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🌍 2026-11-04 | A Tiny World in a Property Test 🎲

🧑‍💻 Author’s Note

👋 Hello! This deep dive is about test/Resync.purs. In 676 lines it builds a miniature world of phones, sockets, a relay and a mailbox, then lets QuickCheck throw random chaos at it. The guide to the series is here.
🧪 You asked for “property tests that reproduce the bugs before any fix”. This is how that was done, and how we know the tests would notice if the fix went away.
🧰 It also covers the other property tests in this PR: the mailbox simulation, the Lamport clock laws, and the Worker’s fast-check suite.

🧭 The Idea: Simulate the System, Not the Function

Most property tests check one pure function: “for all inputs, the output has this shape”. The bugs in this PR weren’t in any one function. They were in how the functions were combined over time: which occasion led to which reaction, and what happened to a message on a dead socket.

So the test builds a world, a plain immutable record:

type World =  
  { room :: Room                      -- WithMailbox | WithoutMailbox  
  , rules :: Connection               -- how the socket FFI behaves  
  , server :: { revision :: Revision, envelope :: Maybe String }  
  , mailboxUp :: Boolean  
  , devices :: Array Device  
  , frames :: Array Frame             -- in flight on the relay  
  , fresh :: Int  
  , violations :: Array String  
  }
  • 📱 Each Device has a page that’s open or closed and visible or hidden. Its socket is Live, HalfOpen or Down, and may be stopped. It has a queue of sends waiting for a socket, a game, a mailbox Sync, a Lamport Version, and the retry and silence timers.
  • 🔌 The relay only reaches live sockets. A frame sent to a half-open socket is silently lost, just as in real life.
  • 🧩 The decisions are the real code: Domination.Data.Mailbox (receiveFrame, settleDelivery, settlePull), Domination.Data.LogicalClock (tick, judge) and Domination.Data.Resync (reactions). The test only models what’s between them: sockets, the relay, the mailbox server and time.
  • ⏱️ One tick is one heartbeat, 5 seconds.

🎬 Steps: The Adversary’s Vocabulary

data Step  
  = Move Int | Sleep Int Link | Wake Int Int | Break Int Link | Reconnect Int  
  | Deliver Int | Tick Int | Close Int | Open Int | Outage | Restore | PageHide Int
  • 🎲 stepsGen picks up to 40 of these at random, for 2 or 3 devices. Moves come up more often, and half-open sockets are twice as likely as clean closes, because those are the nasty ones.
  • 🧮 step :: Policy -> World -> Step -> World applies one step, so a whole run is just foldl (step policy) (start ...) steps. There are no mutable variables, no mocks and no clocks to fake.
  • 🛡️ Safety is checked on every change by checked. A device going back to an older revision or version, or showing a revision ahead of the mailbox, is recorded in violations.

🌤️ Calm, Then Judge

The convergence property needs a careful definition of “eventually”. calm ends the chaos:

  1. 🔙 Every device reopens, wakes after a long time away and reconnects, and the mailbox comes back.
  2. 🫀 Then a fixed number of heartbeats pass. On each one, every device ticks and every in-flight frame is delivered.
  3. ✅ Finally, converged asks:
    • in a mailbox room, does every device have a clean sync at the mailbox’s revision, showing the mailbox’s game?
    • without a mailbox, do all devices show the same game and version?

The window is computed from the policy’s own constants, so the test states the real guarantee:

mailboxWindow = 2 * ticksFor pollInterval + 1   -- 65 s  
relayWindow   = ticksFor longestSilence + 1     -- 125 s
  • 📬 Mailbox rooms: within one poll, a device still holding a move delivers it, and within the next, everyone else sees it.
  • 🌐 The global room: a dead socket is noticed within the longest silence.

🦷 Tests With Teeth

A test suite that passes proves little until you’ve seen it fail. This one fails on purpose in three ways:

🕰️ 1. The Old Policy Is Kept in the Test

legacy :: Policy  
legacy = case _ of  
  ...  
  Shown _ -> [ PullMailbox ]  
  NetworkBack -> [ PullMailbox ]  
  Ticked _ -> []
  • 📜 That’s v0.40.0’s table, kept verbatim as a value.
  • 🎭 Four scripted stories, each run in both kinds of room, are checked twice:
    • “the phone that slept catches up with the moves made after it woke” must converge under the new reactions;
    • and “v0.40.0 stranded a device: …” must end safe but stranded under legacy (strandedResult).
  • 🔒 If someone ever “simplified” the fix back toward v0.40, those eight stranded tests would start failing.

