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feat: Add jsi::ArrayBuffer::getMutableBuffer() #1733

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adf024d
feat: Add `jsi::ArrayBuffer::getMutableBuffer()`
mrousavy Jul 14, 2025
7e0e8fc
whoops
mrousavy Jul 14, 2025
42af352
Update jsi.h
mrousavy Jul 14, 2025
a355b88
Update JSArrayBuffer.cpp
mrousavy Jul 14, 2025
9bda247
Update JSArrayBuffer.cpp
mrousavy Jul 14, 2025
a1d74fd
reinterpret cast
mrousavy Jul 14, 2025
08e002a
fix: Assignment and add tests
mrousavy Jul 14, 2025
39e8e0e
fix: Handle deref
mrousavy Jul 14, 2025
f1d2072
fix: Fix namespace MutableBuffer
mrousavy Jul 14, 2025
46fc3b6
Update JSArrayBuffer.cpp
mrousavy Jul 14, 2025
e0a1dcb
chore: Format
mrousavy Jul 14, 2025
745a535
Update jsi.h
mrousavy Jul 14, 2025
661a78e
fix: Throw in `HermesABIRuntimeWrapper::getMutableBuffer()`
mrousavy Jul 14, 2025
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5 changes: 5 additions & 0 deletions API/hermes/TracingRuntime.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -879,6 +879,11 @@ jsi::ArrayBuffer TracingRuntime::createArrayBuffer(
throw std::logic_error("Cannot create external ArrayBuffers in trace mode.");
}

std::shared_ptr<jsi::MutableBuffer> TracingRuntime::getMutableBuffer(
const jsi::ArrayBuffer &buffer) {
throw std::logic_error("Cannot get external ArrayBuffers in trace mode.");
}

size_t TracingRuntime::size(const jsi::Array &arr) {
// Array size inquiries read from the length property, which is
// non-configurable and thus cannot have side effects.
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2 changes: 2 additions & 0 deletions API/hermes/TracingRuntime.h
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Expand Up @@ -121,6 +121,8 @@ class TracingRuntime : public jsi::RuntimeDecorator<jsi::Runtime> {
jsi::Array createArray(size_t length) override;
jsi::ArrayBuffer createArrayBuffer(
std::shared_ptr<jsi::MutableBuffer> buffer) override;
std::shared_ptr<jsi::MutableBuffer> getMutableBuffer(
const jsi::ArrayBuffer &buffer) override;

size_t size(const jsi::Array &arr) override;
size_t size(const jsi::ArrayBuffer &buf) override;
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21 changes: 21 additions & 0 deletions API/hermes/hermes.cpp
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Expand Up @@ -725,6 +725,8 @@ class HermesRuntimeImpl final : public HermesRuntime,
jsi::Array createArray(size_t length) override;
jsi::ArrayBuffer createArrayBuffer(
std::shared_ptr<jsi::MutableBuffer> buffer) override;
std::shared_ptr<jsi::MutableBuffer> getMutableBuffer(
const jsi::ArrayBuffer &buffer) override;
size_t size(const jsi::Array &) override;
size_t size(const jsi::ArrayBuffer &) override;
uint8_t *data(const jsi::ArrayBuffer &) override;
Expand Down Expand Up @@ -2410,6 +2412,25 @@ uint8_t *HermesRuntimeImpl::data(const jsi::ArrayBuffer &arr) {
return ab->getDataBlock(runtime_);
}

std::shared_ptr<jsi::MutableBuffer> HermesRuntimeImpl::getMutableBuffer(
const jsi::ArrayBuffer &arr) {
auto buf = arrayBufferHandle(arr);
if (LLVM_UNLIKELY(!buf->attached()))
throw jsi::JSINativeException("ArrayBuffer is detached.");

