Counterexample → Test Generator

A model checker trace is a proof that the bug exists — in the model. A test is a proof it exists in the code. The translation is: map each model action back to a code operation, sequence them, and assert the invariant.

This is the model-checking analogue of → bug-reproduction-test-generator.

The mapping problem

The hard part is interleaving. The model checker found a specific schedule that breaks things. Your test has to reproduce that schedule.

Controlling the schedule

Always try single-threaded first. If the model actions are Read(p1), Read(p2), Write(p1), Write(p2), you don't need real threads — just call p1.read(); p2.read(); p1.write(); p2.write(); in sequence. The interleaving that matters is the order of operations on shared state, not actual OS threads.

Step-by-step

1. Extract the action sequence from the trace. Ignore state dumps; you want the action names and parameters between states.
2. Map each action to a code call. Use the mapping from → program-to-tlaplus-spec-generator (or equivalent extraction).
3. Identify shared state. What model variables correspond to real shared objects? Those need to be set up in the fixture.
4. Sequence the calls. One after another, in trace order. If an action was \E p : Foo(p), use the p TLC chose.
5. Translate the invariant. Inv == counter = N becomes assert counter == N.
6. Run it. If the test passes, your model doesn't match your code (see below). If it fails — good, you have a repro.

Worked example

TLC trace (from the lost-update counter):

State 1: counter=0, tmp=[p1|->0, p2|->0], pc=[p1|->"start", p2|->"start"]
State 2: <Read(p1)>    counter=0, tmp=[p1|->0, p2|->0], pc=[p1|->"write", p2|->"start"]
State 3: <Read(p2)>    counter=0, tmp=[p1|->0, p2|->0], pc=[p1|->"write", p2|->"write"]
State 4: <Write(p1)>   counter=1, ...
State 5: <Write(p2)>   counter=1    ← Inv violated: 2 increments, counter should be 2

Action sequence: Read(p1), Read(p2), Write(p1), Write(p2).

Code mapping:

• Read(p) ↔ first half of increment(): tmp = counter
• Write(p) ↔ second half of increment(): lock(); counter = tmp + 1; unlock()

Problem: increment() is one function in code, two actions in the model. To reproduce the interleaving, split it:

Test (Go):

func TestLostUpdate_FromTLC(t *testing.T) {
    counter := 0
    var mu sync.Mutex

    // Simulate the TLC trace: Read(p1), Read(p2), Write(p1), Write(p2)
    // Split increment() into its two atomic halves.
    tmp1 := counter           // Read(p1) -- reads 0
    tmp2 := counter           // Read(p2) -- reads 0
    mu.Lock(); counter = tmp1 + 1; mu.Unlock()   // Write(p1) -- counter=1
    mu.Lock(); counter = tmp2 + 1; mu.Unlock()   // Write(p2) -- counter=1

    // Invariant: two increments → counter == 2
    if counter != 2 {
        t.Fatalf("lost update: 2 increments, counter=%d (TLC trace reproduced)", counter)
    }
}

Test fails with counter=1. Bug reproduced, deterministically, no real threads.

When the test doesn't reproduce

The trace violated the model invariant but the test passes. Three reasons:

A test that doesn't reproduce is good news — it means the model found a bug in itself, not the code. Fix the model.

Do not

• Do not use real threads when a sequential simulation reproduces the interleaving. Real threads make the test flaky and slow.
• Do not assert the entire final state. Assert the invariant. Other state may legitimately differ between model and code.
• Do not skip the test because the model "already proved it." The model is an abstraction — the test confirms the abstraction was faithful.
• Do not leave the test in the suite after the fix without inverting the assertion. Post-fix, the test should pass (counter == 2) — it becomes a regression guard.

Output format

## Action sequence (from trace)
1. <Action(params)>
2. ...

## Code mapping
<Action> ↔ <function/code region>

## Schedule strategy
<single-threaded simulation | barriers | stress loop> — <why>

## Test
<code block — executable>

## Expected
FAIL before fix: <what assertion trips>
PASS after fix:  <same assertion, now holds>