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diff --git a/js/src/builtin/streams/WritableStream.h b/js/src/builtin/streams/WritableStream.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * 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/. */
+
+/* Class WritableStream. */
+
+#ifndef builtin_streams_WritableStream_h
+#define builtin_streams_WritableStream_h
+
+#include "mozilla/Assertions.h"      // MOZ_ASSERT
+#include "mozilla/Attributes.h"      // MOZ_MUST_USE
+#include "mozilla/Casting.h"         // mozilla::AssertedCast
+#include "mozilla/MathAlgorithms.h"  // mozilla::IsPowerOfTwo
+
+#include <stdint.h>  // uint32_t
+
+#include "jstypes.h"          // JS_PUBLIC_API
+#include "js/Class.h"         // JSClass, js::ClassSpec
+#include "js/RootingAPI.h"    // JS::Handle
+#include "js/Value.h"         // JS::{,Int32,Object,Undefined}Value
+#include "vm/List.h"          // js::ListObject
+#include "vm/NativeObject.h"  // js::NativeObject
+
+struct JS_PUBLIC_API JSContext;
+
+namespace js {
+
+class PromiseObject;
+class WritableStreamDefaultController;
+class WritableStreamDefaultWriter;
+
+class WritableStream : public NativeObject {
+ public:
+  enum Slots {
+    /**
+     * A WritableStream's associated controller is always created from under the
+     * stream's constructor and thus cannot be in a different compartment.
+     */
+    Slot_Controller,
+
+    /**
+     * Either |undefined| if no writer has been created yet for |this|, or a
+     * |WritableStreamDefaultWriter| object that writes to this.  Writers are
+     * created under |WritableStream.prototype.getWriter|, which may not be
+     * same-compartment with |this|, so this object may be a wrapper.
+     */
+    Slot_Writer,
+
+    /**
+     * A bit field that stores both [[state]] and the [[backpressure]] spec
+     * fields in a WritableStream::State 32-bit integer.
+     */
+    Slot_State,
+
+    /**
+     * Either |undefined| if this stream hasn't yet started erroring, or an
+     * arbitrary value indicating the reason for the error (e.g. the
+     * reason-value passed to a related |abort(reason)| or |error(e)| function).
+     *
+     * This value can be an arbitrary user-provided value, so it might be a
+     * cross-comaprtment wrapper.
+     */
+    Slot_StoredError,
+
+    /**
+     * Very briefly for newborn writable streams before they are initialized,
+     * |undefined|.
+     *
+     * After initialization, a |ListObject| consisting of the value of the
+     * [[inFlightWriteRequest]] spec field (if it is not |undefined|) followed
+     * by the elements of the [[queue]] List.  |this| and the |ListObject| are
+     * same-compartment.
+     *
+     * After a stream has gone irrevocably into an error state (specifically,
+     * |stream.[[state]]| is "errored") and requests can no longer be enqueued,
+     * |undefined| yet again.
+     *
+     * If the |HaveInFlightWriteRequest| flag is set, the first element of this
+     * List is the non-|undefined| value of [[inFlightWriteRequest]].  If it is
+     * unset, [[inFlightWriteRequest]] has the value |undefined|.
+     */
+    Slot_WriteRequests,
+
+    /**
+     * A slot storing both [[closeRequest]] and [[inFlightCloseRequest]].  This
+     * value is created under |WritableStreamDefaultWriterClose|, so it may be a
+     * wrapper around a promise rather than directly a |PromiseObject|.
+     *
+     * If this slot has the value |undefined|, then [[inFlightCloseRequest]]
+     * and [[closeRequest]] are both |undefined|.  Otherwise one field has the
+     * value |undefined| and the other has the value of this slot, and the value
+     * of the |HaveInFlightCloseRequest| flag indicates which field is set.
+     */
+    Slot_CloseRequest,
+
+    /**
+     * In the spec the [[pendingAbortRequest]] field is either |undefined| or
+     * Record { [[promise]]: Object, [[reason]]: value, [[wasAlreadyErroring]]:
+     * boolean }.  We represent this as follows:
+     *
+     *   1) If Slot_PendingAbortRequestPromise contains |undefined|, then the
+     *      spec field is |undefined|;
+     *   2) Otherwise Slot_PendingAbortRequestPromise contains the value of
+     *      [[pendingAbortRequest]].[[promise]], Slot_PendingAbortRequestReason
+     *      contains the value of [[pendingAbortRequest]].[[reason]], and the
+     *      |PendingAbortRequestWasAlreadyErroring| flag stores the value of
+     *      [[pendingAbortRequest]].[[wasAlreadyErroring]].
