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diff --git a/dom/streams/UnderlyingSinkCallbackHelpers.h b/dom/streams/UnderlyingSinkCallbackHelpers.h
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+++ b/dom/streams/UnderlyingSinkCallbackHelpers.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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/. */
+
+#ifndef mozilla_dom_UnderlyingSinkCallbackHelpers_h
+#define mozilla_dom_UnderlyingSinkCallbackHelpers_h
+
+#include "mozilla/Maybe.h"
+#include "mozilla/Buffer.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/UnderlyingSinkBinding.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsISupports.h"
+#include "nsISupportsImpl.h"
+#include "nsIAsyncOutputStream.h"
+
+/*
+ * See the comment in UnderlyingSourceCallbackHelpers.h!
+ *
+ * A native implementation of these callbacks is however currently not required.
+ */
+namespace mozilla::dom {
+
+class WritableStreamDefaultController;
+
+class UnderlyingSinkAlgorithmsBase : public nsISupports {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(UnderlyingSinkAlgorithmsBase)
+
+  MOZ_CAN_RUN_SCRIPT virtual void StartCallback(
+      JSContext* aCx, WritableStreamDefaultController& aController,
+      JS::MutableHandle<JS::Value> aRetVal, ErrorResult& aRv) = 0;
+
+  MOZ_CAN_RUN_SCRIPT virtual already_AddRefed<Promise> WriteCallback(
+      JSContext* aCx, JS::Handle<JS::Value> aChunk,
+      WritableStreamDefaultController& aController, ErrorResult& aRv) = 0;
+
+  MOZ_CAN_RUN_SCRIPT virtual already_AddRefed<Promise> CloseCallback(
+      JSContext* aCx, ErrorResult& aRv) = 0;
+
+  MOZ_CAN_RUN_SCRIPT virtual already_AddRefed<Promise> AbortCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) = 0;
+
+  // Implement this when you need to release underlying resources immediately
+  // from closed/errored(aborted) streams, without waiting for GC.
+  virtual void ReleaseObjects() {}
+
+ protected:
+  virtual ~UnderlyingSinkAlgorithmsBase() = default;
+};
+
+// https://streams.spec.whatwg.org/#set-up-writable-stream-default-controller-from-underlying-sink
+class UnderlyingSinkAlgorithms final : public UnderlyingSinkAlgorithmsBase {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_INHERITED(
+      UnderlyingSinkAlgorithms, UnderlyingSinkAlgorithmsBase)
+
+  UnderlyingSinkAlgorithms(nsIGlobalObject* aGlobal,
+                           JS::Handle<JSObject*> aUnderlyingSink,
+                           UnderlyingSink& aUnderlyingSinkDict)
+      : mGlobal(aGlobal), mUnderlyingSink(aUnderlyingSink) {
+    // Step 6. (implicit Step 2.)
+    if (aUnderlyingSinkDict.mStart.WasPassed()) {
+      mStartCallback = aUnderlyingSinkDict.mStart.Value();
+    }
+
+    // Step 7. (implicit Step 3.)
+    if (aUnderlyingSinkDict.mWrite.WasPassed()) {
+      mWriteCallback = aUnderlyingSinkDict.mWrite.Value();
+    }
+
+    // Step 8. (implicit Step 4.)
+    if (aUnderlyingSinkDict.mClose.WasPassed()) {
+      mCloseCallback = aUnderlyingSinkDict.mClose.Value();
+    }
+
+    // Step 9. (implicit Step 5.)
