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// The main idea comes from cpython 3.8's `gcmodule.c` [1].
//
// [1]: https://github.com/python/cpython/blob/v3.8.0/Modules/gcmodule.c
// NOTE: Consider adding generation support if necessary. It won't be too hard.
use crate::cc::CcDummy;
use crate::cc::CcDyn;
use crate::cc::GcClone;
use crate::debug;
use crate::ref_count::RefCount;
use crate::ref_count::SingleThreadRefCount;
use crate::Cc;
use crate::Trace;
use std::cell::Cell;
use std::cell::RefCell;
use std::marker::PhantomData;
use std::mem;
use std::ops::Deref;
use std::pin::Pin;
/// Provides advanced explicit control about where to store [`Cc`](type.Cc.html)
/// objects.
///
/// An [`ObjectSpace`](struct.ObjectSpace.html) provides an alternative place for
/// a collection of [`Cc`](type.Cc.html) objects. Those objects are isolated from
/// the default thread-local space used by objects crated via `Cc::new`.
/// The objects in this space can be collected by
/// [`ObjectSpace::collect_cycles()`](struct.ObjectSpace.html#method.collect_cycles),
/// but not [`collect_thread_cycles`](fn.collect_thread_cycles.html).
///
/// Use [`ObjectSpace::create`](struct.ObjectSpace.html#method.create) to
/// create new objects within the space.
///
/// Objects within a space should not refer to objects in a different space.
/// Failing to do so might cause memory leak.
///
/// # Example
///
/// ```
/// use jrsonnet_gcmodule::{Cc, ObjectSpace, Trace};
/// use std::cell::RefCell;
///
/// let mut space = ObjectSpace::default();
/// assert_eq!(space.count_tracked(), 0);
///
/// {
/// type List = Cc<RefCell<Vec<Box<dyn Trace>>>>;
/// let a: List = space.create(Default::default());
/// let b: List = space.create(Default::default());
/// a.borrow_mut().push(Box::new(b.clone()));
/// b.borrow_mut().push(Box::new(a.clone()));
/// }
///
/// assert_eq!(space.count_tracked(), 2);
/// assert_eq!(space.collect_cycles(), 2);
/// ```
pub struct ObjectSpace {
/// Linked list to the tracked objects.
pub(crate) list: RefCell<Pin<Box<GcHeader>>>,
/// Mark `ObjectSpace` as `!Send` and `!Sync`. This enforces thread-exclusive
/// access to the linked list so methods can use `&self` instead of
/// `&mut self`, together with usage of interior mutability.
_phantom: PhantomData<Cc<()>>,
}
/// This is a private type.
pub trait AbstractObjectSpace: 'static + Sized {
type RefCount: RefCount;
type Header;
/// Insert "header" and "value" to the linked list.
fn insert(&self, header: &mut Self::Header, value: &dyn CcDyn);
/// Remove from linked list.
fn remove(header: &Self::Header);
/// Create a `RefCount` object.
fn new_ref_count(&self, tracked: bool) -> Self::RefCount;
fn empty_header(&self) -> Self::Header;
}
impl AbstractObjectSpace for ObjectSpace {
type RefCount = SingleThreadRefCount;
type Header = GcHeader;
fn insert(&self, header: &mut Self::Header, value: &dyn CcDyn) {
let prev: &GcHeader = &self.list.borrow();
debug_assert!(header.next.get().is_null());
let next = prev.next.get();
header.prev.set(prev);
header.next.set(next);
unsafe {
// safety: The linked list is maintained, and pointers are valid.
