lua_vm/

trace_impls.rs

//! Phase-D `Trace` implementations for GC-rooted types defined in this
//! crate. Types in `lua-types` (LuaValue, LuaString, UpVal) have their
//! Trace impls in `lua-types/src/trace_impls.rs` because of Rust's orphan
//! rule.
//!
//! Each impl below is a `todo!("phase-d: trace X")` stub. The
//! panic-driven mega-loop surfaces each one when a runtime path triggers
//! `Heap::full_collect`. Each agent works on ONE type — no family
//! expansion (Trace impls have subtle invariants).
//!
//! Implementation guidance for agents:
//!   1. Read the type definition; enumerate every field
//!   2. For every `Gc<T>`, `GcRef<T>`, or container (Vec/Option/HashMap)
//!      thereof, call `m.mark(field)` or `field.trace(m)` appropriately
//!   3. Skip non-GC fields (primitives, `String`, `Vec<u8>`)
//!   4. Skip "intentionally not traced" fields (weak refs)
//!   5. Reference `reference/lua-5.4.7/src/lgc.c`'s `reallymarkobject`

use lua_gc::{Marker, Trace};
use crate::state::{LuaState, GlobalState};
use crate::string::{LuaStringImpl, LuaUserDataImpl};
use lua_types::{LuaClosure, LuaValue};

/// Phase-B internal richer LuaString. The byte buffer is a Rust `Rc<[u8]>`
/// (not GC-managed); no fields to mark.
impl Trace for LuaStringImpl {
    fn trace(&self, _m: &mut Marker) {}
}

/// Phase-B internal userdata. Both `metatable` and `uv` are currently
/// `Option<()>` / `Vec<()>` stubs — no GC edges to walk yet. Becomes
/// real when userdata machinery lands post-D-1.
impl Trace for LuaUserDataImpl {
    fn trace(&self, _m: &mut Marker) {}
}

impl Trace for LuaState {
    fn trace(&self, m: &mut Marker) {
        // and the open-upvalue list. Trace frame-bounded live ranges instead of
        // every slot up to `ci.top`: that reserved tail can contain stale values
        // from previous calls. Lua locals that sit above the transient `top` are
        // added explicitly from debug local metadata.
        let trace_debug_locals = self.cached_thread_id == self.global.borrow().current_thread_id;
        let mut ci_idx = Some(self.ci);
        while let Some(idx) = ci_idx {
            let ci = &self.call_info[idx.as_usize()];
            let start = ci.func.0 as usize;
            let end_idx = if idx == self.ci {
                self.top.0 as usize
            } else if let Some(next) = ci.next {
                self.call_info[next.as_usize()].func.0 as usize
            } else {
                self.top.0 as usize
            };
            let end = end_idx.min(self.stack.len());
            if start < end {
                for slot in &self.stack[start..end] {
                    slot.val.trace(m);
                }
            }
            if trace_debug_locals && ci.is_lua() {
                if let Some(slot) = self.stack.get(ci.func.0 as usize) {
                    if let LuaValue::Function(LuaClosure::Lua(cl)) = &slot.val {
                        let pc = ci.saved_pc().saturating_sub(1) as i32;
                        let base = ci.func.0 as usize + 1;
                        let mut n = 1i32;
                        while crate::func::get_local_name(&cl.proto, n, pc).is_some() {
                            let idx = base + (n as usize - 1);
                            if let Some(local_slot) = self.stack.get(idx) {
                                local_slot.val.trace(m);
                            }
                            n += 1;
                        }
                    }
                }
            }
            ci_idx = ci.previous;
        }

        for uv in self.openupval.iter() {
            uv.trace(m);
        }

        // PORT NOTE: `global` (Rc<RefCell<GlobalState>>) is reached from the
        // heap's root via GlobalState::trace; tracing it from each thread
        // would re-enter the root and is explicitly excluded.
        // PORT NOTE: `call_info` entries carry pc offsets and stack indices
        // but no direct GcRef fields. The active closure is reached through
        // the stack slot at `ci.func`, already covered by the stack walk.
        // PORT NOTE: `tbclist` holds StackIdx values only; the to-be-closed
        // objects themselves live on the stack and are traced there.
    }
}

impl Trace for GlobalState {
    fn trace(&self, m: &mut Marker) {
        // per-type metatables, and pending finalizers. We expand the set to
        // include preallocated short strings (memerrmsg, tmname[]) and the
        // open-upvalue thread list, both of which the panic-driven Phase-D
        // mega-loop expects to see at the root.

        self.l_registry.trace(m);

        // Values held by Rust-side embedding handles are rooted outside the
        // Lua registry table so handle Drop can unroot without touching the
        // Lua stack/API. They are still ordinary GC roots during marking.
        for value in self.external_roots.iter_values() {
            value.trace(m);
        }

