1 files changed, 1763 insertions, 0 deletions
diff --git a/xpcom/threads/MozPromise.h b/xpcom/threads/MozPromise.h
new file mode 100644
index 0000000000..c0b0addf0a
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+++ b/xpcom/threads/MozPromise.h
@@ -0,0 +1,1763 @@
+/* -*- 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/. */
+
+#if !defined(MozPromise_h_)
+#  define MozPromise_h_
+
+#  include <type_traits>
+#  include <utility>
+
+#  include "mozilla/ErrorNames.h"
+#  include "mozilla/Logging.h"
+#  include "mozilla/Maybe.h"
+#  include "mozilla/Monitor.h"
+#  include "mozilla/Mutex.h"
+#  include "mozilla/RefPtr.h"
+#  include "mozilla/UniquePtr.h"
+#  include "mozilla/Variant.h"
+#  include "nsIDirectTaskDispatcher.h"
+#  include "nsISerialEventTarget.h"
+#  include "nsTArray.h"
+#  include "nsThreadUtils.h"
+
+#  ifdef MOZ_WIDGET_ANDROID
+#    include "mozilla/jni/GeckoResultUtils.h"
+#  endif
+
+#  if MOZ_DIAGNOSTIC_ASSERT_ENABLED
+#    define PROMISE_DEBUG
+#  endif
+
+#  ifdef PROMISE_DEBUG
+#    define PROMISE_ASSERT MOZ_RELEASE_ASSERT
+#  else
+#    define PROMISE_ASSERT(...) \
+      do {                      \
+      } while (0)
+#  endif
+
+#  if DEBUG
+#    include "nsPrintfCString.h"
+#  endif
+
+namespace mozilla {
+
+namespace dom {
+class Promise;
+}
+
+extern LazyLogModule gMozPromiseLog;
+
+#  define PROMISE_LOG(x, ...) \
+    MOZ_LOG(gMozPromiseLog, mozilla::LogLevel::Debug, (x, ##__VA_ARGS__))
+
+namespace detail {
+template <typename F>
+struct MethodTraitsHelper : MethodTraitsHelper<decltype(&F::operator())> {};
+template <typename ThisType, typename Ret, typename... ArgTypes>
+struct MethodTraitsHelper<Ret (ThisType::*)(ArgTypes...)> {
+  using ReturnType = Ret;
+  static const size_t ArgSize = sizeof...(ArgTypes);
+};
+template <typename ThisType, typename Ret, typename... ArgTypes>
+struct MethodTraitsHelper<Ret (ThisType::*)(ArgTypes...) const> {
+  using ReturnType = Ret;
+  static const size_t ArgSize = sizeof...(ArgTypes);
+};
+template <typename ThisType, typename Ret, typename... ArgTypes>
+struct MethodTraitsHelper<Ret (ThisType::*)(ArgTypes...) volatile> {
+  using ReturnType = Ret;
+  static const size_t ArgSize = sizeof...(ArgTypes);
+};
+template <typename ThisType, typename Ret, typename... ArgTypes>
+struct MethodTraitsHelper<Ret (ThisType::*)(ArgTypes...) const volatile> {
+  using ReturnType = Ret;
+  static const size_t ArgSize = sizeof...(ArgTypes);
+};
+template <typename T>
+struct MethodTrait : MethodTraitsHelper<std::remove_reference_t<T>> {};
+
+}  // namespace detail
+
+template <typename MethodType>
+using TakesArgument =
+    std::integral_constant<bool, detail::MethodTrait<MethodType>::ArgSize != 0>;
+
+template <typename MethodType, typename TargetType>
+using ReturnTypeIs =
+    std::is_convertible<typename detail::MethodTrait<MethodType>::ReturnType,
+                        TargetType>;
+
+template <typename ResolveValueT, typename RejectValueT, bool IsExclusive>
+class MozPromise;
+
+template <typename Return>
+struct IsMozPromise : std::false_type {};
+
+template <typename ResolveValueT, typename RejectValueT, bool IsExclusive>
+struct IsMozPromise<MozPromise<ResolveValueT, RejectValueT, IsExclusive>>
+    : std::true_type {};
+
+/*
+ * A promise manages an asynchronous request that may or may not be able to be
+ * fulfilled immediately. When an API returns a promise, the consumer may attach
+ * callbacks to be invoked (asynchronously, on a specified thread) when the
+ * request is either completed (resolved) or cannot be completed (rejected).
+ * Whereas JS promise callbacks are dispatched from Microtask checkpoints,
+ * MozPromises resolution/rejection make a normal round-trip through the event
+ * loop, which simplifies their ordering semantics relative to other native
+ * code.
+ *
+ * MozPromises attempt to mirror the spirit of JS Promises to the extent that
+ * is possible (and desirable) in C++. While the intent is that MozPromises
+ * feel familiar to programmers who are accustomed to their JS-implemented
+ * cousin, we don't shy away from imposing restrictions and adding features that
+ * make sense for the use cases we encounter.
+ *
+ * A MozPromise is ThreadSafe, and may be ->Then()ed on any thread. The Then()
+ * call accepts resolve and reject callbacks, and returns a magic object which
+ * will be implicitly converted to a MozPromise::Request or a MozPromise object
+ * depending on how the return value is used. The magic object serves several
+ * purposes for the consumer.
+ *
+ *   (1) When converting to a MozPromise::Request, it allows the caller to
+ *       cancel the delivery of the resolve/reject value if it has not already
+ *       occurred, via Disconnect() (this must be done on the target thread to
+ *       avoid racing).
+ *
+ *   (2) When converting to a MozPromise (which is called a completion promise),
+ *       it allows promise chaining so ->Then() can be called again to attach
+ *       more resolve and reject callbacks. If the resolve/reject callback
+ *       returns a new MozPromise, that promise is chained to the completion
+ *       promise, such that its resolve/reject value will be forwarded along
+ *       when it arrives. If the resolve/reject callback returns void, the
+ *       completion promise is resolved/rejected with the same value that was
+ *       passed to the callback.
+ *
+ * The MozPromise APIs skirt traditional XPCOM convention by returning nsRefPtrs
+ * (rather than already_AddRefed) from various methods. This is done to allow
+ * elegant chaining of calls without cluttering up the code with intermediate
+ * variables, and without introducing separate API variants for callers that
+ * want a return value (from, say, ->Then()) from those that don't.
