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::collections::VecDeque;
10use std::ffi::{c_char, c_void};
11use std::marker::PhantomData;
12use std::mem;
13use std::ptr;
14
15/// A trait for JS types that can be registered as roots.
16pub trait RootKind {
17    type Vtable;
18    const VTABLE: Self::Vtable;
19    const KIND: JS::RootKind;
20}
21
22impl RootKind for *mut JSObject {
23    type Vtable = ();
24    const VTABLE: Self::Vtable = ();
25    const KIND: JS::RootKind = JS::RootKind::Object;
26}
27
28impl RootKind for *mut JSFunction {
29    type Vtable = ();
30    const VTABLE: Self::Vtable = ();
31    const KIND: JS::RootKind = JS::RootKind::Object;
32}
33
34impl RootKind for *mut JSString {
35    type Vtable = ();
36    const VTABLE: Self::Vtable = ();
37    const KIND: JS::RootKind = JS::RootKind::String;
38}
39
40impl RootKind for *mut JS::Symbol {
41    type Vtable = ();
42    const VTABLE: Self::Vtable = ();
43    const KIND: JS::RootKind = JS::RootKind::Symbol;
44}
45
46impl RootKind for *mut JS::BigInt {
47    type Vtable = ();
48    const VTABLE: Self::Vtable = ();
49    const KIND: JS::RootKind = JS::RootKind::BigInt;
50}
51
52impl RootKind for *mut JSScript {
53    type Vtable = ();
54    const VTABLE: Self::Vtable = ();
55    const KIND: JS::RootKind = JS::RootKind::Script;
56}
57
58impl RootKind for jsid {
59    type Vtable = ();
60    const VTABLE: Self::Vtable = ();
61    const KIND: JS::RootKind = JS::RootKind::Id;
62}
63
64impl RootKind for JS::Value {
65    type Vtable = ();
66    const VTABLE: Self::Vtable = ();
67    const KIND: JS::RootKind = JS::RootKind::Value;
68}
69
70impl<T: Rootable> RootKind for T {
71    type Vtable = *const RootedVFTable;
72    const VTABLE: Self::Vtable = &<Self as Rootable>::VTABLE;
73    const KIND: JS::RootKind = JS::RootKind::Traceable;
74}
75
76/// A vtable for use in RootedTraceable<T>, which must be present for stack roots using
77/// RootKind::Traceable. The C++ tracing implementation uses a virtual trace function
78/// which is only present for C++ Rooted<T> values that use the Traceable root kind.
79#[repr(C)]
80pub struct RootedVFTable {
81    #[cfg(windows)]
82    pub padding: [usize; 1],
83    #[cfg(not(windows))]
84    pub padding: [usize; 2],
85    pub trace: unsafe extern "C" fn(this: *mut c_void, trc: *mut JSTracer, name: *const c_char),
86}
87
88impl RootedVFTable {
89    #[cfg(windows)]
90    pub const PADDING: [usize; 1] = [0];
91    #[cfg(not(windows))]
92    pub const PADDING: [usize; 2] = [0, 0];
93}
94
95/// Marker trait that allows any type that implements the [trace::Traceable] trait to be used
96/// with the [Rooted] type.
97///
98/// `Rooted<T>` relies on dynamic dispatch in C++ when T uses the Traceable RootKind.
99/// This trait initializes the vtable when creating a Rust instance of the Rooted object.
100pub trait Rootable: crate::trace::Traceable + Sized {
101    const VTABLE: RootedVFTable = RootedVFTable {
102        padding: RootedVFTable::PADDING,
103        trace: <Self as Rootable>::trace,
104    };
105
106    unsafe extern "C" fn trace(this: *mut c_void, trc: *mut JSTracer, _name: *const c_char) {
107        let rooted = this as *mut Rooted<Self>;
108        let rooted = rooted.as_mut().unwrap();
109        <Self as crate::trace::Traceable>::trace(&mut rooted.data, trc);
110    }
111}
112
113impl<T: Rootable> Rootable for Option<T> {}
114impl<T: crate::trace::Traceable> Rootable for Vec<T> {}
115impl<T: crate::trace::Traceable> Rootable for VecDeque<T> {}
116
117// The C++ representation of Rooted<T> inherits from StackRootedBase, which
118// contains the actual pointers that get manipulated. The Rust representation
119// also uses the pattern, which is critical to ensuring that the right pointers
120// to Rooted<T> values are used, since some Rooted<T> values are prefixed with
121// a vtable pointer, and we don't want to store pointers to that vtable where
122// C++ expects a StackRootedBase.
