Add ArrayBuffer::get_backing_store() and new_with_backing_store() (#135)

src/array_buffer.rs

@@ -1,6 +1,10 @@
+use crate::support::long;
 use crate::support::Delete;
 use crate::support::Opaque;
+use crate::support::Shared;
+use crate::support::SharedRef;
 use crate::support::UniqueRef;
+use crate::InIsolate;
 use crate::Isolate;
 use crate::Local;
 use crate::ToLocal;
@@ -8,13 +12,18 @@ use crate::ToLocal;
 extern "C" {
   fn v8__ArrayBuffer__Allocator__NewDefaultAllocator() -> *mut Allocator;
   fn v8__ArrayBuffer__Allocator__DELETE(this: &'static mut Allocator);
-
-  fn v8__ArrayBuffer__New(
+  fn v8__ArrayBuffer__New__byte_length(
     isolate: *mut Isolate,
     byte_length: usize,
   ) -> *mut ArrayBuffer;
+  fn v8__ArrayBuffer__New__backing_store(
+    isolate: *mut Isolate,
+    backing_store: *mut SharedRef<BackingStore>,
+  ) -> *mut ArrayBuffer;
   fn v8__ArrayBuffer__ByteLength(self_: *const ArrayBuffer) -> usize;
-
+  fn v8__ArrayBuffer__GetBackingStore(
+    self_: *const ArrayBuffer,
+  ) -> SharedRef<BackingStore>;
   fn v8__ArrayBuffer__NewBackingStore(
     isolate: *mut Isolate,
     byte_length: usize,
@@ -22,6 +31,15 @@ extern "C" {
   fn v8__BackingStore__ByteLength(self_: &BackingStore) -> usize;
   fn v8__BackingStore__IsShared(self_: &BackingStore) -> bool;
   fn v8__BackingStore__DELETE(self_: &mut BackingStore);
+  fn std__shared_ptr__v8__BackingStore__get(
+    ptr: *const SharedRef<BackingStore>,
+  ) -> *mut BackingStore;
+  fn std__shared_ptr__v8__BackingStore__reset(
+    ptr: *mut SharedRef<BackingStore>,
+  );
+  fn std__shared_ptr__v8__BackingStore__use_count(
+    ptr: *const SharedRef<BackingStore>,
+  ) -> long;
 }
 
 /// A thread-safe allocator that V8 uses to allocate |ArrayBuffer|'s memory.
@@ -105,6 +123,18 @@ impl Delete for BackingStore {
   }
 }
 
+impl Shared for BackingStore {
+  fn deref(ptr: *const SharedRef<Self>) -> *mut Self {
+    unsafe { std__shared_ptr__v8__BackingStore__get(ptr) }
+  }
+  fn reset(ptr: *mut SharedRef<Self>) {
+    unsafe { std__shared_ptr__v8__BackingStore__reset(ptr) }
+  }
+  fn use_count(ptr: *const SharedRef<Self>) -> long {
+    unsafe { std__shared_ptr__v8__BackingStore__use_count(ptr) }
+  }
+}
+
 /// An instance of the built-in ArrayBuffer constructor (ES6 draft 15.13.5).
 #[repr(C)]
 pub struct ArrayBuffer(Opaque);
@@ -119,7 +149,19 @@ impl ArrayBuffer {
     byte_length: usize,
   ) -> Local<'sc, ArrayBuffer> {
     let isolate = scope.isolate();
-    let ptr = unsafe { v8__ArrayBuffer__New(isolate, byte_length) };
+    let ptr =
+      unsafe { v8__ArrayBuffer__New__byte_length(isolate, byte_length) };
+    unsafe { scope.to_local(ptr) }.unwrap()
+  }
+
+  pub fn new_with_backing_store<'sc>(
+    scope: &mut impl ToLocal<'sc>,
+    backing_store: &mut SharedRef<BackingStore>,
+  ) -> Local<'sc, ArrayBuffer> {
+    let isolate = scope.isolate();
+    let ptr = unsafe {
+      v8__ArrayBuffer__New__backing_store(isolate, &mut *backing_store)
+    };
     unsafe { scope.to_local(ptr) }.unwrap()
   }
 