+ *
+ * When IsExclusive is true, the MozPromise does a release-mode assertion that
+ * there is at most one call to either Then(...) or ChainTo(...).
+ */
+
+class MozPromiseRefcountable {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(MozPromiseRefcountable)
+ protected:
+  virtual ~MozPromiseRefcountable() = default;
+};
+
+class MozPromiseBase : public MozPromiseRefcountable {
+ public:
+  virtual void AssertIsDead() = 0;
+};
+
+template <typename T>
+class MozPromiseHolder;
+template <typename T>
+class MozPromiseRequestHolder;
+template <typename ResolveValueT, typename RejectValueT, bool IsExclusive>
+class MozPromise : public MozPromiseBase {
+  static const uint32_t sMagic = 0xcecace11;
+
+  // Return a |T&&| to enable move when IsExclusive is true or
+  // a |const T&| to enforce copy otherwise.
+  template <typename T,
+            typename R = std::conditional_t<IsExclusive, T&&, const T&>>
+  static R MaybeMove(T& aX) {
+    return static_cast<R>(aX);
+  }
+
+ public:
+  typedef ResolveValueT ResolveValueType;
+  typedef RejectValueT RejectValueType;
+  class ResolveOrRejectValue {
+   public:
+    template <typename ResolveValueType_>
+    void SetResolve(ResolveValueType_&& aResolveValue) {
+      MOZ_ASSERT(IsNothing());
+      mValue = Storage(VariantIndex<ResolveIndex>{},
+                       std::forward<ResolveValueType_>(aResolveValue));
+    }
+
+    template <typename RejectValueType_>
+    void SetReject(RejectValueType_&& aRejectValue) {
+      MOZ_ASSERT(IsNothing());
+      mValue = Storage(VariantIndex<RejectIndex>{},
+                       std::forward<RejectValueType_>(aRejectValue));
+    }
+
+    template <typename ResolveValueType_>
+    static ResolveOrRejectValue MakeResolve(ResolveValueType_&& aResolveValue) {
+      ResolveOrRejectValue val;
+      val.SetResolve(std::forward<ResolveValueType_>(aResolveValue));
+      return val;
+    }
+
+    template <typename RejectValueType_>
+    static ResolveOrRejectValue MakeReject(RejectValueType_&& aRejectValue) {
+      ResolveOrRejectValue val;
+      val.SetReject(std::forward<RejectValueType_>(aRejectValue));
+      return val;
+    }
+
+    bool IsResolve() const { return mValue.template is<ResolveIndex>(); }
+    bool IsReject() const { return mValue.template is<RejectIndex>(); }
+    bool IsNothing() const { return mValue.template is<NothingIndex>(); }
+
+    const ResolveValueType& ResolveValue() const {
+      return mValue.template as<ResolveIndex>();
+    }
+    ResolveValueType& ResolveValue() {
+      return mValue.template as<ResolveIndex>();
+    }
+    const RejectValueType& RejectValue() const {
+      return mValue.template as<RejectIndex>();
+    }
+    RejectValueType& RejectValue() { return mValue.template as<RejectIndex>(); }
+
+   private:
+    enum { NothingIndex, ResolveIndex, RejectIndex };
+    using Storage = Variant<Nothing, ResolveValueType, RejectValueType>;
+    Storage mValue = Storage(VariantIndex<NothingIndex>{});
+  };
+
+ protected:
+  // MozPromise is the public type, and never constructed directly. Construct
+  // a MozPromise::Private, defined below.
+  MozPromise(const char* aCreationSite, bool aIsCompletionPromise)
+      : mCreationSite(aCreationSite),
+        mMutex("MozPromise Mutex"),
+        mHaveRequest(false),
+        mIsCompletionPromise(aIsCompletionPromise)
+#  ifdef PROMISE_DEBUG
+        ,
+        mMagic4(&mMutex)
+#  endif
+  {
+    PROMISE_LOG("%s creating MozPromise (%p)", mCreationSite, this);
+  }
+
+ public:
+  // MozPromise::Private allows us to separate the public interface (upon which
+  // consumers of the promise may invoke methods like Then()) from the private
+  // interface (upon which the creator of the promise may invoke Resolve() or
+  // Reject()). APIs should create and store a MozPromise::Private (usually
+  // via a MozPromiseHolder), and return a MozPromise to consumers.
+  //
+  // NB: We can include the definition of this class inline once B2G ICS is
+  // gone.
+  class Private;
+
+  template <typename ResolveValueType_>
+  [[nodiscard]] static RefPtr<MozPromise> CreateAndResolve(
+      ResolveValueType_&& aResolveValue, const char* aResolveSite) {
+    static_assert(std::is_convertible_v<ResolveValueType_, ResolveValueT>,
+                  "Resolve() argument must be implicitly convertible to "
+                  "MozPromise's ResolveValueT");
+    RefPtr<typename MozPromise::Private> p =
+        new MozPromise::Private(aResolveSite);
+    p->Resolve(std::forward<ResolveValueType_>(aResolveValue), aResolveSite);
+    return p;
+  }
+
+  template <typename RejectValueType_>
+  [[nodiscard]] static RefPtr<MozPromise> CreateAndReject(
+      RejectValueType_&& aRejectValue, const char* aRejectSite) {
+    static_assert(std::is_convertible_v<RejectValueType_, RejectValueT>,
+                  "Reject() argument must be implicitly convertible to "
+                  "MozPromise's RejectValueT");
+    RefPtr<typename MozPromise::Private> p =
+        new MozPromise::Private(aRejectSite);
+    p->Reject(std::forward<RejectValueType_>(aRejectValue), aRejectSite);
+    return p;
+  }
+
+  template <typename ResolveOrRejectValueType_>
+  [[nodiscard]] static RefPtr<MozPromise> CreateAndResolveOrReject(
+      ResolveOrRejectValueType_&& aValue, const char* aSite) {
+    RefPtr<typename MozPromise::Private> p = new MozPromise::Private(aSite);
+    p->ResolveOrReject(std::forward<ResolveOrRejectValueType_>(aValue), aSite);
+    return p;
+  }
+
+  typedef MozPromise<CopyableTArray<ResolveValueType>, RejectValueType,
+                     IsExclusive>
+      AllPromiseType;
+
+  typedef MozPromise<CopyableTArray<ResolveOrRejectValue>, bool, IsExclusive>
+      AllSettledPromiseType;
+
+ private:
+  class AllPromiseHolder : public MozPromiseRefcountable {
+   public:
+    explicit AllPromiseHolder(size_t aDependentPromises)
+        : mPromise(new typename AllPromiseType::Private(__func__)),
+          mOutstandingPromises(aDependentPromises) {
+      MOZ_ASSERT(aDependentPromises > 0);
+      mResolveValues.SetLength(aDependentPromises);
+    }
+
+    template <typename ResolveValueType_>
+    void Resolve(size_t aIndex, ResolveValueType_&& aResolveValue) {
+      if (!mPromise) {
+        // Already rejected.
