More bugfixes for weak thread finalization

If a thread was held in an upvalue, then it was possible for the runtime
to think that this thread was dead when it was not and reset it.

To prevent this, the runtime has been changed to mark the *targets* of
all live upvalues as live during finalization, even though they are held
through a weak thread ptr.

This requires two-phase finalization, one to make sure all live upvalues
are actually marked as live, *then* one to reset every dead thread.
Multi-phase finalization has always been on the roadmap so this is not
really a problem.

src/finalizers.rs

@@ -7,6 +7,8 @@ use crate::{thread::ThreadInner, Thread};
 pub struct Finalizers<'gc>(Gc<'gc, RefLock<FinalizersState<'gc>>>);
 
 impl<'gc> Finalizers<'gc> {
+    const THREAD_ERR: &'static str = "thread finalization was missed";
+
     pub(crate) fn new(mc: &Mutation<'gc>) -> Self {
         Finalizers(Gc::new(mc, RefLock::default()))
     }
@@ -15,10 +17,29 @@ impl<'gc> Finalizers<'gc> {
         self.0.borrow_mut(mc).threads.push(Gc::downgrade(ptr));
     }
 
+    /// First stage of two-stage finalization.
+    ///
+    /// This stage can cause resurrection, so the arena must be *fully re-marked* before stage two
+    /// (`Finalizers::finalize`).
+    pub(crate) fn prepare(&self, fc: &Finalization<'gc>) {
+        let state = self.0.borrow();
+        for &ptr in &state.threads {
+            let thread = Thread::from_inner(ptr.upgrade(fc).expect(Self::THREAD_ERR));
+            thread.resurrect_live_upvalues(fc).unwrap();
+        }
+    }
+
+    /// Second stage of two-stage finalization.
+    ///
+    /// Assuming stage one was called (`Finalizers::prepare`) and the arena fully re-marked, this
+    /// method will *not* cause any resurrection.
+    ///
+    /// The arena must *immediately* transition to `CollectionPhase::Collecting` afterwards to not
+    /// miss any finalizers.
     pub(crate) fn finalize(&self, fc: &Finalization<'gc>) {
         let mut state = self.0.borrow_mut(fc);
         state.threads.retain(|&ptr| {
-            let ptr = ptr.upgrade(fc).expect("thread finalization was missed");
+            let ptr = ptr.upgrade(fc).expect(Self::THREAD_ERR);
             if Gc::is_dead(fc, ptr) {
                 Thread::from_inner(ptr).reset(fc).unwrap();
                 false

src/lua.rs

@@ -88,7 +88,6 @@ impl<'gc> ops::Deref for Context<'gc> {
 
 pub struct Lua {
     arena: Arena<Rootable![State<'_>]>,
-    finalized: bool,
 }
 
 impl Default for Lua {
@@ -102,7 +101,6 @@ impl Lua {
     pub fn empty() -> Self {
         Lua {
             arena: Arena::<Rootable![State<'_>]>::new(|mc| State::new(mc)),
-            finalized: false,
         }
     }
 
@@ -156,15 +154,17 @@ impl Lua {
 
     /// Finish the current collection cycle completely, calls `gc_arena::Arena::collect_all()`.
     pub fn gc_collect(&mut self) {
-        if !self.finalized {
+        if self.arena.collection_phase() != CollectionPhase::Collecting {
+            self.arena.mark_all().unwrap().finalize(|fc, root| {
+                root.finalizers.prepare(fc);
+            });
             self.arena.mark_all().unwrap().finalize(|fc, root| {
                 root.finalizers.finalize(fc);
             });
         }
 
         self.arena.collect_all();
         assert!(self.arena.collection_phase() == CollectionPhase::Sleeping);
-        self.finalized = false;
     }
 
     pub fn gc_metrics(&self) -> &Metrics {
@@ -190,20 +190,18 @@ impl Lua {
 
         let r = self.arena.mutate(move |mc, state| f(state.ctx(mc)));
         if self.arena.metrics().allocation_debt() > COLLECTOR_GRANULARITY {
-            if self.finalized {
+            if self.arena.collection_phase() == CollectionPhase::Collecting {
                 self.arena.collect_debt();
-
-                if self.arena.collection_phase() == CollectionPhase::Sleeping {
-                    self.finalized = false;
-                }
             } else {
                 if let Some(marked) = self.arena.mark_debt() {
                     marked.finalize(|fc, root| {
+                        root.finalizers.prepare(fc);
+                    });
+                    self.arena.mark_all().unwrap().finalize(|fc, root| {
                         root.finalizers.finalize(fc);
                     });
                     // Immediately transition to `CollectionPhase::Collecting`.
                     self.arena.mark_all().unwrap().start_collecting();
-                    self.finalized = true;
                 }
             }
         }

src/thread/thread.rs

@@ -4,15 +4,17 @@ use std::{
 };
 
 use allocator_api2::vec;
-use gc_arena::{allocator_api::MetricsAlloc, lock::RefLock, Collect, Gc, GcWeak, Mutation};
+use gc_arena::{
+    allocator_api::MetricsAlloc, lock::RefLock, Collect, Finalization, Gc, GcWeak, Mutation,
+};
 use thiserror::Error;
 
 use crate::{
     closure::{UpValue, UpValueState},
     meta_ops,
     types::{RegisterIndex, VarCount},
     BoxSequence, Callback, Closure, Context, Error, FromMultiValue, Fuel, Function, IntoMultiValue,
-    TypeError, VMError, Value,
+    String, Table, TypeError, UserData, VMError, Value,
 };
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -179,6 +181,27 @@ impl<'gc> Thread<'gc> {
         }
     }
 