123#[repr(C)]
124#[derive(Debug)]
125pub struct RootedBase {
126    pub stack: *mut *mut RootedBase,
127    pub prev: *mut RootedBase,
128}
129
130// Annoyingly, bindgen can't cope with SM's use of templates, so we have to roll our own.
131#[repr(C)]
132#[cfg_attr(
133    feature = "crown",
134    crown::unrooted_must_root_lint::allow_unrooted_interior
135)]
136pub struct Rooted<T: RootKind> {
137    pub vtable: T::Vtable,
138    pub base: RootedBase,
139    pub data: T,
140}
141
142/// Trait that provides a GC-safe default value for the given type, if one exists.
143pub trait Initialize: Sized {
144    /// Create a default value. If there is no meaningful default possible, returns None.
145    /// SAFETY:
146    ///   The default must not be a value that can be meaningfully garbage collected.
147    unsafe fn initial() -> Option<Self>;
148}
149
150impl<T> Initialize for Option<T> {
151    unsafe fn initial() -> Option<Self> {
152        Some(None)
153    }
154}
155
156/// A trait for types which can place appropriate GC barriers.
157/// * https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Internals/Garbage_collection#Incremental_marking
158/// * https://dxr.mozilla.org/mozilla-central/source/js/src/gc/Barrier.h
159pub trait GCMethods: Initialize {
160    /// Create a default value
161    unsafe fn initial() -> Self {
162        <Self as Initialize>::initial()
163            .expect("Types used with heap GC methods must have a valid default")
164    }
165
166    /// Place a post-write barrier
167    unsafe fn post_barrier(v: *mut Self, prev: Self, next: Self);
168}
169
170impl Initialize for *mut JSObject {
171    unsafe fn initial() -> Option<*mut JSObject> {
172        Some(ptr::null_mut())
173    }
174}
175
176impl GCMethods for *mut JSObject {
177    unsafe fn post_barrier(v: *mut *mut JSObject, prev: *mut JSObject, next: *mut JSObject) {
178        JS::HeapObjectWriteBarriers(v, prev, next);
179    }
180}
181
182impl Initialize for *mut JSFunction {
183    unsafe fn initial() -> Option<*mut JSFunction> {
184        Some(ptr::null_mut())
185    }
186}
187
188impl GCMethods for *mut JSFunction {
189    unsafe fn post_barrier(v: *mut *mut JSFunction, prev: *mut JSFunction, next: *mut JSFunction) {
190        JS::HeapObjectWriteBarriers(
191            mem::transmute(v),
192            mem::transmute(prev),
193            mem::transmute(next),
194        );
195    }
196}
197
198impl Initialize for *mut JSString {
199    unsafe fn initial() -> Option<*mut JSString> {
200        Some(ptr::null_mut())
201    }
202}
203
204impl GCMethods for *mut JSString {
205    unsafe fn post_barrier(v: *mut *mut JSString, prev: *mut JSString, next: *mut JSString) {
206        JS::HeapStringWriteBarriers(v, prev, next);
207    }
208}
209
210impl Initialize for *mut JS::Symbol {
211    unsafe fn initial() -> Option<*mut JS::Symbol> {
212        Some(ptr::null_mut())
213    }
214}
215
216impl GCMethods for *mut JS::Symbol {
217    unsafe fn post_barrier(_: *mut *mut JS::Symbol, _: *mut JS::Symbol, _: *mut JS::Symbol) {}
218}
219
220impl Initialize for *mut JS::BigInt {
221    unsafe fn initial() -> Option<*mut JS::BigInt> {
222        Some(ptr::null_mut())
223    }
224}
225
226impl GCMethods for *mut JS::BigInt {
227    unsafe fn post_barrier(v: *mut *mut JS::BigInt, prev: *mut JS::BigInt, next: *mut JS::BigInt) {
228        JS::HeapBigIntWriteBarriers(v, prev, next);
229    }
230}
231
232impl Initialize for *mut JSScript {
233    unsafe fn initial() -> Option<*mut JSScript> {
234        Some(ptr::null_mut())
235    }
236}
237
238impl GCMethods for *mut JSScript {