void *context = nullptr;
auto res = vm::JSArrayBuffer::getExternalDataBlock(runtime_, buf, &context);
if (context == nullptr) {
// ArrayBuffer does not hold a MutableBuffer.
return nullptr;
}
checkStatus(res);

auto mutableBuffer =
reinterpret_cast<std::shared_ptr<jsi::MutableBuffer> *>(context);
return *mutableBuffer;
}

jsi::Value HermesRuntimeImpl::getValueAtIndex(const jsi::Array &arr, size_t i) {
vm::GCScope gcScope(runtime_);
if (LLVM_UNLIKELY(i >= size(arr))) {
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4 changes: 4 additions & 0 deletions API/hermes_abi/HermesABIRuntimeWrapper.cpp
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Expand Up @@ -944,6 +944,10 @@ class HermesABIRuntimeWrapper : public Runtime {
return intoJSIArrayBuffer(vtable_->create_arraybuffer_from_external_data(
abiRt_, new MutableBufferWrapper(std::move(buffer))));
}
std::shared_ptr<MutableBuffer> getMutableBuffer(
const ArrayBuffer &buffer) override {
THROW_UNIMPLEMENTED();
}
size_t size(const Array &arr) override {
return vtable_->get_array_length(abiRt_, toABIArray(arr));
}
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4 changes: 4 additions & 0 deletions API/hermes_sandbox/HermesSandboxRuntime.cpp
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Expand Up @@ -2051,6 +2051,10 @@ class HermesSandboxRuntimeImpl : public facebook::hermes::HermesSandboxRuntime,
std::shared_ptr<MutableBuffer> buffer) override {
THROW_UNIMPLEMENTED();
}
std::shared_ptr<MutableBuffer> getMutableBuffer(
const ArrayBuffer &buffer) override {
THROW_UNIMPLEMENTED();
}
size_t size(const Array &arr) override {
return vt_.get_array_length(this, srt_, toSandboxArray(arr).pointer);
}
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8 changes: 8 additions & 0 deletions API/jsi/jsi/decorator.h
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Expand Up @@ -361,6 +361,10 @@ class RuntimeDecorator : public Base, private jsi::Instrumentation {
std::shared_ptr<MutableBuffer> buffer) override {
return plain_.createArrayBuffer(std::move(buffer));
};
std::shared_ptr<MutableBuffer> getMutableBuffer(
const ArrayBuffer &buffer) override {
return plain_.getMutableBuffer(buffer);
}
size_t size(const Array& a) override {
return plain_.size(a);
};
Expand Down Expand Up @@ -894,6 +898,10 @@ class WithRuntimeDecorator : public RuntimeDecorator<Plain, Base> {
std::shared_ptr<MutableBuffer> buffer) override {
return RD::createArrayBuffer(std::move(buffer));
};
std::shared_ptr<MutableBuffer> getMutableBuffer(
const ArrayBuffer &buffer) override {
return RD::getMutableBuffer(buffer);
}
size_t size(const Array& a) override {
Around around{with_};
return RD::size(a);
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11 changes: 10 additions & 1 deletion API/jsi/jsi/jsi.h
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Original file line number Diff line number Diff line change
Expand Up @@ -499,6 +499,8 @@ class JSI_EXPORT Runtime : public ICast {
virtual Array createArray(size_t length) = 0;
virtual ArrayBuffer createArrayBuffer(
std::shared_ptr<MutableBuffer> buffer) = 0;
virtual std::shared_ptr<MutableBuffer> getMutableBuffer(
const ArrayBuffer &buffer) = 0;
virtual size_t size(const Array&) = 0;
virtual size_t size(const ArrayBuffer&) = 0;
virtual uint8_t* data(const ArrayBuffer&) = 0;
Expand Down Expand Up @@ -1216,7 +1218,7 @@ class JSI_EXPORT ArrayBuffer : public Object {
ArrayBuffer(ArrayBuffer&&) = default;
ArrayBuffer& operator=(ArrayBuffer&&) = default;