+    /// For each open upvalue pointing to this thread, if the upvalue itself is live, then resurrect
+    /// the actual value that it is pointing to.
+    ///
+    /// Because open upvalues keep a *weak* pointer to their parent thread, their target values will
+    /// not be properly marked as live until until they are manually marked with this method.
+    pub(crate) fn resurrect_live_upvalues(
+        self,
+        fc: &Finalization<'gc>,
+    ) -> Result<(), BadThreadMode> {
+        // If this thread is not dead, then none of the held stack values can be dead, so we don't
+        // need to resurrect them.
+        if Gc::is_dead(fc, self.0) {
+            let state = self.0.try_borrow().map_err(|_| BadThreadMode {
+                found: ThreadMode::Running,
+                expected: None,
+            })?;
+            state.resurrect_live_upvalues(fc);
+        }
+        Ok(())
+    }
+
     fn check_mode(
         &self,
         mc: &Mutation<'gc>,
@@ -447,7 +470,7 @@ impl<'gc> ThreadState<'gc> {
         for &upval in &self.open_upvalues[start..] {
             match upval.get() {
                 UpValueState::Open(open_upvalue) => {
-                    assert!(open_upvalue.thread.upgrade(mc).unwrap().as_ptr() == this_ptr);
+                    debug_assert!(open_upvalue.thread.upgrade(mc).unwrap().as_ptr() == this_ptr);
                     upval.set(
                         mc,
                         UpValueState::Closed(self.stack[open_upvalue.stack_index]),
@@ -460,6 +483,31 @@ impl<'gc> ThreadState<'gc> {
         self.open_upvalues.truncate(start);
     }
 
+    fn resurrect_live_upvalues(&self, fc: &Finalization<'gc>) {
+        for &upval in &self.open_upvalues {
+            if !Gc::is_dead(fc, UpValue::into_inner(upval)) {
+                match upval.get() {
+                    UpValueState::Open(open_upvalue) => {
+                        match self.stack[open_upvalue.stack_index] {
+                            Value::String(s) => Gc::resurrect(fc, String::into_inner(s)),
+                            Value::Table(t) => Gc::resurrect(fc, Table::into_inner(t)),
+                            Value::Function(Function::Closure(c)) => {
+                                Gc::resurrect(fc, Closure::into_inner(c))
+                            }
+                            Value::Function(Function::Callback(c)) => {
+                                Gc::resurrect(fc, Callback::into_inner(c))
+                            }
+                            Value::Thread(t) => Gc::resurrect(fc, Thread::into_inner(t)),
+                            Value::UserData(u) => Gc::resurrect(fc, UserData::into_inner(u)),
+                            _ => {}
+                        }
+                    }
+                    UpValueState::Closed(_) => panic!("upvalue is not open"),
+                }
+            }
+        }
+    }
+
     fn reset(&mut self, mc: &Mutation<'gc>) {
         self.close_upvalues(mc, 0);
         assert!(self.open_upvalues.is_empty());

tests/table.rs

@@ -34,7 +34,6 @@ fn test_table_iter() {
         assert!(matches!(pairs[5], (Value::String(s), Value::Integer(3)) if s == "3" ));
 
         for (k, _) in table.iter() {
-            dbg!(k);
             table.set(ctx, k, Value::Nil).unwrap();
         }
 

tests/weak_threads.rs

@@ -31,7 +31,49 @@ fn weak_threads_close() -> Result<(), StaticError> {
         lua.gc_collect();
         let executor = lua.try_enter(|ctx| {
             let closure = Closure::load(ctx, None, format!("assert(closure() == {i})").as_bytes())?;
+            Ok(ctx.stash(Executor::start(ctx, closure.into(), ())))
+        })?;
+        lua.execute::<()>(&executor)?;
+    }
+
+    Ok(())
+}
+
+#[test]
+fn live_upvalues_not_dead() -> Result<(), StaticError> {
+    let mut lua = Lua::core();
+
+    let executor = lua.try_enter(|ctx| {
+        let closure = Closure::load(
+            ctx,
+            None,
+            &br#"
+                local co = coroutine.create(function()
+                    local thread = coroutine.create(function()
+                        local i = 1
+                        while true do
+                            coroutine.yield(i)
+                            i = i + 1
+                        end
+                    end)
+
+                    coroutine.yield(function()
+                        return coroutine.continue(thread)
+                    end)
+                end)
+
+                _, go = coroutine.resume(co)
+            "#[..],
+        )?;
+
+        Ok(ctx.stash(Executor::start(ctx, closure.into(), ())))
+    })?;
+    lua.execute::<()>(&executor)?;
 
+    for i in 1..4 {
+        lua.gc_collect();
+        let executor = lua.try_enter(|ctx| {
+            let closure = Closure::load(ctx, None, format!("assert(go() == {i})").as_bytes())?;
             Ok(ctx.stash(Executor::start(ctx, closure.into(), ())))
         })?;
         lua.execute::<()>(&executor)?;