239    unsafe fn post_barrier(v: *mut *mut JSScript, prev: *mut JSScript, next: *mut JSScript) {
240        JS::HeapScriptWriteBarriers(v, prev, next);
241    }
242}
243
244impl Initialize for jsid {
245    unsafe fn initial() -> Option<jsid> {
246        Some(VoidId())
247    }
248}
249
250impl GCMethods for jsid {
251    unsafe fn post_barrier(_: *mut jsid, _: jsid, _: jsid) {}
252}
253
254impl Initialize for JS::Value {
255    unsafe fn initial() -> Option<JS::Value> {
256        Some(JS::Value::default())
257    }
258}
259
260impl GCMethods for JS::Value {
261    unsafe fn post_barrier(v: *mut JS::Value, prev: JS::Value, next: JS::Value) {
262        JS::HeapValueWriteBarriers(v, &prev, &next);
263    }
264}
265
266impl Rootable for JS::PropertyDescriptor {}
267
268impl Initialize for JS::PropertyDescriptor {
269    unsafe fn initial() -> Option<JS::PropertyDescriptor> {
270        Some(JS::PropertyDescriptor::default())
271    }
272}
273
274impl GCMethods for JS::PropertyDescriptor {
275    unsafe fn post_barrier(
276        _: *mut JS::PropertyDescriptor,
277        _: JS::PropertyDescriptor,
278        _: JS::PropertyDescriptor,
279    ) {
280    }
281}
282
283/// A fixed-size array of values, for use inside Rooted<>.
284///
285/// https://searchfox.org/mozilla-central/source/js/public/ValueArray.h#31
286pub struct ValueArray<const N: usize> {
287    elements: [JS::Value; N],
288}
289
290impl<const N: usize> ValueArray<N> {
291    pub fn new(elements: [JS::Value; N]) -> Self {
292        Self { elements }
293    }
294
295    pub fn get_ptr(&self) -> *const JS::Value {
296        self.elements.as_ptr()
297    }
298
299    pub unsafe fn get_mut_ptr(&self) -> *mut JS::Value {
300        self.elements.as_ptr() as *mut _
301    }
302}
303
304impl<const N: usize> Rootable for ValueArray<N> {}
305
306impl<const N: usize> Initialize for ValueArray<N> {
307    unsafe fn initial() -> Option<Self> {
308        Some(Self {
309            elements: [<JS::Value as GCMethods>::initial(); N],
310        })
311    }
312}
313
314/// RootedValueArray roots an internal fixed-size array of Values
315pub type RootedValueArray<const N: usize> = Rooted<ValueArray<N>>;
316
317/// Heap values encapsulate GC concerns of an on-heap reference to a JS
318/// object. This means that every reference to a JS object on heap must
319/// be realized through this structure.
320///
321/// # Safety
322/// For garbage collection to work correctly in SpiderMonkey, modifying the
323/// wrapped value triggers a GC barrier, pointing to the underlying object.
324///
325/// This means that after calling the `set()` function with a non-null or
326/// non-undefined value, the `Heap` wrapper *must not* be moved, since doing
327/// so will invalidate the local reference to wrapped value, still held by
328/// SpiderMonkey.
329///
330/// For safe `Heap` construction with value see `Heap::boxed` function.
331#[cfg_attr(feature = "crown", crown::unrooted_must_root_lint::must_root)]
332#[repr(C)]
333#[derive(Debug)]
334pub struct Heap<T: GCMethods + Copy> {
335    pub ptr: UnsafeCell<T>,
336}
337
338impl<T: GCMethods + Copy> Heap<T> {
339    /// This creates a `Box`-wrapped Heap value. Setting a value inside Heap
340    /// object triggers a barrier, referring to the Heap object location,
341    /// hence why it is not safe to construct a temporary Heap value, assign
342    /// a non-null value and move it (e.g. typical object construction).
343    ///
344    /// Using boxed Heap value guarantees that the underlying Heap value will
345    /// not be moved when constructed.