@@ -127,6 +169,9 @@ impl ArrayBuffer {
   pub fn byte_length(&self) -> usize {
     unsafe { v8__ArrayBuffer__ByteLength(self) }
   }
+  pub fn get_backing_store(&self) -> SharedRef<BackingStore> {
+    unsafe { v8__ArrayBuffer__GetBackingStore(self) }
+  }
 
   /// Returns a new standalone BackingStore that is allocated using the array
   /// buffer allocator of the isolate. The result can be later passed to
@@ -135,8 +180,8 @@ impl ArrayBuffer {
   /// If the allocator returns nullptr, then the function may cause GCs in the
   /// given isolate and re-try the allocation. If GCs do not help, then the
   /// function will crash with an out-of-memory error.
-  pub fn new_backing_store<'sc>(
-    scope: &mut impl ToLocal<'sc>,
+  pub fn new_backing_store(
+    scope: &mut impl InIsolate,
     byte_length: usize,
   ) -> UniqueRef<BackingStore> {
     unsafe {

src/binding.cc

@@ -70,7 +70,6 @@ v8::MaybeLocal<v8::Promise> HostImportModuleDynamicallyCallback(
 }
 
 extern "C" {
-
 void v8__V8__SetFlagsFromCommandLine(int* argc, char** argv) {
   v8::V8::SetFlagsFromCommandLine(argc, argv, true);
 }
@@ -325,6 +324,10 @@ v8::BackingStore* v8__ArrayBuffer__NewBackingStore(v8::Isolate* isolate,
   return u.release();
 }
 
+two_pointers_t v8__ArrayBuffer__GetBackingStore(v8::ArrayBuffer& self) {
+  return make_pod<two_pointers_t>(self.GetBackingStore());
+}
+
 size_t v8__BackingStore__ByteLength(v8::BackingStore& self) {
   return self.ByteLength();
 }
@@ -335,6 +338,21 @@ bool v8__BackingStore__IsShared(v8::BackingStore& self) {
 
 void v8__BackingStore__DELETE(v8::BackingStore& self) { delete &self; }
 
+v8::BackingStore* std__shared_ptr__v8__BackingStore__get(
+    const std::shared_ptr<v8::BackingStore>& ptr) {
+  return ptr.get();
+}
+
+void std__shared_ptr__v8__BackingStore__reset(
+    std::shared_ptr<v8::BackingStore>& ptr) {
+  ptr.reset();
+}
+
+long std__shared_ptr__v8__BackingStore__use_count(
+    const std::shared_ptr<v8::BackingStore>& ptr) {
+  return ptr.use_count();
+}
+
 v8::String* v8__String__NewFromUtf8(v8::Isolate* isolate, const char* data,
                                     v8::NewStringType type, int length) {
   return maybe_local_to_ptr(
@@ -419,11 +437,16 @@ void v8__ArrayBuffer__Allocator__DELETE(v8::ArrayBuffer::Allocator& self) {
   delete &self;
 }
 
-v8::ArrayBuffer* v8__ArrayBuffer__New(v8::Isolate* isolate,
-                                      size_t byte_length) {
+v8::ArrayBuffer* v8__ArrayBuffer__New__byte_length(v8::Isolate* isolate,
+                                                   size_t byte_length) {
   return local_to_ptr(v8::ArrayBuffer::New(isolate, byte_length));
 }
 
