

Reading a failure

Both workflows upload their run directories as artifacts, and write the step count, seed and violations to the job summary. To replay a failure:

gh run download <run-id> -n folio-android
sanderling replay <the downloaded runs directory>

That is the same UI the replay-ui workflow fuzzes. Open the step the summary named, and the Violations tab shows the witness: the property, the reason, and the extractor values at the step that caused it.

When a device leg flakes

Expecting a violation from a single seed is timing-sensitive, most of all on an emulator. Calibration reduces that; it does not remove it. If a platform starts failing across repeated dispatches with "the double-submit bug was NOT found", do not raise the step budget blindly - run a seed sweep with the campaign tool (cmd/internal-tools/campaign), which exists for exactly this, and pin a seed that finds the bug with room to spare. A leg failing with "a predicate threw" is a different problem entirely and no seed will fix it.