# Task 01: CrashPoint Enum + Fault Injection Hooks

## Delivers

A `CrashPoint` enum identifying 8 write-path locations where crashes can occur, a `CrashInjector` struct that triggers controlled panics or early returns at those locations, and test-gated hooks threaded into the signal write path, checkpoint path, cohort ledger, collection index, and co-engagement index. The injector is entirely behind `#[cfg(test)]` -- zero overhead in release builds.

## Complexity: M

## Dependencies

- None (this is the foundation task for the phase)

## Technical Design

### 1. CrashPoint enum

```rust
// tidal/src/testing/crash_injector.rs

/// Locations in the write path where a crash can occur.
///
/// Each variant corresponds to a real boundary between durable and
/// in-memory state transitions. The names encode the operation and
/// whether the crash occurs before or after the durable write.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum CrashPoint {
    /// After WAL append returns but before the signal ledger hot tier is updated.
    /// State: WAL has the event, ledger does not.
    WalPreAggregate,

    /// After signal ledger hot tier is updated but before WAL append confirms.
    /// State: ledger has the update, WAL may or may not.
    /// (In practice, our write path appends WAL first, so this tests the
    /// case where the process dies between WAL confirm and caller return.)
    WalPostAggregate,

    /// Before the checkpoint WriteBatch is committed to the storage engine.
    /// State: in-memory state is live, checkpoint is stale.
    CheckpointPreFlush,

    /// After the checkpoint WriteBatch is committed but before the WAL
    /// checkpoint marker is written.
    /// State: checkpoint is fresh, WAL still has old events.
    CheckpointPostFlush,

    /// During the signal aggregation update (hot tier on_signal call).
    /// State: partial update -- some decay scores updated, others not.
    SignalAggregationUpdate,

    /// During CohortSignalLedger::record() -- after the global ledger
    /// write succeeds but before the cohort write completes.
    CohortLedgerUpdate,

    /// During CollectionIndex::add_item() or create() -- after the
    /// fjall put but before the in-memory bitmap is updated.
    CollectionIndexUpdate,

    /// During CoEngagementIndex::record_positive() -- after some edges
    /// are written but before eviction completes.
    CoEngagementUpdate,
}
```

### 2. CrashInjector struct

```rust
// tidal/src/testing/crash_injector.rs

use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
use std::sync::Arc;

/// Configurable fault injector for crash recovery testing.
///
/// Tracks how many times each crash point has been crossed and triggers
/// a panic when the configured threshold is reached. The panic is caught
/// by the test harness, simulating an unclean process exit.
///
/// Thread-safe: uses atomics for all counters.
pub struct CrashInjector {
    /// Which crash point to trigger on.
    target: CrashPoint,
    /// Fire after this many crossings of the target crash point.
    /// 0 means fire on the first crossing.
    fire_after_n: u64,
    /// Counter of crossings for the target crash point.
    crossing_count: AtomicU64,
    /// Whether the injector has already fired (one-shot).
    fired: AtomicBool,
    /// Whether the injector is armed (can be disarmed for setup phases).
    armed: AtomicBool,
}

impl CrashInjector {
    /// Create a new injector targeting the given crash point.
    ///
    /// `fire_after_n`: trigger on the Nth crossing (0 = first crossing).
    pub fn new(target: CrashPoint, fire_after_n: u64) -> Arc<Self> {
        Arc::new(Self {
            target,
            fire_after_n,
            crossing_count: AtomicU64::new(0),
            fired: AtomicBool::new(false),
            armed: AtomicBool::new(true),
        })
    }

    /// Arm the injector (enable triggering).
    pub fn arm(&self) {
        self.armed.store(true, Ordering::Release);
    }

    /// Disarm the injector (disable triggering without resetting counters).
    pub fn disarm(&self) {
        self.armed.store(false, Ordering::Release);
    }

    /// Whether the injector has fired.
    pub fn has_fired(&self) -> bool {
        self.fired.load(Ordering::Acquire)
    }

    /// Called at each crash point in the write path.
    ///
    /// If this crossing matches the configured trigger condition,
    /// sets `fired = true` and panics with a descriptive message.
    /// The test harness catches the panic via `std::panic::catch_unwind`.
    pub fn maybe_crash(&self, point: CrashPoint) {
        if point != self.target {
            return;
        }
        if !self.armed.load(Ordering::Acquire) {
            return;
        }
        if self.fired.load(Ordering::Acquire) {
            return; // one-shot: already fired
        }

        let count = self.crossing_count.fetch_add(1, Ordering::AcqRel);
        if count >= self.fire_after_n {
            self.fired.store(true, Ordering::Release);
            panic!(
                "CrashInjector: simulated crash at {:?} after {} crossings",
                point,
                count + 1
            );
        }
    }

    /// Return the number of times the target crash point has been crossed.
    pub fn crossing_count(&self) -> u64 {
        self.crossing_count.load(Ordering::Acquire)
    }
}
```