        // PORT NOTE (phase-b-reconcile): The lua-types LuaTable placeholder is
        // storage-less, so `globals` and `loaded` cannot live inside the registry
        // table (see `init_registry`). They are kept as direct GlobalState fields
        // and must be traced explicitly as roots; once the placeholder reconciles
        // with vm::LuaTable, these become reachable via `l_registry` and the two
        // lines below disappear.
        self.globals.trace(m);
        self.loaded.trace(m);

        if let Some(t) = &self.mainthread {
            t.trace(m);
        }

        self.main_thread_value.trace(m);

        if self.current_thread_id != self.main_thread_id {
            if let Some(entry) = self.threads.get(&self.current_thread_id) {
                entry.value.trace(m);
            }
        }

        // Registered coroutines are not roots by registration alone. The
        // post-mark hook traces stacks only for thread handles that were
        // reached from a real root, matching Lua's collectable coroutine
        // semantics.

        for slot in self.mt.iter() {
            if let Some(t) = slot {
                t.trace(m);
            }
        }

        for s in self.tmname.iter() {
            s.trace(m);
        }

        self.memerrmsg.trace(m);

        for th in self.twups.iter() {
            th.trace(m);
        }

        // The short-string intern cache holds `GcRef<LuaString>` values that
        // callers (parser, stdlib) reuse by pointer-equality across
        // `intern_str` calls. C-Lua treats this as a weak table cleared during
        // the atomic weak-table pass (`clearbykeys`); we have no incremental
        // weak-sweep yet, so leaving these untraced would leave the HashMap
        // with dangling `Gc<LuaString>` entries after the very next collect.
        // Trace them as strong roots until the weak-sweep machinery lands.
        for s in self.interned_lt.values() {
            s.trace(m);
        }
        for row in self.strcache.iter() {
            for s in row.iter() {
                s.trace(m);
            }
        }

        // Do not trace `gc_tracked_long_strings` here. That vector is memory
        // accounting metadata, not an owning root. Lua C treats strings as
        // non-weak only when they are reached through a surviving table entry
        // (`iscleared` marks them during weak cleanup); our post-mark weak pass
        // mirrors that by marking string keys/values returned from
        // `prune_weak_dead`. Rooting the whole accounting list would keep dead
        // long strings alive and break gc.lua's weak-string-key checks.

        // Pending finalizers are NOT traced here — that's what lets the mark
        // phase distinguish "still reachable from the user program" from
        // "only kept alive by the finalizer registry". `collect_via_heap`'s
        // post-mark hook checks each entry against the visited set; an
        // unvisited entry is moved to `to_be_finalized` and explicitly
        // marked there so it survives the sweep.
        //
        // `to_be_finalized` IS traced as a strong root: tables in this list
        // are awaiting their `__gc` call but are otherwise dead, and the
        // table (plus its descendants) must survive long enough for the
        // finalizer to run.
        for t in self.to_be_finalized.iter() {
            t.trace(m);
        }

        // Trace suspended parent stacks. When a coroutine is running, any
        // parent threads are suspended and their stacks are not reachable from
        // `threads` (which only holds coroutines, not the main thread). Before
        // `aux_resume` resumes a coroutine it pushes a snapshot of the parent's
        // live stack onto `suspended_parent_stacks` so those GC-managed values
        // remain marked during collections triggered from inside the coroutine.
        for stack_snapshot in self.suspended_parent_stacks.iter() {
            for v in stack_snapshot.iter() {
                v.trace(m);
            }
        }
        for upval_snapshot in self.suspended_parent_open_upvals.iter() {
            for uv in upval_snapshot.iter() {
                uv.trace(m);
            }
        }

        // PORT NOTE: `strt` (the internal LuaStringImpl intern table) is a
        // weak table in C; entries are cleared during the atomic weak-table
        // pass (`clearbykeys`), not marked as roots. The current port has no
        // incremental weak-sweep, but `strt` is keyed by byte-content rather
        // than by `Gc` identity, so a dangling entry there is silently
        // recreated by the next `intern_str` — no UAF, unlike `interned_lt`.
        // PORT NOTE: `fixedgc` holds objects pre-marked fixed/black at
        // allocation (`luaC_fix`); the mark phase never re-visits them, and
        // `dyn Collectable` does not implement `Trace` here.
        // PORT NOTE: `allgc`, `finobj`, `gray`, `grayagain`, `tobefnz`,
        // `weak`, `ephemeron`, `allweak` are GC bookkeeping lists owned by
        // `heap` — they are the universe of allocated objects, not roots.
    }
}

// ──────────────────────────────────────────────────────────────────────────────
// PORT STATUS
//   source:        n/a (GC Trace impls bridging lua-vm and lua-gc)
//   target_crate:  lua-vm
//   confidence:    high
//   todos:         0
//   port_notes:    0
//   unsafe_blocks: 0
//   notes:         Implements lua_gc::Trace for LuaState + GlobalState. C does this via
//                  hand-written mark routines in lgc.c; we use a trait dispatch.
// ──────────────────────────────────────────────────────────────────────────────