+        return;
+      }
+
+      mResolveValues[aIndex].emplace(
+          std::forward<ResolveValueType_>(aResolveValue));
+      if (--mOutstandingPromises == 0) {
+        nsTArray<ResolveValueType> resolveValues;
+        resolveValues.SetCapacity(mResolveValues.Length());
+        for (auto&& resolveValue : mResolveValues) {
+          resolveValues.AppendElement(std::move(resolveValue.ref()));
+        }
+
+        mPromise->Resolve(std::move(resolveValues), __func__);
+        mPromise = nullptr;
+        mResolveValues.Clear();
+      }
+    }
+
+    template <typename RejectValueType_>
+    void Reject(RejectValueType_&& aRejectValue) {
+      if (!mPromise) {
+        // Already rejected.
+        return;
+      }
+
+      mPromise->Reject(std::forward<RejectValueType_>(aRejectValue), __func__);
+      mPromise = nullptr;
+      mResolveValues.Clear();
+    }
+
+    AllPromiseType* Promise() { return mPromise; }
+
+   private:
+    nsTArray<Maybe<ResolveValueType>> mResolveValues;
+    RefPtr<typename AllPromiseType::Private> mPromise;
+    size_t mOutstandingPromises;
+  };
+
+  // Trying to pass ResolveOrRejectValue by value fails static analysis checks,
+  // so we need to use either a const& or an rvalue reference, depending on
+  // whether IsExclusive is true or not.
+  typedef std::conditional_t<IsExclusive, ResolveOrRejectValue&&,
+                             const ResolveOrRejectValue&>
+      ResolveOrRejectValueParam;
+
+  typedef std::conditional_t<IsExclusive, ResolveValueType&&,
+                             const ResolveValueType&>
+      ResolveValueTypeParam;
+
+  typedef std::conditional_t<IsExclusive, RejectValueType&&,
+                             const RejectValueType&>
+      RejectValueTypeParam;
+
+  class AllSettledPromiseHolder : public MozPromiseRefcountable {
+   public:
+    explicit AllSettledPromiseHolder(size_t aDependentPromises)
+        : mPromise(new typename AllSettledPromiseType::Private(__func__)),
+          mOutstandingPromises(aDependentPromises) {
+      MOZ_ASSERT(aDependentPromises > 0);
+      mValues.SetLength(aDependentPromises);
+    }
+
+    void Settle(size_t aIndex, ResolveOrRejectValueParam aValue) {
+      if (!mPromise) {
+        // Already rejected.
+        return;
+      }
+
+      mValues[aIndex].emplace(MaybeMove(aValue));
+      if (--mOutstandingPromises == 0) {
+        nsTArray<ResolveOrRejectValue> values;
+        values.SetCapacity(mValues.Length());
+        for (auto&& value : mValues) {
+          values.AppendElement(std::move(value.ref()));
+        }
+
+        mPromise->Resolve(std::move(values), __func__);
+        mPromise = nullptr;
+        mValues.Clear();
+      }
+    }
+
+    AllSettledPromiseType* Promise() { return mPromise; }
+
+   private:
+    nsTArray<Maybe<ResolveOrRejectValue>> mValues;
+    RefPtr<typename AllSettledPromiseType::Private> mPromise;
+    size_t mOutstandingPromises;
+  };
+
+ public:
+  [[nodiscard]] static RefPtr<AllPromiseType> All(
+      nsISerialEventTarget* aProcessingTarget,
+      nsTArray<RefPtr<MozPromise>>& aPromises) {
+    if (aPromises.Length() == 0) {
+      return AllPromiseType::CreateAndResolve(
+          CopyableTArray<ResolveValueType>(), __func__);
+    }
+
+    RefPtr<AllPromiseHolder> holder = new AllPromiseHolder(aPromises.Length());
+    RefPtr<AllPromiseType> promise = holder->Promise();
+    for (size_t i = 0; i < aPromises.Length(); ++i) {
+      aPromises[i]->Then(
+          aProcessingTarget, __func__,
+          [holder, i](ResolveValueTypeParam aResolveValue) -> void {
+            holder->Resolve(i, MaybeMove(aResolveValue));
+          },
+          [holder](RejectValueTypeParam aRejectValue) -> void {
+            holder->Reject(MaybeMove(aRejectValue));
+          });
+    }
+    return promise;
+  }
+
+  [[nodiscard]] static RefPtr<AllSettledPromiseType> AllSettled(
+      nsISerialEventTarget* aProcessingTarget,
+      nsTArray<RefPtr<MozPromise>>& aPromises) {
+    if (aPromises.Length() == 0) {
+      return AllSettledPromiseType::CreateAndResolve(
+          CopyableTArray<ResolveOrRejectValue>(), __func__);
+    }
+
+    RefPtr<AllSettledPromiseHolder> holder =
+        new AllSettledPromiseHolder(aPromises.Length());
+    RefPtr<AllSettledPromiseType> promise = holder->Promise();
+    for (size_t i = 0; i < aPromises.Length(); ++i) {
+      aPromises[i]->Then(aProcessingTarget, __func__,
+                         [holder, i](ResolveOrRejectValueParam aValue) -> void {
+                           holder->Settle(i, MaybeMove(aValue));
+                         });
+    }
+    return promise;
+  }
+
+  class Request : public MozPromiseRefcountable {
+   public:
+    virtual void Disconnect() = 0;
+
+   protected:
+    Request() : mComplete(false), mDisconnected(false) {}
+    virtual ~Request() = default;
+
+    bool mComplete;
+    bool mDisconnected;
+  };
+
+ protected:
+  /*
+   * A ThenValue tracks a single consumer waiting on the promise. When a
+   * consumer invokes promise->Then(...), a ThenValue is created. Once the
+   * Promise is resolved or rejected, a {Resolve,Reject}Runnable is dispatched,
+   * which invokes the resolve/reject method and then deletes the ThenValue.
