cranpose_core/snapshot_v2/

readonly.rs

//! Read-only snapshot implementation.

use super::*;

/// A read-only snapshot of state at a specific point in time.
///
/// This snapshot cannot be used to modify state. Any attempts to write
/// to state objects while this snapshot is active will fail.
///
/// # Thread Safety
/// Contains `Cell<T>` and `RefCell<T>` which are not `Send`/`Sync`. This is safe because
/// snapshots are stored in thread-local storage and never shared across threads. The `Arc`
/// is used for cheap cloning within a single thread, not for cross-thread sharing.
#[allow(clippy::arc_with_non_send_sync)]
pub struct ReadonlySnapshot {
    state: SnapshotState,
}

impl ReadonlySnapshot {
    /// Create a new read-only snapshot.
    pub fn new(
        id: SnapshotId,
        invalid: SnapshotIdSet,
        read_observer: Option<ReadObserver>,
    ) -> Arc<Self> {
        Arc::new(Self {
            state: SnapshotState::new(id, invalid, read_observer, None, false),
        })
    }

    pub fn snapshot_id(&self) -> SnapshotId {
        self.state.id.get()
    }

    pub fn invalid(&self) -> SnapshotIdSet {
        self.state.invalid.borrow().clone()
    }

    pub fn read_only(&self) -> bool {
        true
    }

    pub fn root_readonly(&self) -> Arc<Self> {
        // Readonly snapshots are always their own root
        ReadonlySnapshot::new(
            self.state.id.get(),
            self.state.invalid.borrow().clone(),
            self.state.read_observer.clone(),
        )
    }

    pub fn enter<T>(&self, f: impl FnOnce() -> T) -> T {
        let previous = current_snapshot();
        set_current_snapshot(Some(AnySnapshot::Readonly(self.root_readonly())));
        let result = f();
        set_current_snapshot(previous);
        result
    }

    pub fn take_nested_snapshot(&self, read_observer: Option<ReadObserver>) -> Arc<Self> {
        let merged_observer = merge_read_observers(read_observer, self.state.read_observer.clone());
        ReadonlySnapshot::new(
            self.state.id.get(),
            self.state.invalid.borrow().clone(),
            merged_observer,
        )
    }

    pub fn has_pending_changes(&self) -> bool {
        false // Read-only snapshots never have changes
    }

    pub fn dispose(&self) {
        self.state.dispose();
    }

    pub fn record_read(&self, state: &dyn StateObject) {
        self.state.record_read(state);
    }

    pub fn record_write(&self, _state: Arc<dyn StateObject>) {
        panic!("Cannot write to a read-only snapshot");
    }

    pub fn is_disposed(&self) -> bool {
        self.state.disposed.get()
    }

    // Internal: set a callback to run when this snapshot is disposed.
    pub(crate) fn set_on_dispose<F>(&self, f: F)
    where
        F: FnOnce() + 'static,
    {
        self.state.set_on_dispose(f);
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::state::{StateObject, PREEXISTING_SNAPSHOT_ID};
    use std::rc::Rc;

    fn mock_state_record() -> Rc<crate::state::StateRecord> {
        crate::state::StateRecord::new(PREEXISTING_SNAPSHOT_ID, (), None)
    }

    // Mock StateObject for testing
    struct MockStateObject;

    impl StateObject for MockStateObject {
        fn object_id(&self) -> crate::state::ObjectId {
            crate::state::ObjectId(0)
        }

        fn first_record(&self) -> Rc<crate::state::StateRecord> {
            mock_state_record()
        }

        fn readable_record(
            &self,
            _snapshot_id: crate::snapshot_id_set::SnapshotId,
            _invalid: &SnapshotIdSet,
        ) -> Rc<crate::state::StateRecord> {
            mock_state_record()
        }

        fn prepend_state_record(&self, _record: Rc<crate::state::StateRecord>) {}

        fn promote_record(
            &self,
            _child_id: crate::snapshot_id_set::SnapshotId,
        ) -> Result<(), &'static str> {
            Ok(())
        }

        fn as_any(&self) -> &dyn std::any::Any {
            self
        }
    }

    #[test]
    fn test_readonly_snapshot_creation() {
        let snapshot = ReadonlySnapshot::new(1, SnapshotIdSet::new(), None);
        assert_eq!(snapshot.snapshot_id(), 1);
        assert!(!snapshot.is_disposed());
    }