### 3. Module structure

```rust
// tidal/src/testing/mod.rs
//! Test-only utilities. Gated behind #[cfg(test)] or the "test-utils" feature.

pub mod crash_injector;
pub use crash_injector::{CrashInjector, CrashPoint};
```

Add to `tidal/src/lib.rs`:

```rust
#[cfg(any(test, feature = "test-utils"))]
pub mod testing;
```

### 4. Thread-local crash injector slot

To avoid threading an `Arc<CrashInjector>` through every function signature in the write path (which would pollute production code), we use a thread-local slot that is only populated during tests.

```rust
// tidal/src/testing/crash_injector.rs

use std::cell::RefCell;

thread_local! {
    static INJECTOR: RefCell<Option<Arc<CrashInjector>>> = const { RefCell::new(None) };
}

/// Install a crash injector for the current thread.
///
/// Only callable from test code. Production builds optimize this away.
pub fn install_injector(injector: Arc<CrashInjector>) {
    INJECTOR.with(|cell| {
        *cell.borrow_mut() = Some(injector);
    });
}

/// Remove the crash injector from the current thread.
pub fn clear_injector() {
    INJECTOR.with(|cell| {
        *cell.borrow_mut() = None;
    });
}

/// Check the crash point against the installed injector (if any).
///
/// In production builds (without #[cfg(test)]), this compiles to nothing.
/// In test builds, it checks the thread-local injector.
#[inline(always)]
pub fn check_crash_point(point: CrashPoint) {
    INJECTOR.with(|cell| {
        if let Some(ref injector) = *cell.borrow() {
            injector.maybe_crash(point);
        }
    });
}
```

### 5. Hook injection sites

Each hook is a single-line call behind `#[cfg(test)]`:

**Signal write path** (`tidal/src/signals/ledger/core.rs`, in `record_signal`):

```rust
// After WAL append, before hot tier update:
#[cfg(any(test, feature = "test-utils"))]
crate::testing::crash_injector::check_crash_point(
    crate::testing::CrashPoint::WalPreAggregate,
);

// After hot tier update:
#[cfg(any(test, feature = "test-utils"))]
crate::testing::crash_injector::check_crash_point(
    crate::testing::CrashPoint::WalPostAggregate,
);
```

**Checkpoint path** (`tidal/src/signals/checkpoint/mod.rs`, in `checkpoint`):

```rust
// Before storage.write_batch(batch):
#[cfg(any(test, feature = "test-utils"))]
crate::testing::crash_injector::check_crash_point(
    crate::testing::CrashPoint::CheckpointPreFlush,
);

// After storage.write_batch(batch) + flush, before returning:
#[cfg(any(test, feature = "test-utils"))]
crate::testing::crash_injector::check_crash_point(
    crate::testing::CrashPoint::CheckpointPostFlush,
);
```

**Signal aggregation** (`tidal/src/signals/hot.rs`, in `on_signal`):

```rust
// Between score updates (between decay_scores[0] and decay_scores[1]):
#[cfg(any(test, feature = "test-utils"))]
crate::testing::crash_injector::check_crash_point(
    crate::testing::CrashPoint::SignalAggregationUpdate,
);
```

**Cohort ledger** (`tidal/src/cohort/ledger.rs`, in `record`):

```rust
// After hot.on_signal, before warm.increment:
#[cfg(any(test, feature = "test-utils"))]
crate::testing::crash_injector::check_crash_point(
    crate::testing::CrashPoint::CohortLedgerUpdate,
);
```

**Collection index** (`tidal/src/db/collections.rs`, in `add_to_collection`):

```rust
// After fjall put, before in-memory bitmap update:
#[cfg(any(test, feature = "test-utils"))]
crate::testing::crash_injector::check_crash_point(
    crate::testing::CrashPoint::CollectionIndexUpdate,
);
```

**Co-engagement index** (`tidal/src/entities/co_engagement.rs`, in `record_positive`):

```rust
// After edge insertion loop, before eviction check:
#[cfg(any(test, feature = "test-utils"))]
crate::testing::crash_injector::check_crash_point(
    crate::testing::CrashPoint::CoEngagementUpdate,
);
```

### 6. Test helper: `run_with_crash`

```rust
// tidal/src/testing/crash_injector.rs

use std::panic::{AssertUnwindSafe, catch_unwind};

/// Execute a closure with a crash injector installed.
///
/// Returns `Ok(T)` if the closure completes normally, or `Err(CrashPoint)`
/// if the injector fired (simulated crash). In the Err case, the
/// injector is automatically cleared from the thread-local slot.
// Takes `&Arc` not `Arc` so callers can inspect `injector.has_fired()` after
// the call without needing to clone first.
pub fn run_with_crash<T, F: FnOnce() -> T>(
    injector: &Arc<CrashInjector>,
    f: F,
) -> Result<T, CrashPoint> {
    let target = injector.target;
    install_injector(Arc::clone(injector));

    let result = catch_unwind(AssertUnwindSafe(f));

    clear_injector();

    match result {
        Ok(val) => Ok(val),
        Err(_) if injector.has_fired() => Err(target),
        Err(payload) => std::panic::resume_unwind(payload),
    }
}
```