+     */
+    Slot_PendingAbortRequestPromise,
+    Slot_PendingAbortRequestReason,
+
+    SlotCount
+  };
+
+ private:
+  enum State : uint32_t {
+    Writable = 0x0000'0000,
+    Closed = 0x0000'0001,
+    Erroring = 0x0000'0002,
+    Errored = 0x0000'0003,
+    StateBits = 0x0000'0003,
+    StateMask = 0x0000'00ff,
+
+    Backpressure = 0x0000'0100,
+    HaveInFlightWriteRequest = 0x0000'0200,
+    HaveInFlightCloseRequest = 0x0000'0400,
+    PendingAbortRequestWasAlreadyErroring = 0x0000'0800,
+    FlagBits = Backpressure | HaveInFlightWriteRequest |
+               HaveInFlightCloseRequest | PendingAbortRequestWasAlreadyErroring,
+    FlagMask = 0x0000'ff00,
+
+    SettableBits = uint32_t(StateBits | FlagBits)
+  };
+
+  bool stateIsInitialized() const { return getFixedSlot(Slot_State).isInt32(); }
+
+  State state() const {
+    MOZ_ASSERT(stateIsInitialized());
+
+    uint32_t v = getFixedSlot(Slot_State).toInt32();
+    MOZ_ASSERT((v & ~SettableBits) == 0);
+
+    return static_cast<State>(v & StateMask);
+  }
+
+  State flags() const {
+    MOZ_ASSERT(stateIsInitialized());
+
+    uint32_t v = getFixedSlot(Slot_State).toInt32();
+    MOZ_ASSERT((v & ~SettableBits) == 0);
+
+    return static_cast<State>(v & FlagMask);
+  }
+
+  void initWritableState() {
+    MOZ_ASSERT(!stateIsInitialized());
+
+    setFixedSlot(Slot_State, JS::Int32Value(Writable));
+
+    MOZ_ASSERT(writable());
+    MOZ_ASSERT(!backpressure());
+  }
+
+  void setState(State newState) {
+    MOZ_ASSERT(stateIsInitialized());
+    MOZ_ASSERT((newState & ~StateBits) == 0);
+    MOZ_ASSERT(newState <= Errored);
+
+#ifdef DEBUG
+    {
+      auto current = state();
+      if (current == Writable) {
+        MOZ_ASSERT(newState == Closed || newState == Erroring);
+      } else if (current == Erroring) {
+        MOZ_ASSERT(newState == Errored || newState == Closed);
+      } else if (current == Closed || current == Errored) {
+        MOZ_ASSERT_UNREACHABLE(
+            "closed/errored stream shouldn't undergo state transitions");
+      } else {
+        MOZ_ASSERT_UNREACHABLE("smashed state bits?");
+      }
+    }
+#endif
+
+    uint32_t newValue = static_cast<uint32_t>(newState) |
+                        (getFixedSlot(Slot_State).toInt32() & FlagMask);
+    setFixedSlot(Slot_State,
+                 JS::Int32Value(mozilla::AssertedCast<int32_t>(newValue)));
+  }
+
+  void setFlag(State flag, bool set) {
+    MOZ_ASSERT(stateIsInitialized());
+    MOZ_ASSERT(mozilla::IsPowerOfTwo(uint32_t(flag)));
+    MOZ_ASSERT((flag & FlagBits) != 0);
+
+    uint32_t v = getFixedSlot(Slot_State).toInt32();
+    MOZ_ASSERT((v & ~SettableBits) == 0);
+
+    uint32_t newValue = set ? (v | flag) : (v & ~flag);
+    setFixedSlot(Slot_State,
+                 JS::Int32Value(mozilla::AssertedCast<int32_t>(newValue)));
+  }
+
+ public:
+  bool writable() const { return state() == Writable; }
+
+  bool closed() const { return state() == Closed; }
+  void setClosed() { setState(Closed); }
+
+  bool erroring() const { return state() == Erroring; }
+  void setErroring() { setState(Erroring); }
+
+  bool errored() const { return state() == Errored; }
+  void setErrored() { setState(Errored); }
+
+  bool backpressure() const { return flags() & Backpressure; }
+  void setBackpressure(bool pressure) { setFlag(Backpressure, pressure); }
+
+  bool haveInFlightWriteRequest() const {
+    return flags() & HaveInFlightWriteRequest;
+  }
+  void setHaveInFlightWriteRequest() {
+    MOZ_ASSERT(!haveInFlightWriteRequest());
+    MOZ_ASSERT(writeRequests()->length() > 0);
+    setFlag(HaveInFlightWriteRequest, true);
+  }
+
+  bool haveInFlightCloseRequest() const {