+   */
+  class ThenValueBase : public Request {
+    friend class MozPromise;
+    static const uint32_t sMagic = 0xfadece11;
+
+   public:
+    class ResolveOrRejectRunnable final
+        : public PrioritizableCancelableRunnable {
+     public:
+      ResolveOrRejectRunnable(ThenValueBase* aThenValue, MozPromise* aPromise)
+          : PrioritizableCancelableRunnable(
+                aPromise->mPriority,
+                "MozPromise::ThenValueBase::ResolveOrRejectRunnable"),
+            mThenValue(aThenValue),
+            mPromise(aPromise) {
+        MOZ_DIAGNOSTIC_ASSERT(!mPromise->IsPending());
+      }
+
+      ~ResolveOrRejectRunnable() {
+        if (mThenValue) {
+          mThenValue->AssertIsDead();
+        }
+      }
+
+      NS_IMETHOD Run() override {
+        PROMISE_LOG("ResolveOrRejectRunnable::Run() [this=%p]", this);
+        mThenValue->DoResolveOrReject(mPromise->Value());
+        mThenValue = nullptr;
+        mPromise = nullptr;
+        return NS_OK;
+      }
+
+      nsresult Cancel() override { return Run(); }
+
+     private:
+      RefPtr<ThenValueBase> mThenValue;
+      RefPtr<MozPromise> mPromise;
+    };
+
+    ThenValueBase(nsISerialEventTarget* aResponseTarget, const char* aCallSite)
+        : mResponseTarget(aResponseTarget), mCallSite(aCallSite) {
+      MOZ_ASSERT(aResponseTarget);
+    }
+
+#  ifdef PROMISE_DEBUG
+    ~ThenValueBase() {
+      mMagic1 = 0;
+      mMagic2 = 0;
+    }
+#  endif
+
+    void AssertIsDead() {
+      PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic);
+      // We want to assert that this ThenValues is dead - that is to say, that
+      // there are no consumers waiting for the result. In the case of a normal
+      // ThenValue, we check that it has been disconnected, which is the way
+      // that the consumer signals that it no longer wishes to hear about the
+      // result. If this ThenValue has a completion promise (which is mutually
+      // exclusive with being disconnectable), we recursively assert that every
+      // ThenValue associated with the completion promise is dead.
+      if (MozPromiseBase* p = CompletionPromise()) {
+        p->AssertIsDead();
+      } else {
+#  ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+        if (MOZ_UNLIKELY(!Request::mDisconnected)) {
+          MOZ_CRASH_UNSAFE_PRINTF(
+              "MozPromise::ThenValue created from '%s' destroyed without being "
+              "either disconnected, resolved, or rejected (dispatchRv: %s)",
+              mCallSite,
+              mDispatchRv ? GetStaticErrorName(*mDispatchRv)
+                          : "not dispatched");
+        }
+#  endif
+      }
+    }
+
+    void Dispatch(MozPromise* aPromise) {
+      PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic);
+      aPromise->mMutex.AssertCurrentThreadOwns();
+      MOZ_ASSERT(!aPromise->IsPending());
+
+      nsCOMPtr<nsIRunnable> r = new ResolveOrRejectRunnable(this, aPromise);
+      PROMISE_LOG(
+          "%s Then() call made from %s [Runnable=%p, Promise=%p, ThenValue=%p] "
+          "%s dispatch",
+          aPromise->mValue.IsResolve() ? "Resolving" : "Rejecting", mCallSite,
+          r.get(), aPromise, this,
+          aPromise->mUseSynchronousTaskDispatch ? "synchronous"
+          : aPromise->mUseDirectTaskDispatch    ? "directtask"
+                                                : "normal");
+
+      if (aPromise->mUseSynchronousTaskDispatch &&
+          mResponseTarget->IsOnCurrentThread()) {
+        PROMISE_LOG("ThenValue::Dispatch running task synchronously [this=%p]",
+                    this);
+        r->Run();
+        return;
+      }
+
+      if (aPromise->mUseDirectTaskDispatch &&
+          mResponseTarget->IsOnCurrentThread()) {
+        PROMISE_LOG(
+            "ThenValue::Dispatch dispatch task via direct task queue [this=%p]",
+            this);
+        nsCOMPtr<nsIDirectTaskDispatcher> dispatcher =
+            do_QueryInterface(mResponseTarget);
+        if (dispatcher) {
+          SetDispatchRv(dispatcher->DispatchDirectTask(r.forget()));
+          return;
+        }
+        NS_WARNING(
+            nsPrintfCString(
+                "Direct Task dispatching not available for thread \"%s\"",
+                PR_GetThreadName(PR_GetCurrentThread()))
+                .get());
+        MOZ_DIAGNOSTIC_ASSERT(
+            false,
+            "mResponseTarget must implement nsIDirectTaskDispatcher for direct "
+            "task dispatching");
+      }
+
+      // Promise consumers are allowed to disconnect the Request object and
+      // then shut down the thread or task queue that the promise result would
+      // be dispatched on. So we unfortunately can't assert that promise
+      // dispatch succeeds. :-(
+      // We do record whether or not it succeeds so that if the ThenValueBase is
+      // then destroyed and it was not disconnected, we can include that
+      // information in the assertion message.
+      SetDispatchRv(mResponseTarget->Dispatch(r.forget()));
+    }
+
+    void Disconnect() override {
+      MOZ_DIAGNOSTIC_ASSERT(mResponseTarget->IsOnCurrentThread());
+      MOZ_DIAGNOSTIC_ASSERT(!Request::mComplete);
+      Request::mDisconnected = true;
+
+      // We could support rejecting the completion promise on disconnection, but
+      // then we'd need to have some sort of default reject value. The use cases
+      // of disconnection and completion promise chaining seem pretty
+      // orthogonal, so let's use assert against it.
