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: submitMovesBalanceByAtMostTypedAmount, 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, and every number here says which host it was measured on, because the hosts do not agree. On an M3 mac driving iOS 26.1 simulators, ios seed 7 convicts at step 97-101, 11 runs out of 11 from a cleared install, each on both properties and each with the balance moving by exactly twice the typed amount: 199 typed, 39800 cents moved, one account's transaction count rising by two against a window holding one submit. Web seed 3 convicts at step 185-187 on that mac and at step 192 on the ubuntu runner. Both legs run a 240-step budget.

What those numbers assume is a cleared starting state, and that is the only thing that moved them. Measured four ways on one simulator, seed 7 convicts at step 97 from a fresh install with clear-state on, at 100 from a fresh install with it off, and at 97 from a dirty container with it on. It walks 240 steps clean exactly once: dirty container, clear-state off, where the app opens already signed in on the previous run's accounts and the walk diverges at step 1. The leg therefore clears state for itself rather than relying on how it was called.

Do not read a mac number as a statement about CI, but the ios leg does now convict there. It had never done so before 2026-08-16, through every dispatch, and no seed was ever the reason. submitCommitsOneTransactionPerAction counted every tap on TxnSubmit toward its window, including taps the app refuses because the amount field is empty, and more than half of a typical window was those. Measured on recorded android runs: 35 taps against a real budget of 16, and 42 against 17. A window that wide cannot attribute anything, which is why seed 28 reached the bug at the step it convicts at locally and was still not judged.

submitCouldCommit stopped counting them, and the numbers moved a long way:

leg      before                          after (run 31898888205)
ios      240 steps clean, every time     convicts step 59, detected 60
web      step 192 on the ubuntu runner   convicts step 185, detected 186
android  never reached AddTransaction    healthy over 200 steps, reached it

The ios witness at that conviction reads one account's transaction count rising from 0 to 7 against a window holding 6 submits, with applied: true on the action and is_error unset. Seven transactions from six submits is one double submit, which is the bug the leg exists to find.

On the calibration mac the same seed now convicts around step 48, twice in a row, where it used to convict at 97-101. Treat both as approximate: the point is that the window is now tight enough to attribute a submit, not that any particular step number is pinned. A run that fails is worth reading before it is worth recalibrating.

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

That step number was measured on a local emulator with animations ON, and the CI job sets disable-animations: true, so it does not describe the CI leg. The worry that follows is that zeroing the 700ms Compose fade would stop the leg exercising the cross-fade wait entirely. The first real dispatch says otherwise: its 200-step trace carries 4 transitional steps, so the wait still fires, just far less often than it does locally. Treat the android number as an order of magnitude, not a pin. It is a health gate, so nothing keys on it.

Repeating the ios leg by hand needs nothing special now, because the run clears the app's state itself. It used to: just ios installs over the top without uninstalling and folio's signed-in session survives that, so a repeat under the old --clear-data=false opened on the previous run's Home screen and diverged at step 1. That is how the leg came to look dead while the app and the seed were both fine, and it is worth recognising: a leg that reports "the double-submit bug was NOT found" from a machine that has been running the app all day is describing the machine.

The ios leg clears state and passes no --ios-app-path, which is deliberate: without an app path the driver wipes the app's data container instead of reinstalling, and the reinstall is the path that races FrontBoard. simctl uninstall + install followed straight away by the XCTest runner's own launch has failed with app.folio is unknown to FrontBoard about half the time on the host that reported it. That race is untouched and still open; the leg simply does not take that path. It did not reproduce here at all, in 20 consecutive reinstall-and-launch cycles on iOS 26.1, 10 of them reinstalling on top of a live app, so any fix for it has to be developed on a host that can still show it failing.

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.

Sweep in the leg's own configuration, though. The campaign tool and the ios leg now clear state the same way, so a swept seed means what the leg means, but the starting frame is not a detail you can skip checking: while the leg still passed --clear-data=false, seed 14 convicted at step 17 in 2 campaign runs out of 2 and in 0 leg-shaped runs out of 3. Prefer the earliest conviction on offer over the first one found, too. A run reproduces its trajectory on another host only for as long as every snapshot agrees, and every step of prefix is another chance for it not to: seeds convicting at steps 33, 60, 114, 187 and 189 all turned up within the first 30, so an early one is usually there to be found.

A short prefix is necessary and not sufficient, though, and ios is the standing counter-example: seed 28 has the shortest prefix on offer, reproduced its walk on the runner exactly, reached the bug at step 32, and still did not convict, because the window the counting invariant had to judge it in was 117 steps wide. Sweeping selects for a seed that reaches the bug. It cannot select for one whose walk also closes the window, so when a property needs a window, check what the window looked like and not only that the conviction happened.