+    return flags() & HaveInFlightCloseRequest;
+  }
+
+  bool hasController() const {
+    return !getFixedSlot(Slot_Controller).isUndefined();
+  }
+  inline WritableStreamDefaultController* controller() const;
+  inline void setController(WritableStreamDefaultController* controller);
+  void clearController() {
+    setFixedSlot(Slot_Controller, JS::UndefinedValue());
+  }
+
+  bool hasWriter() const { return !getFixedSlot(Slot_Writer).isUndefined(); }
+  bool isLocked() const { return hasWriter(); }
+  void setWriter(JSObject* writer) {
+    MOZ_ASSERT(!hasWriter());
+    setFixedSlot(Slot_Writer, JS::ObjectValue(*writer));
+  }
+  void clearWriter() { setFixedSlot(Slot_Writer, JS::UndefinedValue()); }
+
+  JS::Value storedError() const { return getFixedSlot(Slot_StoredError); }
+  void setStoredError(JS::Handle<JS::Value> value) {
+    setFixedSlot(Slot_StoredError, value);
+  }
+  void clearStoredError() {
+    setFixedSlot(Slot_StoredError, JS::UndefinedValue());
+  }
+
+  JS::Value inFlightWriteRequest() const {
+    MOZ_ASSERT(stateIsInitialized());
+
+    // The in-flight write request is the first element of |writeRequests()| --
+    // if there is a request in flight.
+    if (haveInFlightWriteRequest()) {
+      MOZ_ASSERT(writeRequests()->length() > 0);
+      return writeRequests()->get(0);
+    }
+
+    return JS::UndefinedValue();
+  }
+
+  void clearInFlightWriteRequest(JSContext* cx);
+
+  JS::Value closeRequest() const {
+    JS::Value v = getFixedSlot(Slot_CloseRequest);
+    if (v.isUndefined()) {
+      // In principle |haveInFlightCloseRequest()| only distinguishes whether
+      // the close-request slot is [[closeRequest]] or [[inFlightCloseRequest]].
+      // In practice, for greater implementation strictness to try to head off
+      // more bugs, we require that the HaveInFlightCloseRequest flag be unset
+      // when [[closeRequest]] and [[inFlightCloseRequest]] are both undefined.
+      MOZ_ASSERT(!haveInFlightCloseRequest());
+      return JS::UndefinedValue();
+    }
+
+    if (!haveInFlightCloseRequest()) {
+      return v;
+    }
+
+    return JS::UndefinedValue();
+  }
+
+  void setCloseRequest(JSObject* closeRequest) {
+    MOZ_ASSERT(!haveCloseRequestOrInFlightCloseRequest());
+    setFixedSlot(Slot_CloseRequest, JS::ObjectValue(*closeRequest));
+    MOZ_ASSERT(!haveInFlightCloseRequest());
+  }
+
+  void clearCloseRequest() {
+    MOZ_ASSERT(!haveInFlightCloseRequest());
+    MOZ_ASSERT(!getFixedSlot(Slot_CloseRequest).isUndefined());
+    setFixedSlot(Slot_CloseRequest, JS::UndefinedValue());
+  }
+
+  JS::Value inFlightCloseRequest() const {
+    JS::Value v = getFixedSlot(Slot_CloseRequest);
+    if (v.isUndefined()) {
+      // In principle |haveInFlightCloseRequest()| only distinguishes whether
+      // the close-request slot is [[closeRequest]] or [[inFlightCloseRequest]].
+      // In practice, for greater implementation strictness to try to head off
+      // more bugs, we require that the HaveInFlightCloseRequest flag be unset
+      // when [[closeRequest]] and [[inFlightCloseRequest]] are both undefined.
+      MOZ_ASSERT(!haveInFlightCloseRequest());
+      return JS::UndefinedValue();
+    }
+
+    if (haveInFlightCloseRequest()) {
+      return v;
+    }
+
+    return JS::UndefinedValue();
+  }
+
+  bool haveCloseRequestOrInFlightCloseRequest() const {
+    // Slot_CloseRequest suffices to store both [[closeRequest]] and
+    // [[inFlightCloseRequest]], with the precisely-set field determined by
+    // |haveInFlightCloseRequest()|.  If both are undefined, then per above, for
+    // extra implementation rigor, |haveInFlightCloseRequest()| will be false,
+    // so additionally assert that.