+      MOZ_DIAGNOSTIC_ASSERT(!CompletionPromise());
+    }
+
+   protected:
+    virtual MozPromiseBase* CompletionPromise() const = 0;
+    virtual void DoResolveOrRejectInternal(ResolveOrRejectValue& aValue) = 0;
+
+    void DoResolveOrReject(ResolveOrRejectValue& aValue) {
+      PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic);
+      MOZ_DIAGNOSTIC_ASSERT(mResponseTarget->IsOnCurrentThread());
+      Request::mComplete = true;
+      if (Request::mDisconnected) {
+        PROMISE_LOG(
+            "ThenValue::DoResolveOrReject disconnected - bailing out [this=%p]",
+            this);
+        return;
+      }
+
+      // Invoke the resolve or reject method.
+      DoResolveOrRejectInternal(aValue);
+    }
+
+    void SetDispatchRv(nsresult aRv) {
+#  ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+      mDispatchRv = Some(aRv);
+#  endif
+    }
+
+    nsCOMPtr<nsISerialEventTarget>
+        mResponseTarget;  // May be released on any thread.
+#  ifdef PROMISE_DEBUG
+    uint32_t mMagic1 = sMagic;
+#  endif
+    const char* mCallSite;
+#  ifdef PROMISE_DEBUG
+    uint32_t mMagic2 = sMagic;
+#  endif
+#  ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    Maybe<nsresult> mDispatchRv;
+#  endif
+  };
+
+  /*
+   * We create two overloads for invoking Resolve/Reject Methods so as to
+   * make the resolve/reject value argument "optional".
+   */
+  template <typename ThisType, typename MethodType, typename ValueType>
+  static std::enable_if_t<TakesArgument<MethodType>::value,
+                          typename detail::MethodTrait<MethodType>::ReturnType>
+  InvokeMethod(ThisType* aThisVal, MethodType aMethod, ValueType&& aValue) {
+    return (aThisVal->*aMethod)(std::forward<ValueType>(aValue));
+  }
+
+  template <typename ThisType, typename MethodType, typename ValueType>
+  static std::enable_if_t<!TakesArgument<MethodType>::value,
+                          typename detail::MethodTrait<MethodType>::ReturnType>
+  InvokeMethod(ThisType* aThisVal, MethodType aMethod, ValueType&& aValue) {
+    return (aThisVal->*aMethod)();
+  }
+
+  // Called when promise chaining is supported.
+  template <bool SupportChaining, typename ThisType, typename MethodType,
+            typename ValueType, typename CompletionPromiseType>
+  static std::enable_if_t<SupportChaining, void> InvokeCallbackMethod(
+      ThisType* aThisVal, MethodType aMethod, ValueType&& aValue,
+      CompletionPromiseType&& aCompletionPromise) {
+    auto p = InvokeMethod(aThisVal, aMethod, std::forward<ValueType>(aValue));
+    if (aCompletionPromise) {
+      p->ChainTo(aCompletionPromise.forget(), "<chained completion promise>");
+    }
+  }
+
+  // Called when promise chaining is not supported.
+  template <bool SupportChaining, typename ThisType, typename MethodType,
+            typename ValueType, typename CompletionPromiseType>
+  static std::enable_if_t<!SupportChaining, void> InvokeCallbackMethod(
+      ThisType* aThisVal, MethodType aMethod, ValueType&& aValue,
+      CompletionPromiseType&& aCompletionPromise) {
+    MOZ_DIAGNOSTIC_ASSERT(
+        !aCompletionPromise,
+        "Can't do promise chaining for a non-promise-returning method.");
+    InvokeMethod(aThisVal, aMethod, std::forward<ValueType>(aValue));
+  }
+
+  template <typename>
+  class ThenCommand;
+
+  template <typename...>
+  class ThenValue;
+
+  template <typename ThisType, typename ResolveMethodType,
+            typename RejectMethodType>
+  class ThenValue<ThisType*, ResolveMethodType, RejectMethodType>
+      : public ThenValueBase {
+    friend class ThenCommand<ThenValue>;
+
+    using R1 = typename RemoveSmartPointer<
+        typename detail::MethodTrait<ResolveMethodType>::ReturnType>::Type;
+    using R2 = typename RemoveSmartPointer<
+        typename detail::MethodTrait<RejectMethodType>::ReturnType>::Type;
+    using SupportChaining =
+        std::integral_constant<bool, IsMozPromise<R1>::value &&
+                                         std::is_same_v<R1, R2>>;
+
+    // Fall back to MozPromise when promise chaining is not supported to make
+    // code compile.
+    using PromiseType =
+        std::conditional_t<SupportChaining::value, R1, MozPromise>;
+
+   public:
+    ThenValue(nsISerialEventTarget* aResponseTarget, ThisType* aThisVal,
+              ResolveMethodType aResolveMethod, RejectMethodType aRejectMethod,
+              const char* aCallSite)
+        : ThenValueBase(aResponseTarget, aCallSite),
+          mThisVal(aThisVal),
+          mResolveMethod(aResolveMethod),
+          mRejectMethod(aRejectMethod) {}
+
+    void Disconnect() override {
+      ThenValueBase::Disconnect();
+
+      // If a Request has been disconnected, we don't guarantee that the
+      // resolve/reject runnable will be dispatched. Null out our refcounted
+      // this-value now so that it's released predictably on the dispatch
+      // thread.
+      mThisVal = nullptr;
+    }
+
+   protected:
+    MozPromiseBase* CompletionPromise() const override {
+      return mCompletionPromise;
+    }
+
+    void DoResolveOrRejectInternal(ResolveOrRejectValue& aValue) override {
+      if (aValue.IsResolve()) {
+        InvokeCallbackMethod<SupportChaining::value>(
+            mThisVal.get(), mResolveMethod, MaybeMove(aValue.ResolveValue()),
+            std::move(mCompletionPromise));
+      } else {
+        InvokeCallbackMethod<SupportChaining::value>(
+            mThisVal.get(), mRejectMethod, MaybeMove(aValue.RejectValue()),
+            std::move(mCompletionPromise));
+      }
+
+      // Null out mThisVal after invoking the callback so that any references
+      // are released predictably on the dispatch thread. Otherwise, it would be
+      // released on whatever thread last drops its reference to the ThenValue,
+      // which may or may not be ok.
