[api] Add a SharedArrayBuffersEnabled callback.

- Adds a SharedArrayBuffersEnabled callback and uses it to
  enable/disable SABs per context. The feature flag is used
  if no callback is registered.

Bug: chromium:923807
Change-Id: I4d3472fcd79b158cb50dc98793aece6dbbb81d93
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2316901Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69057}

include/v8.h

@@ -7506,6 +7506,9 @@ typedef Local<String> (*WasmLoadSourceMapCallback)(Isolate* isolate,
 // --- Callback for checking if WebAssembly Simd is enabled ---
 typedef bool (*WasmSimdEnabledCallback)(Local<Context> context);
 
+// --- Callback for checking if SharedArrayBuffers are enabled ---
+typedef bool (*SharedArrayBuffersEnabledCallback)(Local<Context> context);
+
 // --- Garbage Collection Callbacks ---
 
 /**
@@ -9373,6 +9376,9 @@ class V8_EXPORT Isolate {
 
   void SetWasmSimdEnabledCallback(WasmSimdEnabledCallback callback);
 
+  void SetSharedArrayBuffersEnabledCallback(
+      SharedArrayBuffersEnabledCallback callback);
+
   /**
   * Check if V8 is dead and therefore unusable.  This is the case after
   * fatal errors such as out-of-memory situations.

src/api/api.cc

@@ -8989,6 +8989,10 @@ CALLBACK_SETTER(WasmLoadSourceMapCallback, WasmLoadSourceMapCallback,
 CALLBACK_SETTER(WasmSimdEnabledCallback, WasmSimdEnabledCallback,
                 wasm_simd_enabled_callback)
 
+CALLBACK_SETTER(SharedArrayBuffersEnabledCallback,
+                SharedArrayBuffersEnabledCallback,
+                shared_array_buffers_enabled_callback)
+
 void Isolate::AddNearHeapLimitCallback(v8::NearHeapLimitCallback callback,
                                        void* data) {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);

src/execution/isolate.cc

@@ -2634,6 +2634,14 @@ bool Isolate::IsWasmSimdEnabled(Handle<Context> context) {
   return FLAG_experimental_wasm_simd;
 }
 
+bool Isolate::AreSharedArrayBuffersEnabled(Handle<Context> context) {
+  if (shared_array_buffers_enabled_callback()) {
+    v8::Local<v8::Context> api_context = v8::Utils::ToLocal(context);
+    return shared_array_buffers_enabled_callback()(api_context);
+  }
+  return FLAG_harmony_sharedarraybuffer;
+}
+
 Handle<Context> Isolate::GetIncumbentContext() {
   JavaScriptFrameIterator it(this);
 

src/execution/isolate.h

@@ -410,6 +410,8 @@ using DebugObjectCache = std::vector<Handle<HeapObject>>;
   V(WasmThreadsEnabledCallback, wasm_threads_enabled_callback, nullptr)        \
   V(WasmLoadSourceMapCallback, wasm_load_source_map_callback, nullptr)         \
   V(WasmSimdEnabledCallback, wasm_simd_enabled_callback, nullptr)              \
+  V(SharedArrayBuffersEnabledCallback, shared_array_buffers_enabled_callback,  \
+    nullptr)                                                                   \
   /* State for Relocatable. */                                                 \
   V(Relocatable*, relocatable_top, nullptr)                                    \
   V(DebugObjectCache*, string_stream_debug_object_cache, nullptr)              \
@@ -634,6 +636,7 @@ class V8_EXPORT_PRIVATE Isolate final : private HiddenFactory {
 
   bool AreWasmThreadsEnabled(Handle<Context> context);
   bool IsWasmSimdEnabled(Handle<Context> context);
+  bool AreSharedArrayBuffersEnabled(Handle<Context> context);
 
   THREAD_LOCAL_TOP_ADDRESS(Object, pending_exception)
 

src/init/bootstrapper.cc

@@ -4137,7 +4137,7 @@ void Genesis::InitializeGlobal_harmony_atomics_waitasync() {
 }
 
 void Genesis::InitializeGlobal_harmony_sharedarraybuffer() {
-  if (!FLAG_harmony_sharedarraybuffer) return;
+  if (!isolate()->AreSharedArrayBuffersEnabled(native_context())) return;
 
   Handle<JSGlobalObject> global(native_context()->global_object(), isolate());