rune/core/gc/

context.rs

use super::GcState;
use super::Trace;
use crate::core::object::GcString;
use crate::core::object::LispHashTable;
use crate::core::object::{Gc, IntoObject, Object, UninternedSymbolMap, WithLifetime};
use bumpalo::collections::Vec as GcVec;
use std::cell::{Cell, RefCell};
use std::fmt::Debug;
use std::ops::Deref;
use std::sync::atomic::AtomicBool;

/// A global store of all gc roots. This struct should be passed to the [Context]
/// when it is created.
#[derive(Default, Debug)]
pub(crate) struct RootSet {
    pub(super) roots: RefCell<Vec<*const dyn Trace>>,
}

#[expect(dead_code)]
// These types are only stored here so they can be dropped
pub(in crate::core) enum DropStackElem {
    String(String),
    ByteString(Vec<u8>),
    Vec(Vec<Object<'static>>),
}

/// A block of allocations. This type should be owned by [Context] and not used
/// directly.
#[derive(Default)]
pub(crate) struct Block<const CONST: bool> {
    pub(in crate::core) objects: bumpalo::Bump,
    // Allocations that will be dropped when the objects are moved. At that time
    // the allocation will get copied into the GC heap. This let's us avoid an
    // extra copy of memory when a vector is first made an object. The
    // allocation can continue to live outside of the GC heap until we copy the
    // object.
    pub(in crate::core) drop_stack: RefCell<Vec<DropStackElem>>,
    // We don't yet have a hashmap that supports allocators, so we need to keep
    // track of the memory and free it only after the table is garbage
    // collected. Kind of a hack.
    pub(in crate::core) lisp_hashtables: RefCell<Vec<*const LispHashTable>>,
    pub(in crate::core) uninterned_symbol_map: UninternedSymbolMap,
}

unsafe impl<const C: bool> Send for Block<C> {}

/// Owns all allocations and creates objects. All objects have
/// a lifetime tied to the borrow of their `Context`. When the
/// `Context` goes out of scope, no objects should be accessible.
pub(crate) struct Context<'rt> {
    pub(crate) block: Block<false>,
    root_set: &'rt RootSet,
    next_limit: usize,
}

impl Drop for Context<'_> {
    fn drop(&mut self) {
        self.garbage_collect(true);
        if self.block.objects.allocated_bytes() == 0 {
            return;
        }
        if std::thread::panicking() {
            eprintln!("Error: Context was dropped while still holding data");
        } else {
            panic!("Error: Context was dropped while still holding data");
        }
    }
}

thread_local! {
    /// Ensure there is only one context per thread.
    static SINGLETON_CHECK: Cell<bool> = const { Cell::new(false) };
}

/// Ensure there is only one global context.
static GLOBAL_CHECK: AtomicBool = AtomicBool::new(false);

impl Block<true> {
    pub(crate) fn new_global() -> Self {
        use std::sync::atomic::Ordering::SeqCst as Ord;
        assert!(GLOBAL_CHECK.compare_exchange(false, true, Ord, Ord).is_ok());
        Self::default()
    }
}

impl Block<false> {
    pub(crate) fn new_local() -> Self {
        Self::assert_unique();
        Self::default()
    }

    pub(crate) fn new_local_unchecked() -> Self {
        Self::default()
    }

    pub(crate) fn assert_unique() {
        SINGLETON_CHECK.with(|x| {
            assert!(!x.get(), "There was already and active context when this context was created");
            x.set(true);
        });
    }
}

impl<const CONST: bool> Block<CONST> {
    pub(crate) fn add<'ob, T, Tx>(&'ob self, obj: T) -> Object<'ob>
    where
        T: IntoObject<Out<'ob> = Tx>,
        Gc<Tx>: Into<Object<'ob>>,
    {
        obj.into_obj(self).into()
    }

    pub(crate) fn add_as<'ob, T, Tx, V>(&'ob self, obj: T) -> Gc<V>
    where
        T: IntoObject<Out<'ob> = Tx>,
        Gc<Tx>: Into<Gc<V>>,
    {
        obj.into_obj(self).into()
    }

    /// Create a new String whose backing storage is already part of the GC
    /// heap. Does not require dropping when moved during garbage collection
    /// (unlike std::string).
    pub(crate) fn string_with_capacity(&self, cap: usize) -> GcString<'_> {
        GcString::with_capacity_in(cap, &self.objects)
    }