346    #[cfg_attr(feature = "crown", expect(crown::unrooted_must_root))]
347    pub fn boxed(v: T) -> Box<Heap<T>>
348    where
349        Heap<T>: Default,
350    {
351        let boxed = Box::new(Heap::default());
352        boxed.set(v);
353        boxed
354    }
355
356    pub fn set(&self, v: T) {
357        unsafe {
358            let ptr = self.ptr.get();
359            let prev = *ptr;
360            *ptr = v;
361            T::post_barrier(ptr, prev, v);
362        }
363    }
364
365    pub fn get(&self) -> T {
366        unsafe { *self.ptr.get() }
367    }
368
369    pub fn get_unsafe(&self) -> *mut T {
370        self.ptr.get()
371    }
372
373    /// Retrieves a Handle to the underlying value.
374    ///
375    /// # Safety
376    ///
377    /// This is only safe to do on a rooted object (which Heap is not, it needs
378    /// to be additionally rooted), like RootedGuard, so use this only if you
379    /// know what you're doing.
380    ///
381    /// # Notes
382    ///
383    /// Since Heap values need to be informed when a change to underlying
384    /// value is made (e.g. via `get()`), this does not allow to create
385    /// MutableHandle objects, which can bypass this and lead to crashes.
386    pub unsafe fn handle(&self) -> JS::Handle<T> {
387        JS::Handle::from_marked_location(self.ptr.get() as *const _)
388    }
389}
390
391impl<T> Default for Heap<*mut T>
392where
393    *mut T: GCMethods + Copy,
394{
395    fn default() -> Heap<*mut T> {
396        Heap {
397            ptr: UnsafeCell::new(ptr::null_mut()),
398        }
399    }
400}
401
402impl Default for Heap<JS::Value> {
403    fn default() -> Heap<JS::Value> {
404        Heap {
405            ptr: UnsafeCell::new(JS::Value::default()),
406        }
407    }
408}
409
410impl<T: GCMethods + Copy> Drop for Heap<T> {
411    fn drop(&mut self) {
412        unsafe {
413            let ptr = self.ptr.get();
414            T::post_barrier(ptr, *ptr, <T as GCMethods>::initial());
415        }
416    }
417}
418
419impl<T: GCMethods + Copy + PartialEq> PartialEq for Heap<T> {
420    fn eq(&self, other: &Self) -> bool {
421        self.get() == other.get()
422    }
423}
424
425/// Trait for things that can be converted to handles
426/// For any type `T: IntoHandle` we have an implementation of `From<T>`
427/// for `MutableHandle<T::Target>`. This is a way round the orphan
428/// rule.
429pub trait IntoHandle {
430    /// The type of the handle
431    type Target;
432
433    /// Convert this object to a handle.
434    fn into_handle(self) -> JS::Handle<Self::Target>;
435}
436
437pub trait IntoMutableHandle: IntoHandle {
438    /// Convert this object to a mutable handle.
439    fn into_handle_mut(self) -> JS::MutableHandle<Self::Target>;
440}
441
442impl<T: IntoHandle> From<T> for JS::Handle<T::Target> {
443    fn from(value: T) -> Self {
444        value.into_handle()
445    }
446}
447
448impl<T: IntoMutableHandle> From<T> for JS::MutableHandle<T::Target> {
449    fn from(value: T) -> Self {
450        value.into_handle_mut()
451    }
452}
453
454/// Methods for a CustomAutoRooter
455#[repr(C)]
456pub struct CustomAutoRooterVFTable {
457    #[cfg(windows)]
458    pub padding: [usize; 1],
459    #[cfg(not(windows))]
460    pub padding: [usize; 2],
461    pub trace: unsafe extern "C" fn(this: *mut c_void, trc: *mut JSTracer),
462}
463
464impl CustomAutoRooterVFTable {
465    #[cfg(windows)]
466    pub const PADDING: [usize; 1] = [0];
467    #[cfg(not(windows))]
468    pub const PADDING: [usize; 2] = [0, 0];
469}
470
471#[repr(C)]
472#[derive(Copy, Clone)]
473pub struct StackGCVector<T, AllocPolicy = js::TempAllocPolicy>(PhantomData<(T, AllocPolicy)>, u8);
mozjs_sys/jsgc.rs

mozjs_sys/
jsgc.rs