(*next).prev.set(header);
// safety: To access vtable pointer. Test by test_gc_header_value.
let fat_ptr: [*mut (); 2] = mem::transmute(value);
header.ccdyn_vptr = fat_ptr[1];
}
prev.next.set(header);
}
#[inline]
fn remove(header: &Self::Header) {
let header: &GcHeader = header;
debug_assert!(!header.next.get().is_null());
debug_assert!(!header.prev.get().is_null());
let next = header.next.get();
let prev = header.prev.get();
// safety: The linked list is maintained. Pointers in it are valid.
unsafe {
(*prev).next.set(next);
(*next).prev.set(prev);
}
header.next.set(std::ptr::null_mut());
}
#[inline]
fn new_ref_count(&self, tracked: bool) -> Self::RefCount {
SingleThreadRefCount::new(tracked)
}
#[inline]
fn empty_header(&self) -> Self::Header {
GcHeader::empty()
}
}
impl Default for ObjectSpace {
/// Constructs an empty [`ObjectSpace`](struct.ObjectSpace.html).
fn default() -> Self {
let header = new_gc_list();
Self {
list: RefCell::new(header),
_phantom: PhantomData,
}
}
}
impl ObjectSpace {
/// Count objects tracked by this [`ObjectSpace`](struct.ObjectSpace.html).
pub fn count_tracked(&self) -> usize {
let list: &GcHeader = &self.list.borrow();
let mut count = 0;
visit_list(list, |_| count += 1);
count
}
/// Collect cyclic garbage tracked by this [`ObjectSpace`](struct.ObjectSpace.html).
/// Return the number of objects collected.
pub fn collect_cycles(&self) -> usize {
let list: &GcHeader = &self.list.borrow();
collect_list(list, ())
}
/// Constructs a new [`Cc<T>`](type.Cc.html) in this
/// [`ObjectSpace`](struct.ObjectSpace.html).
///
/// The returned object should only refer to objects in the same space.
/// Otherwise the collector might fail to collect cycles.
pub fn create<T: Trace>(&self, value: T) -> Cc<T> {
// `&mut self` ensures thread-exclusive access.
Cc::new_in_space(value, self)
}
/// Leak all objects allocated in this space
pub fn leak(&self) {
*self.list.borrow_mut() = new_gc_list();
}
// TODO: Consider implementing "merge" or method to collect multiple spaces
// together, to make it easier to support generational collection.
}
impl Drop for ObjectSpace {
fn drop(&mut self) {
self.collect_cycles();
}
}
pub trait Linked {
fn next(&self) -> *const Self;
fn prev(&self) -> *const Self;
fn set_prev(&self, other: *const Self);
/// Get the trait object to operate on the actual `CcBox`.
fn value(&self) -> &dyn CcDyn;
}
/// Internal metadata used by the cycle collector.
#[cfg_attr(target_pointer_width = "32", repr(C, align(8)))]
#[cfg_attr(not(target_pointer_width = "32"), repr(C))]
pub struct GcHeader {
pub(crate) next: Cell<*const GcHeader>,
pub(crate) prev: Cell<*const GcHeader>,
/// Vtable of (`&CcBox<T> as &dyn CcDyn`)
pub(crate) ccdyn_vptr: *const (),
}
impl Linked for GcHeader {
#[inline]
fn next(&self) -> *const Self {
self.next.get()
}
#[inline]
fn prev(&self) -> *const Self {
self.prev.get()
}
#[inline]
fn set_prev(&self, other: *const Self) {
self.prev.set(other)
}
#[inline]
fn value(&self) -> &dyn CcDyn {
// safety: To build trait object from self and vtable pointer.
// Test by test_gc_header_value_consistency().
unsafe {
let fat_ptr: (*const (), *const ()) =
((self as *const Self).offset(1) as _, self.ccdyn_vptr);
mem::transmute(fat_ptr)
}
}
}
impl GcHeader {
/// Create an empty header.
pub(crate) fn empty() -> Self {
Self {
next: Cell::new(std::ptr::null()),
prev: Cell::new(std::ptr::null()),
ccdyn_vptr: CcDummy::ccdyn_vptr(),
}
}
}
/// Collect cyclic garbage in the current thread created by
/// [`Cc::new`](type.Cc.html#method.new).
/// Return the number of objects collected.
pub fn collect_thread_cycles() -> usize {
debug::log(|| ("collect", "collect_thread_cycles"));
THREAD_OBJECT_SPACE.with(|list| list.collect_cycles())
}
/// Count number of objects tracked by the collector in the current thread
/// created by [`Cc::new`](type.Cc.html#method.new).