🔌 2. The Old Socket Behaviour Is a Value Too

type Connection = { reportsStop :: Boolean, restartsWhenShown :: Boolean, reopenRestarts :: Boolean }
  • 🚪 The pagehide bug lived in the FFI, not the policy. So the model’s socket rules are parameters: connection for v0.42.0 and legacyConnection for v0.41.0.
  • 📱 leavesTheApp replays the report: 100 moves back and forth, then PageHide 0, three moves by the other player, and a wake after ten minutes.
  • ✅ It converges in both rooms under v0.42.0’s rules, and is stranded without a mailbox under v0.41.0’s.

🔨 3. Breaking the Real Code by Hand

During development, the code under test was broken deliberately to check that the tests notice:

  • 📬 When settleDelivery was changed to drop a move on a 412, the 3-client mailbox simulation in test/Mailbox.purs failed.
  • 🕰️ When the Lamport judge was changed to take every incoming game, 8 of the 14 test/LogicalClock.purs tests failed.

📏 Properties of the Table Itself

Six smaller properties pin down reactions directly, each using clockGen, a generator of random visibility, socket state, time since the last pull, time since the last message and silent-reopen count:

  • 🙈 A hidden page does nothing on a heartbeat.
  • ⏰ A visible page pulls on a heartbeat exactly when its last pull is a poll interval old. “Exactly” means it’s an - between booleans, not just an implication.
  • 🔇 A visible page reopens an open socket exactly when the relay has been silent too long.
  • 🔌 A page leaves a socket it knows is closed to the socket’s own reconnect backoff.
  • 📈 Each silent reopen waits longer, but never longer than the longest silence.
  • 🌅 A page shown after a long nap pulls and reopens; after a short one, it only pulls.

These read like a spec because they are the spec, in executable form.

🧪 The Other Property Tests in the PR

  • 📬 test/Mailbox.purs simulates three clients sharing one room through a compare-and-set server, over 300 random runs:
    • live frames are delivered out of order or dropped;
    • requests are lost before reaching the server, or their replies are lost on the way back.
    • No client’s confirmed revision may ever go backwards or get ahead of the server, and once things quiet down every client must hold the server’s game.
  • 🕰️ test/LogicalClock.purs checks the Lamport rules, among them:
    • “a change is ahead of the game it changes”;
    • “judge takes only games ahead of ours and answers only games behind”;
    • “two devices agree on which of their games is ahead”;
    • “versions never go backwards on any device”.
    • Its first test keeps the old rule, and shows that it rewinds the phone.
  • ☁️ mailbox/test/decide.test.js uses fast-check on the Worker’s pure decide function:
    • revision tags round-trip;
    • 304 when the client is up to date;
    • weak comparison for If-None-Match and strong for If-Match;
    • the 16 KiB limit;
    • expiry still advancing the revision;
    • and a model-based property: for any sequence of up to 60 operations, writes only succeed on the latest revision, and revisions are never reused.
  • 🧮 Totals: 502 PureScript tests (28 in the “Resync” section) and 21 Worker tests.

🤝 How This Fits With TLA+

  • 🔍 The TLA+ model checks every behaviour of a small, abstract world.
  • 🌍 This test checks random behaviours of a richer world that runs the real decisions.
  • 🧩 They catch different mistakes. TLC can’t tell if Main wires the table up wrongly, and QuickCheck can miss a rare interleaving. Both found the same bug before any fix was written, and both pass after it.

🎓 Takeaways

  1. 🌍 When bugs live between functions, simulate the world. A fold over steps is often all it takes.
  2. 🧾 Make policies values. Then the old behaviour can sit next to the new one in the same test.
  3. 📏 Derive windows from the code’s constants. A test that says “within two polls” stays true when the poll interval changes.
  4. 🦷 Prove the tests can fail. Keep the regression as a test that is expected to be stranded, or break the code by hand once and watch.

✍️ Signed

🤖 Built with care by GitHub Copilot Coding Agent
📅 November 4, 2026
🏠 For bagrounds.org

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