mozjs_sys/
jsgc.rs

1/* This Source Code Form is subject to the terms of the Mozilla Public
2 * License, v. 2.0. If a copy of the MPL was not distributed with this
3 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
4
5use crate::jsapi::{js, JS};
6use crate::jsapi::{jsid, JSFunction, JSObject, JSScript, JSString, JSTracer};
7use crate::jsid::VoidId;
8use std::cell::UnsafeCell;
9use std::ffi::{c_char, c_void};
10use std::marker::PhantomData;
11use std::mem;
12use std::ptr;
13
14/// A trait for JS types that can be registered as roots.
15pub trait RootKind {
16    type Vtable;
17    const VTABLE: Self::Vtable;
18    const KIND: JS::RootKind;
19}
20
21impl RootKind for *mut JSObject {
22    type Vtable = ();
23    const VTABLE: Self::Vtable = ();
24    const KIND: JS::RootKind = JS::RootKind::Object;
25}
26
27impl RootKind for *mut JSFunction {
28    type Vtable = ();
29    const VTABLE: Self::Vtable = ();
30    const KIND: JS::RootKind = JS::RootKind::Object;
31}
32
33impl RootKind for *mut JSString {
34    type Vtable = ();
35    const VTABLE: Self::Vtable = ();
36    const KIND: JS::RootKind = JS::RootKind::String;
37}
38
39impl RootKind for *mut JS::Symbol {
40    type Vtable = ();
41    const VTABLE: Self::Vtable = ();
42    const KIND: JS::RootKind = JS::RootKind::Symbol;
43}
44
45impl RootKind for *mut JS::BigInt {
46    type Vtable = ();
47    const VTABLE: Self::Vtable = ();
48    const KIND: JS::RootKind = JS::RootKind::BigInt;
49}
50
51impl RootKind for *mut JSScript {
52    type Vtable = ();
53    const VTABLE: Self::Vtable = ();
54    const KIND: JS::RootKind = JS::RootKind::Script;
55}
56
57impl RootKind for jsid {
58    type Vtable = ();
59    const VTABLE: Self::Vtable = ();
60    const KIND: JS::RootKind = JS::RootKind::Id;
61}
62
63impl RootKind for JS::Value {
64    type Vtable = ();
65    const VTABLE: Self::Vtable = ();
66    const KIND: JS::RootKind = JS::RootKind::Value;
67}
68
69impl<T: Rootable> RootKind for T {
70    type Vtable = *const RootedVFTable;
71    const VTABLE: Self::Vtable = &<Self as Rootable>::VTABLE;
72    const KIND: JS::RootKind = JS::RootKind::Traceable;
73}
74
75/// A vtable for use in RootedTraceable<T>, which must be present for stack roots using
76/// RootKind::Traceable. The C++ tracing implementation uses a virtual trace function
77/// which is only present for C++ Rooted<T> values that use the Traceable root kind.
78#[repr(C)]
79pub struct RootedVFTable {
80    #[cfg(windows)]
81    pub padding: [usize; 1],
82    #[cfg(not(windows))]
83    pub padding: [usize; 2],
84    pub trace: unsafe extern "C" fn(this: *mut c_void, trc: *mut JSTracer, name: *const c_char),
85}
86
87impl RootedVFTable {
88    #[cfg(windows)]
89    pub const PADDING: [usize; 1] = [0];
90    #[cfg(not(windows))]
91    pub const PADDING: [usize; 2] = [0, 0];
92}
93
94/// Marker trait that allows any type that implements the [trace::Traceable] trait to be used
95/// with the [Rooted] type.
96///
97/// `Rooted<T>` relies on dynamic dispatch in C++ when T uses the Traceable RootKind.
98/// This trait initializes the vtable when creating a Rust instance of the Rooted object.