ArrayBuffer(Runtime& runtime, std::shared_ptr<MutableBuffer> buffer)
ArrayBuffer(Runtime &runtime, std::shared_ptr<MutableBuffer> buffer)
: ArrayBuffer(runtime.createArrayBuffer(std::move(buffer))) {}

/// \return the size of the ArrayBuffer storage. This is not affected by
Expand All @@ -1234,6 +1236,13 @@ class JSI_EXPORT ArrayBuffer : public Object {
return runtime.data(*this);
}

/// \return the underlying MutableBuffer if this ArrayBuffer
/// was created with one.
/// This returns nullptr if it does not carry a MutableBuffer.
std::shared_ptr<MutableBuffer> getMutableBuffer(Runtime& runtime) const {
return runtime.getMutableBuffer(*this);
}

private:
friend class Object;
friend class Value;
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8 changes: 8 additions & 0 deletions include/hermes/VM/JSArrayBuffer.h
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Expand Up @@ -79,6 +79,14 @@ class JSArrayBuffer final : public JSObject {
void *context,
FinalizeNativeStatePtr finalizePtr);

/// Gets the external data block that this JSArrayBuffer holds if there is
/// any. If this JSArrayBuffer does not hold external data, this returns an
/// error.
static ExecutionStatus getExternalDataBlock(
Runtime &runtime,
Handle<JSArrayBuffer> self,
void **context);

/// Retrieves a pointer to the held buffer.
/// \return A pointer to the buffer owned by this object. This can be null
/// if the ArrayBuffer is empty.
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26 changes: 26 additions & 0 deletions lib/VM/JSArrayBuffer.cpp
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Expand Up @@ -274,5 +274,31 @@ ExecutionStatus JSArrayBuffer::setExternalDataBlock(
return ExecutionStatus::RETURNED;
}

ExecutionStatus JSArrayBuffer::getExternalDataBlock(
Runtime &runtime,
Handle<JSArrayBuffer> self,
void **context) {
assert(self->attached() && "Buffer must be attached");

NamedPropertyDescriptor desc;
bool exists = JSObject::getOwnNamedDescriptor(
self,
runtime,
Predefined::getSymbolID(
Predefined::InternalPropertyArrayBufferExternalFinalizer),
desc);
if (!exists) {
// JSArrayBuffer does not hold an external data block
return ExecutionStatus::EXCEPTION;
}
// Raw pointers below.
NoAllocScope scope(runtime);
NativeState *ns = vmcast<NativeState>(
JSObject::getNamedSlotValueUnsafe(*self, runtime, desc)
.getObject(runtime));
*context = ns->context();
return ExecutionStatus::RETURNED;
}

} // namespace vm
} // namespace hermes
20 changes: 20 additions & 0 deletions unittests/API/APITest.cpp
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Expand Up @@ -199,6 +199,26 @@ TEST_F(HermesRuntimeTestMethodsTest, ExternalArrayBufferTest) {
rt->instrumentation().collectGarbage("");
EXPECT_TRUE(weakBuf.expired());
}

{
auto buf = std::make_shared<FixedBuffer>();
for (uint32_t i = 0; i < buf->arr.size(); i++)
buf->arr[i] = i;
auto arrayBuffer = ArrayBuffer(*rt, buf);
auto roundtrip = eval(
R"#(
(function (buf) {
return buf;
})
)#");
Value result =
roundtrip.asObject(*rt).asFunction(*rt).call(*rt, arrayBuffer);
ArrayBuffer arrayBufferAgain =
result.asObject(*rt).getArrayBuffer(*rt);
std::shared_ptr<MutableBuffer> mutableBuffer =
arrayBufferAgain.getMutableBuffer(*rt);
EXPECT_TRUE(mutableBuffer == buf);
}
}

TEST_F(HermesRuntimeTestMethodsTest, DetachedArrayBuffer) {
Expand Down
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