+v8::ArrayBuffer* v8__ArrayBuffer__New__backing_store(
+    v8::Isolate* isolate, std::shared_ptr<v8::BackingStore>& backing_store) {
+  return local_to_ptr(v8::ArrayBuffer::New(isolate, backing_store));
+}
+
 size_t v8__ArrayBuffer__ByteLength(v8::ArrayBuffer& self) {
   return self.ByteLength();
 }

src/lib.rs

@@ -74,6 +74,7 @@ pub use snapshot::{FunctionCodeHandling, SnapshotCreator, StartupData};
 pub use string::NewStringType;
 pub use string::String;
 pub use support::MaybeBool;
+pub use support::SharedRef;
 pub use support::UniqueRef;
 pub use try_catch::{TryCatch, TryCatchScope};
 pub use uint8_array::Uint8Array;

src/support.h

@@ -2,6 +2,7 @@
 #define SUPPORT_H_
 
 #include <algorithm>
+#include <array>
 #include <cassert>
 #include <memory>
 #include <new>
@@ -11,9 +12,8 @@
 #include "v8/include/v8.h"
 
 // Check assumptions made in binding code.
-// TODO(ry) re-enable the following
-// static_assert(sizeof(bool) == sizeof(uint8_t));
-// static_assert(sizeof(std::unique_ptr<void>) == sizeof(void*));
+static_assert(sizeof(bool) == sizeof(uint8_t), "");
+static_assert(sizeof(std::unique_ptr<void>) == sizeof(void*), "");
 
 namespace support {
 template <class T>
@@ -35,6 +35,44 @@ void construct_in_place(uninit_t<T>& buf, Args... args) {
       construct_in_place_helper<T, Args...>(buf, std::forward<Args>(args)...);
 }
 
+template <class P>
+struct make_pod {
+  template <class V>
+  inline make_pod(V&& value) : pod_(helper<V>(value)) {}
+  template <class V>
+  inline make_pod(const V& value) : pod_(helper<V>(value)) {}
+  inline operator P() { return pod_; }
+
+ private:
+  P pod_;
+
+  template <class V>
+  union helper {
+    static_assert(std::is_pod<P>::value, "type P must a pod type");
+    static_assert(sizeof(V) <= sizeof(P),
+                  "type P must be at least as big as type V");
+    static_assert(alignof(V) <= alignof(P),
+                  "alignment of type P must be compatible with that of type V");
+
+    inline helper(V&& value) : value_(value), padding_() {}
+    inline helper(const V& value) : value_(value), padding_() {}
+    inline ~helper() {}
+
+    inline operator P() {
+      // Do a memcpy here avoid undefined behavior.
+      P result;
+      memcpy(&result, this, sizeof result);
+      return result;
+    }
+
+   private:
+    struct {
+      V value_;
+      char padding_[sizeof(P) - sizeof(V)];
+    };
+  };
+};
+
 // The C-ABI compatible equivalent of V8's Maybe<bool>.
 enum class MaybeBool { JustFalse = 0, JustTrue = 1, Nothing = 2 };
 
@@ -87,6 +125,13 @@ inline static v8::Global<T> ptr_to_global(T* ptr) {
   return global;
 }
 
+// Because, for some reason, Clang complains that `std::aray<void*, 2` is an
+// incomplete type, incompatible with C linkage.
+struct two_pointers_t {
+  void* a;
+  void* b;
+};
+
 }  // namespace support
 
 #endif  // SUPPORT_H_

src/support.rs

@@ -8,6 +8,7 @@ use std::ptr::NonNull;
 
 pub use std::os::raw::c_char as char;
 pub use std::os::raw::c_int as int;
+pub use std::os::raw::c_long as long;
 
 pub type Opaque = [usize; 0];
 