+      mThisVal = nullptr;
+    }
+
+   private:
+    RefPtr<ThisType>
+        mThisVal;  // Only accessed and refcounted on dispatch thread.
+    ResolveMethodType mResolveMethod;
+    RejectMethodType mRejectMethod;
+    RefPtr<typename PromiseType::Private> mCompletionPromise;
+  };
+
+  template <typename ThisType, typename ResolveRejectMethodType>
+  class ThenValue<ThisType*, ResolveRejectMethodType> : public ThenValueBase {
+    friend class ThenCommand<ThenValue>;
+
+    using R1 = typename RemoveSmartPointer<typename detail::MethodTrait<
+        ResolveRejectMethodType>::ReturnType>::Type;
+    using SupportChaining =
+        std::integral_constant<bool, IsMozPromise<R1>::value>;
+
+    // Fall back to MozPromise when promise chaining is not supported to make
+    // code compile.
+    using PromiseType =
+        std::conditional_t<SupportChaining::value, R1, MozPromise>;
+
+   public:
+    ThenValue(nsISerialEventTarget* aResponseTarget, ThisType* aThisVal,
+              ResolveRejectMethodType aResolveRejectMethod,
+              const char* aCallSite)
+        : ThenValueBase(aResponseTarget, aCallSite),
+          mThisVal(aThisVal),
+          mResolveRejectMethod(aResolveRejectMethod) {}
+
+    void Disconnect() override {
+      ThenValueBase::Disconnect();
+
+      // If a Request has been disconnected, we don't guarantee that the
+      // resolve/reject runnable will be dispatched. Null out our refcounted
+      // this-value now so that it's released predictably on the dispatch
+      // thread.
+      mThisVal = nullptr;
+    }
+
+   protected:
+    MozPromiseBase* CompletionPromise() const override {
+      return mCompletionPromise;
+    }
+
+    void DoResolveOrRejectInternal(ResolveOrRejectValue& aValue) override {
+      InvokeCallbackMethod<SupportChaining::value>(
+          mThisVal.get(), mResolveRejectMethod, MaybeMove(aValue),
+          std::move(mCompletionPromise));
+
+      // Null out mThisVal after invoking the callback so that any references
+      // are released predictably on the dispatch thread. Otherwise, it would be
+      // released on whatever thread last drops its reference to the ThenValue,
+      // which may or may not be ok.
+      mThisVal = nullptr;
+    }
+
+   private:
+    RefPtr<ThisType>
+        mThisVal;  // Only accessed and refcounted on dispatch thread.
+    ResolveRejectMethodType mResolveRejectMethod;
+    RefPtr<typename PromiseType::Private> mCompletionPromise;
+  };
+
+  // NB: We could use std::function here instead of a template if it were
+  // supported. :-(
+  template <typename ResolveFunction, typename RejectFunction>
+  class ThenValue<ResolveFunction, RejectFunction> : public ThenValueBase {
+    friend class ThenCommand<ThenValue>;
+
+    using R1 = typename RemoveSmartPointer<
+        typename detail::MethodTrait<ResolveFunction>::ReturnType>::Type;
+    using R2 = typename RemoveSmartPointer<
+        typename detail::MethodTrait<RejectFunction>::ReturnType>::Type;
+    using SupportChaining =
+        std::integral_constant<bool, IsMozPromise<R1>::value &&
+                                         std::is_same_v<R1, R2>>;
+
+    // Fall back to MozPromise when promise chaining is not supported to make
+    // code compile.
+    using PromiseType =
+        std::conditional_t<SupportChaining::value, R1, MozPromise>;
+
+   public:
+    ThenValue(nsISerialEventTarget* aResponseTarget,
+              ResolveFunction&& aResolveFunction,
+              RejectFunction&& aRejectFunction, const char* aCallSite)
+        : ThenValueBase(aResponseTarget, aCallSite) {
+      mResolveFunction.emplace(std::move(aResolveFunction));
+      mRejectFunction.emplace(std::move(aRejectFunction));
+    }
+
+    void Disconnect() override {
+      ThenValueBase::Disconnect();
+
+      // If a Request has been disconnected, we don't guarantee that the
+      // resolve/reject runnable will be dispatched. Destroy our callbacks
+      // now so that any references in closures are released predictable on
+      // the dispatch thread.
+      mResolveFunction.reset();
+      mRejectFunction.reset();
+    }
+
+   protected:
+    MozPromiseBase* CompletionPromise() const override {
+      return mCompletionPromise;
+    }
+
+    void DoResolveOrRejectInternal(ResolveOrRejectValue& aValue) override {
+      // Note: The usage of InvokeCallbackMethod here requires that
+      // ResolveFunction/RejectFunction are capture-lambdas (i.e. anonymous
+      // classes with ::operator()), since it allows us to share code more
+      // easily. We could fix this if need be, though it's quite easy to work
+      // around by just capturing something.
+      if (aValue.IsResolve()) {
+        InvokeCallbackMethod<SupportChaining::value>(
+            mResolveFunction.ptr(), &ResolveFunction::operator(),
+            MaybeMove(aValue.ResolveValue()), std::move(mCompletionPromise));
+      } else {
+        InvokeCallbackMethod<SupportChaining::value>(
+            mRejectFunction.ptr(), &RejectFunction::operator(),
+            MaybeMove(aValue.RejectValue()), std::move(mCompletionPromise));
+      }
+
+      // Destroy callbacks after invocation so that any references in closures
+      // are released predictably on the dispatch thread. Otherwise, they would
+      // be released on whatever thread last drops its reference to the
+      // ThenValue, which may or may not be ok.
+      mResolveFunction.reset();
+      mRejectFunction.reset();
+    }
+
+   private:
+    Maybe<ResolveFunction>
+        mResolveFunction;  // Only accessed and deleted on dispatch thread.
+    Maybe<RejectFunction>
+        mRejectFunction;  // Only accessed and deleted on dispatch thread.