99pub trait Rootable: crate::trace::Traceable + Sized {
100    const VTABLE: RootedVFTable = RootedVFTable {
101        padding: RootedVFTable::PADDING,
102        trace: <Self as Rootable>::trace,
103    };
104
105    unsafe extern "C" fn trace(this: *mut c_void, trc: *mut JSTracer, _name: *const c_char) {
106        let rooted = this as *mut Rooted<Self>;
107        let rooted = rooted.as_mut().unwrap();
108        <Self as crate::trace::Traceable>::trace(&mut rooted.data, trc);
109    }
110}
111
112impl<T: Rootable> Rootable for Option<T> {}
113
114// The C++ representation of Rooted<T> inherits from StackRootedBase, which
115// contains the actual pointers that get manipulated. The Rust representation
116// also uses the pattern, which is critical to ensuring that the right pointers
117// to Rooted<T> values are used, since some Rooted<T> values are prefixed with
118// a vtable pointer, and we don't want to store pointers to that vtable where
119// C++ expects a StackRootedBase.
120#[repr(C)]
121#[derive(Debug)]
122pub struct RootedBase {
123    pub stack: *mut *mut RootedBase,
124    pub prev: *mut RootedBase,
125}
126
127// Annoyingly, bindgen can't cope with SM's use of templates, so we have to roll our own.
128#[repr(C)]
129#[cfg_attr(
130    feature = "crown",
131    crown::unrooted_must_root_lint::allow_unrooted_interior
132)]
133pub struct Rooted<T: RootKind> {
134    pub vtable: T::Vtable,
135    pub base: RootedBase,
136    pub data: T,
137}
138
139/// Trait that provides a GC-safe default value for the given type, if one exists.
140pub trait Initialize: Sized {
141    /// Create a default value. If there is no meaningful default possible, returns None.
142    /// SAFETY:
143    ///   The default must not be a value that can be meaningfully garbage collected.
144    unsafe fn initial() -> Option<Self>;
145}
146
147impl<T> Initialize for Option<T> {
148    unsafe fn initial() -> Option<Self> {
149        Some(None)
150    }
151}
152
153/// A trait for types which can place appropriate GC barriers.
154/// * https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Internals/Garbage_collection#Incremental_marking
155/// * https://dxr.mozilla.org/mozilla-central/source/js/src/gc/Barrier.h
156pub trait GCMethods: Initialize {
157    /// Create a default value
158    unsafe fn initial() -> Self {
159        <Self as Initialize>::initial()
160            .expect("Types used with heap GC methods must have a valid default")
161    }
162
163    /// Place a post-write barrier
164    unsafe fn post_barrier(v: *mut Self, prev: Self, next: Self);
165}
166
167impl Initialize for *mut JSObject {
168    unsafe fn initial() -> Option<*mut JSObject> {
169        Some(ptr::null_mut())
170    }
171}
172
173impl GCMethods for *mut JSObject {
174    unsafe fn post_barrier(v: *mut *mut JSObject, prev: *mut JSObject, next: *mut JSObject) {
175        JS::HeapObjectWriteBarriers(v, prev, next);
176    }
177}
178
179impl Initialize for *mut JSFunction {
180    unsafe fn initial() -> Option<*mut JSFunction> {
181        Some(ptr::null_mut())
182    }
183}
184
185impl GCMethods for *mut JSFunction {
186    unsafe fn post_barrier(v: *mut *mut JSFunction, prev: *mut JSFunction, next: *mut JSFunction) {
187        JS::HeapObjectWriteBarriers(
188            mem::transmute(v),
189            mem::transmute(prev),
190            mem::transmute(next),
191        );
192    }
193}
194
195impl Initialize for *mut JSString {
196    unsafe fn initial() -> Option<*mut JSString> {
197        Some(ptr::null_mut())
198    }
199}
200
201impl GCMethods for *mut JSString {
202    unsafe fn post_barrier(v: *mut *mut JSString, prev: *mut JSString, next: *mut JSString) {
203        JS::HeapStringWriteBarriers(v, prev, next);
204    }
205}
206
207impl Initialize for *mut JS::Symbol {
208    unsafe fn initial() -> Option<*mut JS::Symbol> {
209        Some(ptr::null_mut())
210    }
211}
212
213impl GCMethods for *mut JS::Symbol {
214    unsafe fn post_barrier(_: *mut *mut JS::Symbol, _: *mut JS::Symbol, _: *mut JS::Symbol) {}
215}
216
217impl Initialize for *mut JS::BigInt {