+    if (getFixedSlot(Slot_CloseRequest).isUndefined()) {
+      MOZ_ASSERT(!haveInFlightCloseRequest());
+      return false;
+    }
+
+    return true;
+  }
+
+  void convertCloseRequestToInFlightCloseRequest() {
+    MOZ_ASSERT(stateIsInitialized());
+    MOZ_ASSERT(!haveInFlightCloseRequest());
+    setFlag(HaveInFlightCloseRequest, true);
+    MOZ_ASSERT(haveInFlightCloseRequest());
+  }
+
+  void clearInFlightCloseRequest() {
+    MOZ_ASSERT(stateIsInitialized());
+    MOZ_ASSERT(haveInFlightCloseRequest());
+    MOZ_ASSERT(!getFixedSlot(Slot_CloseRequest).isUndefined());
+
+    // As noted above, for greater rigor we require HaveInFlightCloseRequest be
+    // unset when [[closeRequest]] and [[inFlightCloseRequest]] are both
+    // undefined.
+    setFlag(HaveInFlightCloseRequest, false);
+    setFixedSlot(Slot_CloseRequest, JS::UndefinedValue());
+  }
+
+  ListObject* writeRequests() const {
+    MOZ_ASSERT(!getFixedSlot(Slot_WriteRequests).isUndefined(),
+               "shouldn't be accessing [[writeRequests]] on a newborn and "
+               "uninitialized stream, or on a stream that's errored and no "
+               "longer has any write requests");
+    return &getFixedSlot(Slot_WriteRequests).toObject().as<ListObject>();
+  }
+  void clearWriteRequests() {
+    // Setting [[writeRequests]] to an empty List in the irrevocably-in-error
+    // case (in which [[writeRequests]] is never again accessed) is optimized to
+    // just clearing the field.  See the comment on the slot constant above.
+    MOZ_ASSERT(stateIsInitialized());
+    MOZ_ASSERT(!haveInFlightWriteRequest(),
+               "must clear the in-flight request flag before clearing "
+               "requests");
+    setFixedSlot(Slot_WriteRequests, JS::UndefinedValue());
+  }
+
+  bool hasPendingAbortRequest() const {
+    MOZ_ASSERT(stateIsInitialized());
+    return !getFixedSlot(Slot_PendingAbortRequestPromise).isUndefined();
+  }
+  JSObject* pendingAbortRequestPromise() const {
+    MOZ_ASSERT(hasPendingAbortRequest());
+    return &getFixedSlot(Slot_PendingAbortRequestPromise).toObject();
+  }
+  JS::Value pendingAbortRequestReason() const {
+    MOZ_ASSERT(hasPendingAbortRequest());
+    return getFixedSlot(Slot_PendingAbortRequestReason);
+  }
+  bool pendingAbortRequestWasAlreadyErroring() const {
+    MOZ_ASSERT(hasPendingAbortRequest());
+    return flags() & PendingAbortRequestWasAlreadyErroring;
+  }
+
+  void setPendingAbortRequest(JSObject* promise, const JS::Value& reason,
+                              bool wasAlreadyErroring) {
+    MOZ_ASSERT(!hasPendingAbortRequest());
+    MOZ_ASSERT(!(flags() & PendingAbortRequestWasAlreadyErroring));
+    setFixedSlot(Slot_PendingAbortRequestPromise, JS::ObjectValue(*promise));
+    setFixedSlot(Slot_PendingAbortRequestReason, reason);
+    setFlag(PendingAbortRequestWasAlreadyErroring, wasAlreadyErroring);
+  }
+
+  void clearPendingAbortRequest() {
+    MOZ_ASSERT(stateIsInitialized());
+    MOZ_ASSERT(hasPendingAbortRequest());
+
+    // [[pendingAbortRequest]] is { [[promise]], [[reason]] } in the spec but
+    // separate slots in our implementation, so both must be cleared.
+    setFixedSlot(Slot_PendingAbortRequestPromise, JS::UndefinedValue());
+    setFixedSlot(Slot_PendingAbortRequestReason, JS::UndefinedValue());
+  }
+
+  static MOZ_MUST_USE WritableStream* create(
+      JSContext* cx, void* nsISupportsObject_alreadyAddreffed = nullptr,
+      JS::Handle<JSObject*> proto = nullptr);
+
+  static bool constructor(JSContext* cx, unsigned argc, JS::Value* vp);
+
+  static const ClassSpec classSpec_;
+  static const JSClass class_;
+  static const ClassSpec protoClassSpec_;
+  static const JSClass protoClass_;
+};
+
+}  // namespace js
+
+#endif  // builtin_streams_WritableStream_h