    #[test]
    fn test_readonly_snapshot_is_valid() {
        let invalid = SnapshotIdSet::new().set(5);
        let snapshot = ReadonlySnapshot::new(10, invalid, None);

        let any_snapshot = AnySnapshot::Readonly(snapshot.clone());
        assert!(any_snapshot.is_valid(1));
        assert!(any_snapshot.is_valid(10));
        assert!(!any_snapshot.is_valid(5)); // Invalid
        assert!(!any_snapshot.is_valid(11)); // Future
    }

    #[test]
    fn test_readonly_snapshot_no_pending_changes() {
        let snapshot = ReadonlySnapshot::new(1, SnapshotIdSet::new(), None);
        assert!(!snapshot.has_pending_changes());
    }

    #[test]
    fn test_readonly_snapshot_enter() {
        let snapshot = ReadonlySnapshot::new(1, SnapshotIdSet::new(), None);

        set_current_snapshot(None);
        assert!(current_snapshot().is_none());

        snapshot.enter(|| {
            let current = current_snapshot();
            assert!(current.is_some());
            assert_eq!(current.unwrap().snapshot_id(), 1);
        });

        assert!(current_snapshot().is_none());
    }

    #[test]
    fn test_readonly_snapshot_enter_restores_previous() {
        let snapshot1 = ReadonlySnapshot::new(1, SnapshotIdSet::new(), None);
        let snapshot2 = ReadonlySnapshot::new(2, SnapshotIdSet::new(), None);

        snapshot1.enter(|| {
            snapshot2.enter(|| {
                let current = current_snapshot();
                assert_eq!(current.unwrap().snapshot_id(), 2);
            });

            let current = current_snapshot();
            assert_eq!(current.unwrap().snapshot_id(), 1);
        });
    }

    #[test]
    fn test_readonly_snapshot_nested() {
        let parent = ReadonlySnapshot::new(1, SnapshotIdSet::new(), None);
        let nested = parent.take_nested_snapshot(None);

        assert_eq!(nested.snapshot_id(), 1); // Same ID
    }

    #[test]
    fn test_readonly_snapshot_read_observer() {
        use std::sync::{Arc as StdArc, Mutex};

        let read_count = StdArc::new(Mutex::new(0));
        let read_count_clone = read_count.clone();

        let observer = Arc::new(move |_: &dyn StateObject| {
            *read_count_clone.lock().unwrap() += 1;
        });

        let snapshot = ReadonlySnapshot::new(1, SnapshotIdSet::new(), Some(observer));
        let mock_state = MockStateObject;

        snapshot.record_read(&mock_state);
        snapshot.record_read(&mock_state);

        assert_eq!(*read_count.lock().unwrap(), 2);
    }

    #[test]
    fn test_readonly_snapshot_nested_with_observer() {
        use std::sync::{Arc as StdArc, Mutex};

        let parent_reads = StdArc::new(Mutex::new(0));
        let parent_reads_clone = parent_reads.clone();
        let parent_observer = Arc::new(move |_: &dyn StateObject| {
            *parent_reads_clone.lock().unwrap() += 1;
        });

        let nested_reads = StdArc::new(Mutex::new(0));
        let nested_reads_clone = nested_reads.clone();
        let nested_observer = Arc::new(move |_: &dyn StateObject| {
            *nested_reads_clone.lock().unwrap() += 1;
        });

        let parent = ReadonlySnapshot::new(1, SnapshotIdSet::new(), Some(parent_observer));
        let nested = parent.take_nested_snapshot(Some(nested_observer));

        let mock_state = MockStateObject;

        // Reading in nested snapshot should call both observers
        nested.record_read(&mock_state);

        assert_eq!(*parent_reads.lock().unwrap(), 1);
        assert_eq!(*nested_reads.lock().unwrap(), 1);
    }

    #[test]
    #[should_panic(expected = "Cannot write to a read-only snapshot")]
    fn test_readonly_snapshot_write_panics() {
        let snapshot = ReadonlySnapshot::new(1, SnapshotIdSet::new(), None);
        let mock_state = Arc::new(MockStateObject);
        snapshot.record_write(mock_state);
    }

    #[test]
    fn test_readonly_snapshot_dispose() {
        let snapshot = ReadonlySnapshot::new(1, SnapshotIdSet::new(), None);
        assert!(!snapshot.is_disposed());

        snapshot.dispose();
        assert!(snapshot.is_disposed());
    }
}