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diff --git a/js/public/ArrayBufferMaybeShared.h b/js/public/ArrayBufferMaybeShared.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Functions for working with either ArrayBuffer or SharedArrayBuffer objects
+ * in agnostic fashion.
+ */
+
+#ifndef js_ArrayBufferMaybeShared_h
+#define js_ArrayBufferMaybeShared_h
+
+#include <stdint.h>  // uint8_t, uint32_t
+
+#include "jstypes.h"  // JS_PUBLIC_API
+
+struct JS_PUBLIC_API JSContext;
+class JS_PUBLIC_API JSObject;
+
+namespace JS {
+
+class JS_PUBLIC_API AutoRequireNoGC;
+
+// TYPE TESTING
+
+/**
+ * Check whether obj supports the JS::GetArrayBufferMaybeShared* APIs.  Note
+ * that this may return false if a security wrapper is encountered that denies
+ * the unwrapping. If this test succeeds, then it is safe to call the various
+ * predicate and accessor JSAPI calls defined below.
+ */
+extern JS_PUBLIC_API bool IsArrayBufferObjectMaybeShared(JSObject* obj);
+
+// ACCESSORS
+
+/*
+ * Test for ArrayBufferMaybeShared subtypes and return the unwrapped object if
+ * so, else nullptr. Never throws.
+ */
+extern JS_PUBLIC_API JSObject* UnwrapArrayBufferMaybeShared(JSObject* obj);
+
+/**
+ * Get the length, sharedness, and data from an ArrayBufferMaybeShared subtypes.
+ *
+ * The computed length and data pointer may be invalidated by a GC or by an
+ * unshared array buffer becoming detached. Callers must take care not to
+ * perform any actions that could trigger a GC or result in an unshared array
+ * buffer becoming detached. If such actions nonetheless must be performed,
+ * callers should perform this call a second time (and sensibly handle results
+ * that may be different from those returned the first time). (Sharedness is an
+ * immutable characteristic of an array buffer or shared array buffer, so that
+ * boolean remains valid across GC or detaching.)
+ *
+ * |obj| must be an ArrayBufferMaybeShared subtype: an ArrayBuffer or a
+ * SharedArrayBuffer.
+ *
+ * |*length| will be set to bytes in the buffer.
+ *
+ * |*isSharedMemory| will be set to true if it is a SharedArrayBuffer, otherwise
+ * to false.
+ *
+ * |*data| will be set to a pointer to the bytes in the buffer.
+ */
+extern JS_PUBLIC_API void GetArrayBufferMaybeSharedLengthAndData(
+    JSObject* obj, size_t* length, bool* isSharedMemory, uint8_t** data);
+
+/**
+ * Return a pointer to the start of the array buffer's data, and indicate
+ * whether the data is from a shared array buffer through an outparam.
+ *
+ * The returned data pointer may be invalidated by a GC or by an unshared array
+ * buffer becoming detached. Callers must take care not to perform any actions
+ * that could trigger a GC or result in an unshared array buffer becoming
+ * detached. If such actions nonetheless must be performed, callers should
+ * perform this call a second time (and sensibly handle results that may be
+ * different from those returned the first time). (Sharedness is an immutable
+ * characteristic of an array buffer or shared array buffer, so that boolean
+ * remains valid across GC or detaching.)
+ *
+ * |obj| must have passed a JS::IsArrayBufferObjectMaybeShared test, or somehow
+ * be known that it would pass such a test: it is an ArrayBuffer or
+ * SharedArrayBuffer or a wrapper of an ArrayBuffer/SharedArrayBuffer, and the
+ * unwrapping will succeed.
+ *
+ * |*isSharedMemory| will be set to true if the typed array maps a
+ * SharedArrayBuffer, otherwise to false.
+ */
+extern JS_PUBLIC_API uint8_t* GetArrayBufferMaybeSharedData(
+    JSObject* obj, bool* isSharedMemory, const AutoRequireNoGC&);
+
+/**
+ * Returns whether the passed array buffer is 'large': its byteLength >= 2 GB.
+ *
+ * |obj| must pass a JS::IsArrayBufferObjectMaybeShared test.
+ */
+extern JS_PUBLIC_API bool IsLargeArrayBufferMaybeShared(JSObject* obj);
+
+}  // namespace JS
+
+#endif /* js_ArrayBufferMaybeShared_h */