    /// Create a new Vec whose backing storage is already part of the GC
    /// heap. Does not require dropping when moved during garbage collection
    /// (unlike std::vec).
    pub(crate) fn vec_new(&self) -> GcVec<'_, Object<'_>> {
        GcVec::new_in(&self.objects)
    }

    pub(crate) fn vec_with_capacity(&self, cap: usize) -> GcVec<'_, Object<'_>> {
        GcVec::with_capacity_in(cap, &self.objects)
    }
}

impl<'ob, 'rt> Context<'rt> {
    const MIN_GC_BYTES: usize = 2000;
    const GC_GROWTH_FACTOR: usize = 12; // divide by 10
    pub(crate) fn new(roots: &'rt RootSet) -> Self {
        Self { block: Block::new_local(), root_set: roots, next_limit: Self::MIN_GC_BYTES }
    }

    pub(crate) fn from_block(block: Block<false>, roots: &'rt RootSet) -> Self {
        Block::assert_unique();
        Context { block, root_set: roots, next_limit: Self::MIN_GC_BYTES }
    }

    pub(crate) fn bind<T>(&'ob self, obj: T) -> <T as WithLifetime<'ob>>::Out
    where
        T: WithLifetime<'ob>,
    {
        unsafe { obj.with_lifetime() }
    }

    pub(crate) fn get_root_set(&'ob self) -> &'rt RootSet {
        self.root_set
    }

    pub(crate) fn garbage_collect(&mut self, force: bool) {
        let bytes = self.block.objects.allocated_bytes();
        if cfg!(not(test)) && !force && bytes < self.next_limit {
            return;
        }

        let mut state = GcState::new();
        for x in self.root_set.roots.borrow().iter() {
            // SAFETY: The contract of root structs will ensure that it removes
            // itself from this list before it drops.
            unsafe {
                (**x).trace(&mut state);
            }
        }

        state.trace_stack();

        self.next_limit = (state.to_space.allocated_bytes() * Self::GC_GROWTH_FACTOR) / 10;
        self.block.drop_stack.borrow_mut().clear();
        // Find all hashtables that have not been moved (i.e. They are no longer
        // accessible) and drop them. Otherwise, update the object pointer.
        self.block.lisp_hashtables.borrow_mut().retain_mut(|ptr| {
            let table = unsafe { &**ptr };
            if let Some(fwd) = table.forwarding_ptr() {
                *ptr = fwd.as_ptr().cast::<LispHashTable>();
                true
            } else {
                unsafe { std::ptr::drop_in_place(*ptr as *mut LispHashTable) };
                false
            }
        });

        self.block.objects = state.to_space;
    }
}

impl Deref for Context<'_> {
    type Target = Block<false>;

    fn deref(&self) -> &Self::Target {
        &self.block
    }
}

impl AsRef<Block<false>> for Context<'_> {
    fn as_ref(&self) -> &Block<false> {
        &self.block
    }
}

impl<const CONST: bool> Drop for Block<CONST> {
    // Only one block can exist in a thread at a time. This part of that
    // contract.
    fn drop(&mut self) {
        SINGLETON_CHECK.with(|s| {
            assert!(s.get(), "Context singleton check was overwritten");
            s.set(false);
        });
    }
}

#[cfg(test)]
mod test {
    use rune_core::macros::{list, rebind, root};

    use crate::core::{
        cons::Cons,
        object::{HashTable, ObjectType, Symbol},
    };

    use super::*;
    fn bind_to_mut<'ob>(cx: &'ob mut Context) -> Object<'ob> {
        cx.add("invariant")
    }

    #[test]
    fn test_reborrow() {
        let roots = &RootSet::default();
        let cx = &mut Context::new(roots);
        let obj = rebind!(bind_to_mut(cx));
        _ = "foo".into_obj(cx);
        assert_eq!(obj, "invariant");
    }

    #[test]
    fn test_garbage_collect() {
        let roots = &RootSet::default();
        let cx = &mut Context::new(roots);
        root!(vec, new(Vec), cx);
        cx.garbage_collect(true);
        let cons = list!["foo", 1, false, "end"; cx];
        vec.push(cons);
        cx.garbage_collect(true);
    }

    #[test]
    fn test_move_values() {
        let roots = &RootSet::default();
        let cx = &mut Context::new(roots);
        let int = cx.add(1);
        let float = cx.add(1.5);
        let cons: Object = Cons::new(int, float, cx).into();
        let string = cx.add("string");
        let symbol = cx.add(Symbol::new_uninterned("sym", cx));
        println!("sym: {:?}", symbol.into_raw());
        let mut table = HashTable::default();
        table.insert(symbol, string);
        let _ = table.get(&symbol).unwrap();
        root!(symbol, cx);
        let table = cx.add(table);
        let vec = vec![cons, table];
        let vec = cx.add(vec);
        root!(vec, cx);
        cx.garbage_collect(true);
        let vec = vec.bind(cx);
        let ObjectType::Vec(vec) = vec.untag() else { unreachable!() };
        let ObjectType::Cons(cons) = vec[0].get().untag() else { unreachable!() };
        let ObjectType::HashTable(table) = vec[1].get().untag() else { unreachable!() };
        let key = symbol.bind(cx);
        println!("key: {:?}", key.into_raw());
        let val = table.get(symbol.bind(cx)).unwrap();
        let ObjectType::String(string) = val.untag() else { unreachable!() };
        let ObjectType::Int(int) = cons.car().untag() else { unreachable!() };
        let ObjectType::Float(float) = cons.cdr().untag() else { unreachable!() };
        assert_eq!(string, "string");
        assert_eq!(**float, 1.5);
        assert_eq!(int, 1);
    }
}