/// Return the number of objects tracked.
pub fn count_thread_tracked() -> usize {
THREAD_OBJECT_SPACE.with(|list| list.count_tracked())
}
thread_local!(pub(crate) static THREAD_OBJECT_SPACE: ObjectSpace = ObjectSpace::default());
/// Acquire reference to thread-local global object space
pub fn with_thread_object_space<R>(handler: impl FnOnce(&ObjectSpace) -> R) -> R {
THREAD_OBJECT_SPACE.with(handler)
}
/// Create an empty linked list with a dummy GcHeader.
pub(crate) fn new_gc_list() -> Pin<Box<GcHeader>> {
let pinned = Box::pin(GcHeader::empty());
let header: &GcHeader = pinned.deref();
header.prev.set(header);
header.next.set(header);
pinned
}
/// Scan the specified linked list. Collect cycles.
pub(crate) fn collect_list<L: Linked, K>(list: &L, lock: K) -> usize {
update_refs(list);
subtract_refs(list);
release_unreachable(list, lock)
}
/// Visit the linked list.
pub(crate) fn visit_list<'a, L: Linked>(list: &'a L, mut func: impl FnMut(&'a L)) {
// Skip the first dummy entry.
let mut ptr = list.next();
while ptr as *const _ != list as *const _ {
// The linked list is maintained so the pointer is valid.
let header: &L = unsafe { &*ptr };
ptr = header.next();
func(header);
}
}
const PREV_MASK_COLLECTING: usize = 1;
const PREV_MASK_VISITED: usize = 2;
const PREV_SHIFT: u32 = 2;
/// Temporarily use `GcHeader.prev` as `gc_ref_count`.
/// Idea comes from https://bugs.python.org/issue33597.
fn update_refs<L: Linked>(list: &L) {
visit_list(list, |header| {
let ref_count = header.value().gc_ref_count();
// It's possible that the ref_count becomes 0 in a multi-thread context:
// thread 1> drop()
// thread 1> drop() -> dec_ref()
// thread 2> collect_cycles() # take linked list lock
// thread 1> drop() -> drop_ccbox() # blocked by the linked list lock
// thread 2> observe that ref_count is 0, but T is not dropped yet.
// In such case just ignore the object by not marking it as COLLECTING.
if ref_count > 0 {
let shifted = (ref_count << PREV_SHIFT) | PREV_MASK_COLLECTING;
header.set_prev(shifted as _);
} else {
debug_assert!(header.prev() as usize & PREV_MASK_COLLECTING == 0);
}
});
}
/// Subtract ref counts in `GcHeader.prev` by calling the non-recursive
/// `Trace::trace` on every track objects.
///
/// After this, potential unreachable objects will have ref count down
/// to 0. If vertexes in a connected component _all_ have ref count 0,
/// they are unreachable and can be released.
fn subtract_refs<L: Linked>(list: &L) {
let mut tracer = |header: *const ()| {
// safety: The type is known to be GcHeader.
let header = unsafe { &*(header as *const L) };
if is_collecting(header) {
debug_assert!(
!is_unreachable(header),
"bug: object {} becomes unreachable while trying to dec_ref (is Trace impl correct?)",
debug_name(header)
);
edit_gc_ref_count(header, -1);
}
};
visit_list(list, |header| {
set_visited(header);
header.value().gc_traverse(&mut tracer);
});
}
/// Mark objects as reachable recursively. So ref count 0 means unreachable
/// values. This also removes the COLLECTING flag for reachable objects so
/// unreachable objects all have the COLLECTING flag set.
fn mark_reachable<L: Linked>(list: &L) {
fn revive<L: Linked>(header: *const ()) {
// safety: The type is known to be GcHeader.
let header = unsafe { &*(header as *const L) };
// hasn't visited?
if is_collecting(header) {
unset_collecting(header);
if is_unreachable(header) {
edit_gc_ref_count(header, 1); // revive
}
header.value().gc_traverse(&mut revive::<L>); // revive recursively
}
}
visit_list(list, |header| {
if is_collecting(header) && !is_unreachable(header) {
unset_collecting(header);
header.value().gc_traverse(&mut revive::<L>)
}
});
}
/// Release unreachable objects in the linked list.