+    RefPtr<typename PromiseType::Private> mCompletionPromise;
+  };
+
+  template <typename ResolveRejectFunction>
+  class ThenValue<ResolveRejectFunction> : public ThenValueBase {
+    friend class ThenCommand<ThenValue>;
+
+    using R1 = typename RemoveSmartPointer<
+        typename detail::MethodTrait<ResolveRejectFunction>::ReturnType>::Type;
+    using SupportChaining =
+        std::integral_constant<bool, IsMozPromise<R1>::value>;
+
+    // Fall back to MozPromise when promise chaining is not supported to make
+    // code compile.
+    using PromiseType =
+        std::conditional_t<SupportChaining::value, R1, MozPromise>;
+
+   public:
+    ThenValue(nsISerialEventTarget* aResponseTarget,
+              ResolveRejectFunction&& aResolveRejectFunction,
+              const char* aCallSite)
+        : ThenValueBase(aResponseTarget, aCallSite) {
+      mResolveRejectFunction.emplace(std::move(aResolveRejectFunction));
+    }
+
+    void Disconnect() override {
+      ThenValueBase::Disconnect();
+
+      // If a Request has been disconnected, we don't guarantee that the
+      // resolve/reject runnable will be dispatched. Destroy our callbacks
+      // now so that any references in closures are released predictable on
+      // the dispatch thread.
+      mResolveRejectFunction.reset();
+    }
+
+   protected:
+    MozPromiseBase* CompletionPromise() const override {
+      return mCompletionPromise;
+    }
+
+    void DoResolveOrRejectInternal(ResolveOrRejectValue& aValue) override {
+      // Note: The usage of InvokeCallbackMethod here requires that
+      // ResolveRejectFunction is capture-lambdas (i.e. anonymous
+      // classes with ::operator()), since it allows us to share code more
+      // easily. We could fix this if need be, though it's quite easy to work
+      // around by just capturing something.
+      InvokeCallbackMethod<SupportChaining::value>(
+          mResolveRejectFunction.ptr(), &ResolveRejectFunction::operator(),
+          MaybeMove(aValue), std::move(mCompletionPromise));
+
+      // Destroy callbacks after invocation so that any references in closures
+      // are released predictably on the dispatch thread. Otherwise, they would
+      // be released on whatever thread last drops its reference to the
+      // ThenValue, which may or may not be ok.
+      mResolveRejectFunction.reset();
+    }
+
+   private:
+    Maybe<ResolveRejectFunction>
+        mResolveRejectFunction;  // Only accessed and deleted on dispatch
+                                 // thread.
+    RefPtr<typename PromiseType::Private> mCompletionPromise;
+  };
+
+ public:
+  void ThenInternal(already_AddRefed<ThenValueBase> aThenValue,
+                    const char* aCallSite) {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    RefPtr<ThenValueBase> thenValue = aThenValue;
+    MutexAutoLock lock(mMutex);
+    MOZ_DIAGNOSTIC_ASSERT(
+        !IsExclusive || !mHaveRequest,
+        "Using an exclusive promise in a non-exclusive fashion");
+    mHaveRequest = true;
+    PROMISE_LOG("%s invoking Then() [this=%p, aThenValue=%p, isPending=%d]",
+                aCallSite, this, thenValue.get(), (int)IsPending());
+    if (!IsPending()) {
+      thenValue->Dispatch(this);
+    } else {
+      mThenValues.AppendElement(thenValue.forget());
+    }
+  }
+
+ protected:
+  /*
+   * A command object to store all information needed to make a request to
+   * the promise. This allows us to delay the request until further use is
+   * known (whether it is ->Then() again for more promise chaining or ->Track()
+   * to terminate chaining and issue the request).
+   *
+   * This allows a unified syntax for promise chaining and disconnection
+   * and feels more like its JS counterpart.
+   */
+  template <typename ThenValueType>
+  class ThenCommand {
+    // Allow Promise1::ThenCommand to access the private constructor,
+    // Promise2::ThenCommand(ThenCommand&&).
+    template <typename, typename, bool>
+    friend class MozPromise;
+
+    using PromiseType = typename ThenValueType::PromiseType;
+    using Private = typename PromiseType::Private;
+
+    ThenCommand(const char* aCallSite,
+                already_AddRefed<ThenValueType> aThenValue,
+                MozPromise* aReceiver)
+        : mCallSite(aCallSite), mThenValue(aThenValue), mReceiver(aReceiver) {}
+
+    ThenCommand(ThenCommand&& aOther) = default;
+
+   public:
+    ~ThenCommand() {
+      // Issue the request now if the return value of Then() is not used.
+      if (mThenValue) {
+        mReceiver->ThenInternal(mThenValue.forget(), mCallSite);
+      }
+    }
+
+    // Allow RefPtr<MozPromise> p = somePromise->Then();
+    //       p->Then(thread1, ...);
+    //       p->Then(thread2, ...);
+    operator RefPtr<PromiseType>() {
+      static_assert(
+          ThenValueType::SupportChaining::value,
+          "The resolve/reject callback needs to return a RefPtr<MozPromise> "
+          "in order to do promise chaining.");
+
+      // mCompletionPromise must be created before ThenInternal() to avoid race.
+      RefPtr<Private> p =
+          new Private("<completion promise>", true /* aIsCompletionPromise */);
+      mThenValue->mCompletionPromise = p;
+      // Note ThenInternal() might nullify mCompletionPromise before return.
+      // So we need to return p instead of mCompletionPromise.
+      mReceiver->ThenInternal(mThenValue.forget(), mCallSite);
+      return p;
+    }
+
+    template <typename... Ts>
+    auto Then(Ts&&... aArgs) -> decltype(std::declval<PromiseType>().Then(
+        std::forward<Ts>(aArgs)...)) {
+      return static_cast<RefPtr<PromiseType>>(*this)->Then(
+          std::forward<Ts>(aArgs)...);
+    }
+
+    void Track(MozPromiseRequestHolder<MozPromise>& aRequestHolder) {
+      aRequestHolder.Track(do_AddRef(mThenValue));
+      mReceiver->ThenInternal(mThenValue.forget(), mCallSite);
+    }
+
+    // Allow calling ->Then() again for more promise chaining or ->Track() to
+    // end chaining and track the request for future disconnection.