+    if (aUnderlyingSinkDict.mAbort.WasPassed()) {
+      mAbortCallback = aUnderlyingSinkDict.mAbort.Value();
+    }
+
+    mozilla::HoldJSObjects(this);
+  };
+
+  MOZ_CAN_RUN_SCRIPT void StartCallback(
+      JSContext* aCx, WritableStreamDefaultController& aController,
+      JS::MutableHandle<JS::Value> aRetVal, ErrorResult& aRv) override;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> WriteCallback(
+      JSContext* aCx, JS::Handle<JS::Value> aChunk,
+      WritableStreamDefaultController& aController, ErrorResult& aRv) override;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> CloseCallback(
+      JSContext* aCx, ErrorResult& aRv) override;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> AbortCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) override;
+
+ protected:
+  ~UnderlyingSinkAlgorithms() override { mozilla::DropJSObjects(this); }
+
+ private:
+  // Virtually const, but are cycle collected
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+  JS::Heap<JSObject*> mUnderlyingSink;
+  MOZ_KNOWN_LIVE RefPtr<UnderlyingSinkStartCallback> mStartCallback;
+  MOZ_KNOWN_LIVE RefPtr<UnderlyingSinkWriteCallback> mWriteCallback;
+  MOZ_KNOWN_LIVE RefPtr<UnderlyingSinkCloseCallback> mCloseCallback;
+  MOZ_KNOWN_LIVE RefPtr<UnderlyingSinkAbortCallback> mAbortCallback;
+};
+
+// https://streams.spec.whatwg.org/#writablestream-set-up
+// Wrappers defined by the "Set up" methods in the spec.
+// (closeAlgorithmWrapper, abortAlgorithmWrapper)
+// This helps you just return nullptr when 1) the algorithm is synchronous, or
+// 2) an error occurred, as this wrapper will return a resolved or rejected
+// promise respectively.
+// Note that StartCallback is only for JS consumers to access the
+// controller, and thus is no-op here since native consumers can call
+// `ErrorNative()` etc. without direct controller access.
+class UnderlyingSinkAlgorithmsWrapper : public UnderlyingSinkAlgorithmsBase {
+ public:
+  void StartCallback(JSContext* aCx,
+                     WritableStreamDefaultController& aController,
+                     JS::MutableHandle<JS::Value> aRetVal,
+                     ErrorResult& aRv) final {
+    // Step 1: Let startAlgorithm be an algorithm that returns undefined.
+    aRetVal.setUndefined();
+  }
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> CloseCallback(
+      JSContext* aCx, ErrorResult& aRv) final;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> AbortCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) final;
+
+  virtual already_AddRefed<Promise> CloseCallbackImpl(JSContext* aCx,
+                                                      ErrorResult& aRv) {
+    // (closeAlgorithm is optional, give null by default)
+    return nullptr;
+  }
+
+  virtual already_AddRefed<Promise> AbortCallbackImpl(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) {
+    // (abortAlgorithm is optional, give null by default)
+    return nullptr;
+  }
+};
+
+class WritableStreamToOutput final : public UnderlyingSinkAlgorithmsWrapper,
+                                     public nsIOutputStreamCallback {
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIOUTPUTSTREAMCALLBACK
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(WritableStreamToOutput,
+                                           UnderlyingSinkAlgorithmsBase)
+
+  WritableStreamToOutput(nsIGlobalObject* aParent,
+                         nsIAsyncOutputStream* aOutput)
+      : mWritten(0), mParent(aParent), mOutput(aOutput) {}
+
+  // Streams algorithms
+
+  already_AddRefed<Promise> WriteCallback(
+      JSContext* aCx, JS::Handle<JS::Value> aChunk,
+      WritableStreamDefaultController& aController, ErrorResult& aRv) override;
+
+  // No CloseCallbackImpl() since ReleaseObjects() will call Close()
+
+  already_AddRefed<Promise> AbortCallbackImpl(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) override;
+
+  void ReleaseObjects() override;
+
+ private:
+  ~WritableStreamToOutput() override = default;
+
+  void ClearData() {
+    mData = Nothing();
+    mPromise = nullptr;
+    mWritten = 0;
+  }
+
+  uint32_t mWritten;
+  nsCOMPtr<nsIGlobalObject> mParent;
+  nsCOMPtr<nsIAsyncOutputStream> mOutput;
+  RefPtr<Promise> mPromise;  // Resolved when entirely written
+  Maybe<Buffer<uint8_t>> mData;
+};
+
+}  // namespace mozilla::dom
+
+#endif