fn release_unreachable<L: Linked, K>(list: &L, lock: K) -> usize {
// Mark reachable objects. For example, A refers B. A's gc_ref_count
// is 1 while B's gc_ref_count is 0. In this case B should be revived
// by A's non-zero gc_ref_count.
mark_reachable(list);
let mut count = 0;
// Count unreachable objects. This is an optimization to avoid realloc.
visit_list(list, |header| {
if is_unreachable(header) {
count += 1;
}
});
debug::log(|| ("collect", format!("{} unreachable objects", count)));
// Build a list of what to drop. The collecting steps change the linked list
// so `visit_list` cannot be used.
//
// Here we keep extra references to the `CcBox<T>` to keep them alive. This
// ensures metadata fields like `ref_count` is available.
let mut to_drop: Vec<Box<dyn GcClone>> = Vec::with_capacity(count);
visit_list(list, |header| {
if is_unreachable(header) {
to_drop.push(header.value().gc_clone());
}
});
// Restore "prev" so deleting nodes from the linked list can work.
restore_prev(list);
// Drop the lock so deref() can work, reference counts and the linked list
// can be changed. This is needed because gc_drop_t might change the ref
// counts. This is okay for linked list because objects has been cloned
// to a separate `to_drop` list and the original linked list is no longer
// used.
drop(lock);
// Drop the reference to the list so we don't reuse it.
// drop(list);
#[cfg(feature = "debug")]
{
crate::debug::GC_DROPPING.with(|d| d.set(true));
}
// Drop `T` without releasing memory of `CcBox<T>`. This might trigger some
// recursive drops of other `Cc<T>`. `CcBox<T>` need to stay alive so
// `Cc<T>::drop` can read the ref count metadata.
for value in to_drop.iter() {
value.gc_drop_t();
}
// At this point the only references to the `CcBox<T>`s are inside the
// `to_drop` list. Dropping `to_drop` would release the memory.
for value in to_drop.iter() {
let ref_count = value.gc_ref_count();
assert_eq!(
ref_count, 1,
concat!(
"bug: unexpected ref-count after dropping cycles\n",
"This usually indicates a buggy Trace or Drop implementation."
)
);
}
#[cfg(feature = "debug")]
{
crate::debug::GC_DROPPING.with(|d| d.set(false));
}
count
}
/// Restore `GcHeader.prev` as a pointer used in the linked list.
fn restore_prev<L: Linked>(list: &L) {
let mut prev = list;
visit_list(list, |header| {
header.set_prev(prev);
prev = header;
});
}
fn is_unreachable<L: Linked>(header: &L) -> bool {
let prev = header.prev() as usize;
is_collecting(header) && (prev >> PREV_SHIFT) == 0
}
pub(crate) fn is_collecting<L: Linked>(header: &L) -> bool {
let prev = header.prev() as usize;
(prev & PREV_MASK_COLLECTING) != 0
}
fn set_visited<L: Linked>(header: &L) -> bool {
let prev = header.prev() as usize;
let visited = (prev & PREV_MASK_VISITED) != 0;
debug_assert!(
!visited,
"bug: double visit: {} (is Trace impl correct?)",
debug_name(header)
);
let new_prev = prev | PREV_MASK_VISITED;
header.set_prev(new_prev as _);
visited
}
fn unset_collecting<L: Linked>(header: &L) {
let prev = header.prev() as usize;
let new_prev = (prev & PREV_MASK_COLLECTING) ^ prev;
header.set_prev(new_prev as _);
}
fn edit_gc_ref_count<L: Linked>(header: &L, delta: isize) {
let prev = header.prev() as isize;
let new_prev = prev + (1 << PREV_SHIFT) * delta;
header.set_prev(new_prev as _);
}
#[allow(unused_variables)]
fn debug_name<L: Linked>(header: &L) -> String {
#[cfg(feature = "debug")]
{
return header.value().gc_debug_name();
}
#[cfg(not(feature = "debug"))]
{
"(enable gcmodule \"debug\" feature for debugging)".to_string()
}
}