218    unsafe fn initial() -> Option<*mut JS::BigInt> {
219        Some(ptr::null_mut())
220    }
221}
222
223impl GCMethods for *mut JS::BigInt {
224    unsafe fn post_barrier(v: *mut *mut JS::BigInt, prev: *mut JS::BigInt, next: *mut JS::BigInt) {
225        JS::HeapBigIntWriteBarriers(v, prev, next);
226    }
227}
228
229impl Initialize for *mut JSScript {
230    unsafe fn initial() -> Option<*mut JSScript> {
231        Some(ptr::null_mut())
232    }
233}
234
235impl GCMethods for *mut JSScript {
236    unsafe fn post_barrier(v: *mut *mut JSScript, prev: *mut JSScript, next: *mut JSScript) {
237        JS::HeapScriptWriteBarriers(v, prev, next);
238    }
239}
240
241impl Initialize for jsid {
242    unsafe fn initial() -> Option<jsid> {
243        Some(VoidId())
244    }
245}
246
247impl GCMethods for jsid {
248    unsafe fn post_barrier(_: *mut jsid, _: jsid, _: jsid) {}
249}
250
251impl Initialize for JS::Value {
252    unsafe fn initial() -> Option<JS::Value> {
253        Some(JS::Value::default())
254    }
255}
256
257impl GCMethods for JS::Value {
258    unsafe fn post_barrier(v: *mut JS::Value, prev: JS::Value, next: JS::Value) {
259        JS::HeapValueWriteBarriers(v, &prev, &next);
260    }
261}
262
263impl Rootable for JS::PropertyDescriptor {}
264
265impl Initialize for JS::PropertyDescriptor {
266    unsafe fn initial() -> Option<JS::PropertyDescriptor> {
267        Some(JS::PropertyDescriptor::default())
268    }
269}
270
271impl GCMethods for JS::PropertyDescriptor {
272    unsafe fn post_barrier(
273        _: *mut JS::PropertyDescriptor,
274        _: JS::PropertyDescriptor,
275        _: JS::PropertyDescriptor,
276    ) {
277    }
278}
279
280/// A fixed-size array of values, for use inside Rooted<>.
281///
282/// https://searchfox.org/mozilla-central/source/js/public/ValueArray.h#31
283pub struct ValueArray<const N: usize> {
284    elements: [JS::Value; N],
285}
286
287impl<const N: usize> ValueArray<N> {
288    pub fn new(elements: [JS::Value; N]) -> Self {
289        Self { elements }
290    }
291
292    pub fn get_ptr(&self) -> *const JS::Value {
293        self.elements.as_ptr()
294    }
295
296    pub unsafe fn get_mut_ptr(&self) -> *mut JS::Value {
297        self.elements.as_ptr() as *mut _
298    }
299}
300
301impl<const N: usize> Rootable for ValueArray<N> {}
302
303impl<const N: usize> Initialize for ValueArray<N> {
304    unsafe fn initial() -> Option<Self> {
305        Some(Self {
306            elements: [<JS::Value as GCMethods>::initial(); N],
307        })
308    }
309}
310
311/// RootedValueArray roots an internal fixed-size array of Values
312pub type RootedValueArray<const N: usize> = Rooted<ValueArray<N>>;
313
314/// Heap values encapsulate GC concerns of an on-heap reference to a JS
315/// object. This means that every reference to a JS object on heap must
316/// be realized through this structure.
317///
318/// # Safety
319/// For garbage collection to work correctly in SpiderMonkey, modifying the
320/// wrapped value triggers a GC barrier, pointing to the underlying object.
321///
322/// This means that after calling the `set()` function with a non-null or
323/// non-undefined value, the `Heap` wrapper *must not* be moved, since doing
324/// so will invalidate the local reference to wrapped value, still held by
325/// SpiderMonkey.
326///
327/// For safe `Heap` construction with value see `Heap::boxed` function.
328#[cfg_attr(feature = "crown", crown::unrooted_must_root_lint::must_root)]
329#[repr(C)]
330#[derive(Debug)]
331pub struct Heap<T: GCMethods + Copy> {
332    pub ptr: UnsafeCell<T>,
333}
334
335impl<T: GCMethods + Copy> Heap<T> {
336    /// This creates a `Box`-wrapped Heap value. Setting a value inside Heap
337    /// object triggers a barrier, referring to the Heap object location,
338    /// hence why it is not safe to construct a temporary Heap value, assign
339    /// a non-null value and move it (e.g. typical object construction).
340    ///
341    /// Using boxed Heap value guarantees that the underlying Heap value will
342    /// not be moved when constructed.