## Acceptance Criteria

- [ ] `CrashPoint` enum with 8 variants: `WalPreAggregate`, `WalPostAggregate`, `CheckpointPreFlush`, `CheckpointPostFlush`, `SignalAggregationUpdate`, `CohortLedgerUpdate`, `CollectionIndexUpdate`, `CoEngagementUpdate`
- [ ] `CrashInjector::new(target, fire_after_n)` creates a one-shot injector
- [ ] `CrashInjector::maybe_crash(point)` panics when the crossing count reaches `fire_after_n`
- [ ] `CrashInjector::arm()` / `disarm()` control whether the injector can fire
- [ ] `CrashInjector::has_fired()` returns whether the injector has triggered
- [ ] Thread-local `install_injector` / `clear_injector` / `check_crash_point` functions
- [ ] `run_with_crash(injector, closure)` helper catches injector panics and returns `Err(CrashPoint)`
- [ ] Hook sites added at all 8 write-path locations behind `#[cfg(any(test, feature = "test-utils"))]`
- [ ] `tidal/src/testing/mod.rs` module gated behind `#[cfg(any(test, feature = "test-utils"))]`
- [ ] Zero overhead in release builds -- all hooks compile away
- [ ] Unit tests: `injector_fires_at_threshold`, `injector_one_shot`, `injector_disarm_prevents_fire`, `run_with_crash_returns_err_on_fire`, `run_with_crash_returns_ok_on_complete`
- [ ] `cargo clippy -D warnings` and `cargo fmt` pass

## Test Strategy

```rust
#[cfg(test)]
#[allow(clippy::unwrap_used)]
mod tests {
    use super::*;

    #[test]
    fn injector_fires_at_threshold() {
        let inj = CrashInjector::new(CrashPoint::WalPreAggregate, 3);
        // Crossings 0, 1, 2 -- should not fire.
        inj.maybe_crash(CrashPoint::WalPreAggregate);
        inj.maybe_crash(CrashPoint::WalPreAggregate);
        inj.maybe_crash(CrashPoint::WalPreAggregate);
        assert!(!inj.has_fired());
        // Wait -- fire_after_n=3 means fire when count >= 3.
        // Crossing 0 (count=0), 1 (count=1), 2 (count=2) don't fire.
        // Crossing 3 (count=3) fires.
        let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
            inj.maybe_crash(CrashPoint::WalPreAggregate);
        }));
        assert!(result.is_err());
        assert!(inj.has_fired());
    }

    #[test]
    fn injector_ignores_wrong_crash_point() {
        let inj = CrashInjector::new(CrashPoint::WalPreAggregate, 0);
        // Different crash point -- should not fire.
        inj.maybe_crash(CrashPoint::CheckpointPreFlush);
        inj.maybe_crash(CrashPoint::CohortLedgerUpdate);
        assert!(!inj.has_fired());
    }

    #[test]
    fn injector_one_shot() {
        let inj = CrashInjector::new(CrashPoint::WalPreAggregate, 0);
        let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
            inj.maybe_crash(CrashPoint::WalPreAggregate);
        }));
        assert!(result.is_err());
        assert!(inj.has_fired());

        // Second crossing should not panic (one-shot).
        inj.maybe_crash(CrashPoint::WalPreAggregate); // should not panic
    }

    #[test]
    fn injector_disarm_prevents_fire() {
        let inj = CrashInjector::new(CrashPoint::WalPreAggregate, 0);
        inj.disarm();
        inj.maybe_crash(CrashPoint::WalPreAggregate); // should not panic
        assert!(!inj.has_fired());

        inj.arm();
        let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
            inj.maybe_crash(CrashPoint::WalPreAggregate);
        }));
        assert!(result.is_err());
    }

    #[test]
    fn run_with_crash_returns_ok_on_complete() {
        let inj = CrashInjector::new(CrashPoint::WalPreAggregate, 1000);
        let result = run_with_crash(inj, || 42);
        assert_eq!(result, Ok(42));
    }

    #[test]
    fn run_with_crash_returns_err_on_fire() {
        let inj = CrashInjector::new(CrashPoint::WalPreAggregate, 0);
        let result = run_with_crash(inj, || {
            check_crash_point(CrashPoint::WalPreAggregate);
            42
        });
        assert_eq!(result, Err(CrashPoint::WalPreAggregate));
    }
}
```