@@ -137,6 +138,61 @@ where
   }
 }
 
+pub trait Shared
+where
+  Self: Sized + 'static,
+{
+  fn deref(shared_ptr: *const SharedRef<Self>) -> *mut Self;
+  fn reset(shared_ptr: *mut SharedRef<Self>);
+  fn use_count(shared_ptr: *const SharedRef<Self>) -> long;
+}
+
+/// Wrapper around a C++ shared_ptr. The shared_ptr is assumed to contain a
+/// value and not be null.
+#[repr(C)]
+#[derive(Debug)]
+pub struct SharedRef<T>([*mut Opaque; 2], PhantomData<T>)
+where
+  T: Shared;
+
+impl<T> SharedRef<T>
+where
+  T: Shared,
+{
+  pub fn use_count(&self) -> long {
+    <T as Shared>::use_count(self)
+  }
+}
+
+impl<T> Deref for SharedRef<T>
+where
+  T: Shared,
+{
+  // TODO: Maybe this should deref to UnsafeCell<T>?
+  type Target = T;
+  fn deref(&self) -> &T {
+    unsafe { &*<T as Shared>::deref(self) }
+  }
+}
+
+impl<T> DerefMut for SharedRef<T>
+where
+  T: Shared,
+{
+  fn deref_mut(&mut self) -> &mut T {
+    unsafe { &mut *<T as Shared>::deref(self) }
+  }
+}
+
+impl<T> Drop for SharedRef<T>
+where
+  T: Shared,
+{
+  fn drop(&mut self) {
+    <T as Shared>::reset(self);
+  }
+}
+
 #[repr(C)]
 #[derive(Debug, PartialEq)]
 pub enum MaybeBool {

tests/test_api.rs

@@ -227,6 +227,61 @@ fn array_buffer() {
   drop(locker);
 }
 
+#[test]
+fn array_buffer_with_shared_backing_store() {
+  setup();
+  let mut params = v8::Isolate::create_params();
+  params.set_array_buffer_allocator(v8::new_default_allocator());
+  let isolate = v8::Isolate::new(params);
+  let mut locker = v8::Locker::new(&isolate);
+  {
+    let mut hs = v8::HandleScope::new(&mut locker);
+    let scope = hs.enter();
+
+    let mut context = v8::Context::new(scope);
+    context.enter();
+
+    let ab1 = v8::ArrayBuffer::new(scope, 42);
+    assert_eq!(42, ab1.byte_length());
+
+    let bs1 = ab1.get_backing_store();
+    assert_eq!(ab1.byte_length(), bs1.byte_length());
+    assert_eq!(2, v8::SharedRef::use_count(&bs1));
+
+    let bs2 = ab1.get_backing_store();
+    assert_eq!(ab1.byte_length(), bs2.byte_length());
+    assert_eq!(3, v8::SharedRef::use_count(&bs1));
+    assert_eq!(3, v8::SharedRef::use_count(&bs2));
+
+    let mut bs3 = ab1.get_backing_store();
+    assert_eq!(ab1.byte_length(), bs3.byte_length());
+    assert_eq!(4, v8::SharedRef::use_count(&bs1));
+    assert_eq!(4, v8::SharedRef::use_count(&bs2));
+    assert_eq!(4, v8::SharedRef::use_count(&bs3));
+
+    drop(bs2);
+    assert_eq!(3, v8::SharedRef::use_count(&bs1));
+    assert_eq!(3, v8::SharedRef::use_count(&bs3));
+
+    drop(bs1);
+    assert_eq!(2, v8::SharedRef::use_count(&bs3));
+
+    let ab2 = v8::ArrayBuffer::new_with_backing_store(scope, &mut bs3);
+    assert_eq!(ab1.byte_length(), ab2.byte_length());
+    assert_eq!(3, v8::SharedRef::use_count(&bs3));
+
+    let bs4 = ab2.get_backing_store();
+    assert_eq!(ab2.byte_length(), bs4.byte_length());
+    assert_eq!(4, v8::SharedRef::use_count(&bs4));
+    assert_eq!(4, v8::SharedRef::use_count(&bs3));
+
+    drop(bs3);
+    assert_eq!(3, v8::SharedRef::use_count(&bs4));
+
+    context.exit();
+  }
+}
+
 fn v8_str<'sc>(
   scope: &mut impl v8::ToLocal<'sc>,
   s: &str,