343    pub fn boxed(v: T) -> Box<Heap<T>>
344    where
345        Heap<T>: Default,
346    {
347        let boxed = Box::new(Heap::default());
348        boxed.set(v);
349        boxed
350    }
351
352    pub fn set(&self, v: T) {
353        unsafe {
354            let ptr = self.ptr.get();
355            let prev = *ptr;
356            *ptr = v;
357            T::post_barrier(ptr, prev, v);
358        }
359    }
360
361    pub fn get(&self) -> T {
362        unsafe { *self.ptr.get() }
363    }
364
365    pub fn get_unsafe(&self) -> *mut T {
366        self.ptr.get()
367    }
368
369    /// Retrieves a Handle to the underlying value.
370    ///
371    /// # Safety
372    ///
373    /// This is only safe to do on a rooted object (which Heap is not, it needs
374    /// to be additionally rooted), like RootedGuard, so use this only if you
375    /// know what you're doing.
376    ///
377    /// # Notes
378    ///
379    /// Since Heap values need to be informed when a change to underlying
380    /// value is made (e.g. via `get()`), this does not allow to create
381    /// MutableHandle objects, which can bypass this and lead to crashes.
382    pub unsafe fn handle(&self) -> JS::Handle<T> {
383        JS::Handle::from_marked_location(self.ptr.get() as *const _)
384    }
385}
386
387impl<T> Default for Heap<*mut T>
388where
389    *mut T: GCMethods + Copy,
390{
391    fn default() -> Heap<*mut T> {
392        Heap {
393            ptr: UnsafeCell::new(ptr::null_mut()),
394        }
395    }
396}
397
398impl Default for Heap<JS::Value> {
399    fn default() -> Heap<JS::Value> {
400        Heap {
401            ptr: UnsafeCell::new(JS::Value::default()),
402        }
403    }
404}
405
406impl<T: GCMethods + Copy> Drop for Heap<T> {
407    fn drop(&mut self) {
408        unsafe {
409            let ptr = self.ptr.get();
410            T::post_barrier(ptr, *ptr, <T as GCMethods>::initial());
411        }
412    }
413}
414
415impl<T: GCMethods + Copy + PartialEq> PartialEq for Heap<T> {
416    fn eq(&self, other: &Self) -> bool {
417        self.get() == other.get()
418    }
419}
420
421/// Trait for things that can be converted to handles
422/// For any type `T: IntoHandle` we have an implementation of `From<T>`
423/// for `MutableHandle<T::Target>`. This is a way round the orphan
424/// rule.
425pub trait IntoHandle {
426    /// The type of the handle
427    type Target;
428
429    /// Convert this object to a handle.
430    fn into_handle(self) -> JS::Handle<Self::Target>;
431}
432
433pub trait IntoMutableHandle: IntoHandle {
434    /// Convert this object to a mutable handle.
435    fn into_handle_mut(self) -> JS::MutableHandle<Self::Target>;
436}
437
438impl<T: IntoHandle> From<T> for JS::Handle<T::Target> {
439    fn from(value: T) -> Self {
440        value.into_handle()
441    }
442}
443
444impl<T: IntoMutableHandle> From<T> for JS::MutableHandle<T::Target> {
445    fn from(value: T) -> Self {
446        value.into_handle_mut()
447    }
448}
449
450/// Methods for a CustomAutoRooter
451#[repr(C)]
452pub struct CustomAutoRooterVFTable {
453    #[cfg(windows)]
454    pub padding: [usize; 1],
455    #[cfg(not(windows))]
456    pub padding: [usize; 2],
457    pub trace: unsafe extern "C" fn(this: *mut c_void, trc: *mut JSTracer),
458}
459
460impl CustomAutoRooterVFTable {
461    #[cfg(windows)]
462    pub const PADDING: [usize; 1] = [0];
463    #[cfg(not(windows))]
464    pub const PADDING: [usize; 2] = [0, 0];
465}
466
467#[repr(C)]
468#[derive(Copy, Clone)]
469pub struct StackGCVector<T, AllocPolicy = js::TempAllocPolicy>(PhantomData<(T, AllocPolicy)>, u8);
mozjs_sys/jsgc.rs

mozjs_sys/
jsgc.rs
