santosomar/general-secure-coding-agent-skills/counterexample-debugger

Counterexample Debugger

A counterexample is a proof that a property fails — a concrete sequence of states that leads to the violation. The trace is correct by construction; the job is to make it understandable.

Triage by source

The compression algorithm

Raw traces are unreadable because they show every variable at every step. Compress:

1. Diff. For each step i → i+1, show only variables that changed.
2. Project. Drop variables not mentioned in the violated property and not data-flowing into ones that are.
3. Collapse stutter. If five consecutive steps change nothing relevant (the model is busy-waiting on something you projected away), collapse to (... 5 irrelevant steps ...).
4. Name the steps. If the model has actions/transitions, label each step with the action that fired. Step 3 [ClientSend] beats Step 3.

A 40-state TLC trace usually compresses to 5–8 relevant transitions.

Narrative translation

After compression, narrate the trace as a story:

• Setup: What's the initial state? (Usually trivial — all zeros/empty)
• Trigger: Which step first moves toward the bad state? This is often NOT step 1.
• Point of no return: Which step makes the violation inevitable? Every step before this is reversible; every step after is doomed.
• Violation: Which step actually breaks the property, and what's the state at that moment?

Worked example (TLC)

Property: Inv == (lock_holder /= NULL) => (waiters = {}) (If someone holds the lock, no one should be waiting — i.e., the lock is supposed to be fair.)

Raw trace: 12 states, 7 variables each. 84 lines.

Compressed:

Step 1 [Init]       lock_holder=NULL  waiters={}  pc=<<idle,idle>>
Step 4 [Acquire(1)] lock_holder=1                                    ← trigger
Step 7 [Wait(2)]                      waiters={2}                    ← point of no return
Step 8 [CheckInv]   VIOLATION: lock_holder=1 ∧ waiters={2}

Narrative: Process 1 acquires the lock. Three steps later — before process 1 releases — process 2 enters the wait set. The invariant assumed acquire-release would be atomic from the waiters' perspective. It isn't.

Map to code: Step 7's Wait(2) action corresponds to Lock::wait() at lock.c:44. The check-then-wait in that function isn't guarded.

Lasso-shaped traces (liveness)

Liveness counterexamples (<>[]P failures) are lassos: a finite prefix followed by a cycle that repeats forever. TLC marks the loop-back point with Back to state N.

• The prefix tells you how the system got into the trap.
• The cycle tells you what it does forever instead of making progress.
• The fix is almost always in the cycle, not the prefix.

Edge cases

• Single-state counterexample: The initial state violates the invariant. You don't have a bug in a transition — your initial state is wrong (or your invariant is).
• Trace longer than the model's diameter: TLC found the violation via BFS, so this shouldn't happen — unless you're running DFS/simulation mode. Switch to BFS; the minimal trace is the readable one.
• Counterexample involves a fairness assumption you forgot to state: The "bug" is that you didn't say WF_vars(SomeAction). The trace will show an enabled action that never fires.
• The counterexample is correct behavior: Sometimes the property is wrong, not the system. If the narrative sounds reasonable, re-read the property.

Do not

• Do not dump the raw trace on the user. Always compress first.
• Do not fix the first step that looks suspicious. Find the point of no return — that's where the fix goes.
• Do not assume model-level state maps 1:1 to code-level state. The model is an abstraction; confirm the mapping.
• Do not turn the trace into a test without understanding it first. → counterexample-to-test-generator consumes the narrative, not the raw trace.

Output format

## Property violated
<property, restated in English>
 
## Compressed trace
Step <N> [<action>]  <changed vars only>   ← <annotation: trigger / PNR / violation>
...
 
## Narrative
<setup → trigger → point of no return → violation, in prose>
 
## Source mapping
Step <N> ↔ <file>:<line>  (<function>)
 
## Suggested fix location
<file>:<line> — <what about this code allows step N>