+    ThenCommand* operator->() { return this; }
+
+   private:
+    const char* mCallSite;
+    RefPtr<ThenValueType> mThenValue;
+    RefPtr<MozPromise> mReceiver;
+  };
+
+ public:
+  template <typename ThisType, typename... Methods,
+            typename ThenValueType = ThenValue<ThisType*, Methods...>,
+            typename ReturnType = ThenCommand<ThenValueType>>
+  ReturnType Then(nsISerialEventTarget* aResponseTarget, const char* aCallSite,
+                  ThisType* aThisVal, Methods... aMethods) {
+    RefPtr<ThenValueType> thenValue =
+        new ThenValueType(aResponseTarget, aThisVal, aMethods..., aCallSite);
+    return ReturnType(aCallSite, thenValue.forget(), this);
+  }
+
+  template <typename... Functions,
+            typename ThenValueType = ThenValue<Functions...>,
+            typename ReturnType = ThenCommand<ThenValueType>>
+  ReturnType Then(nsISerialEventTarget* aResponseTarget, const char* aCallSite,
+                  Functions&&... aFunctions) {
+    RefPtr<ThenValueType> thenValue =
+        new ThenValueType(aResponseTarget, std::move(aFunctions)..., aCallSite);
+    return ReturnType(aCallSite, thenValue.forget(), this);
+  }
+
+  void ChainTo(already_AddRefed<Private> aChainedPromise,
+               const char* aCallSite) {
+    MutexAutoLock lock(mMutex);
+    MOZ_DIAGNOSTIC_ASSERT(
+        !IsExclusive || !mHaveRequest,
+        "Using an exclusive promise in a non-exclusive fashion");
+    mHaveRequest = true;
+    RefPtr<Private> chainedPromise = aChainedPromise;
+    PROMISE_LOG(
+        "%s invoking Chain() [this=%p, chainedPromise=%p, isPending=%d]",
+        aCallSite, this, chainedPromise.get(), (int)IsPending());
+
+    // We want to use the same type of dispatching method with the chained
+    // promises.
+
+    // We need to ensure that the UseSynchronousTaskDispatch branch isn't taken
+    // at compilation time to ensure we're not triggering the static_assert in
+    // UseSynchronousTaskDispatch method. if constexpr (IsExclusive) ensures
+    // that.
+    if (mUseDirectTaskDispatch) {
+      chainedPromise->UseDirectTaskDispatch(aCallSite);
+    } else if constexpr (IsExclusive) {
+      if (mUseSynchronousTaskDispatch) {
+        chainedPromise->UseSynchronousTaskDispatch(aCallSite);
+      }
+    } else {
+      chainedPromise->SetTaskPriority(mPriority, aCallSite);
+    }
+
+    if (!IsPending()) {
+      ForwardTo(chainedPromise);
+    } else {
+      mChainedPromises.AppendElement(chainedPromise);
+    }
+  }
+
+#  ifdef MOZ_WIDGET_ANDROID
+  // Creates a C++ MozPromise from its Java counterpart, GeckoResult.
+  [[nodiscard]] static RefPtr<MozPromise> FromGeckoResult(
+      java::GeckoResult::Param aGeckoResult) {
+    using jni::GeckoResultCallback;
+    RefPtr<Private> p = new Private("GeckoResult Glue", false);
+    auto resolve = GeckoResultCallback::CreateAndAttach<ResolveValueType>(
+        [p](ResolveValueType&& aArg) {
+          p->Resolve(MaybeMove(aArg), __func__);
+        });
+    auto reject = GeckoResultCallback::CreateAndAttach<RejectValueType>(
+        [p](RejectValueType&& aArg) { p->Reject(MaybeMove(aArg), __func__); });
+    aGeckoResult->NativeThen(resolve, reject);
+    return p;
+  }
+#  endif
+
+  // Creates a C++ MozPromise from its JS counterpart, dom::Promise.
+  // FromDomPromise currently only supports primitive types (int8/16/32, float,
+  // double) And the reject value type must be a nsresult.
+  // To use, please include MozPromiseInlines.h
+  static RefPtr<MozPromise> FromDomPromise(dom::Promise* aDOMPromise);
+
+  // Note we expose the function AssertIsDead() instead of IsDead() since
+  // checking IsDead() is a data race in the situation where the request is not
+  // dead. Therefore we enforce the form |Assert(IsDead())| by exposing
+  // AssertIsDead() only.
+  void AssertIsDead() override {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    for (auto&& then : mThenValues) {
+      then->AssertIsDead();
+    }
+    for (auto&& chained : mChainedPromises) {
+      chained->AssertIsDead();
+    }
+  }
+
+  bool IsResolved() const { return mValue.IsResolve(); }
+
+ protected:
+  bool IsPending() const { return mValue.IsNothing(); }
+
+  ResolveOrRejectValue& Value() {
+    // This method should only be called once the value has stabilized. As
+    // such, we don't need to acquire the lock here.
+    MOZ_DIAGNOSTIC_ASSERT(!IsPending());
+    return mValue;
+  }
+
+  void DispatchAll() {
+    mMutex.AssertCurrentThreadOwns();
+    for (auto&& thenValue : mThenValues) {
+      thenValue->Dispatch(this);
+    }
+    mThenValues.Clear();
+
+    for (auto&& chainedPromise : mChainedPromises) {
+      ForwardTo(chainedPromise);
+    }
+    mChainedPromises.Clear();
+  }
+
+  void ForwardTo(Private* aOther) {
+    MOZ_ASSERT(!IsPending());
+    if (mValue.IsResolve()) {
+      aOther->Resolve(MaybeMove(mValue.ResolveValue()), "<chained promise>");
+    } else {
+      aOther->Reject(MaybeMove(mValue.RejectValue()), "<chained promise>");
+    }
+  }
+
+  virtual ~MozPromise() {
+    PROMISE_LOG("MozPromise::~MozPromise [this=%p]", this);
+    AssertIsDead();
+    // We can't guarantee a completion promise will always be revolved or
+    // rejected since ResolveOrRejectRunnable might not run when dispatch fails.
+    if (!mIsCompletionPromise) {
+      MOZ_ASSERT(!IsPending());
+      MOZ_ASSERT(mThenValues.IsEmpty());
+      MOZ_ASSERT(mChainedPromises.IsEmpty());
+    }
+#  ifdef PROMISE_DEBUG
+    mMagic1 = 0;
+    mMagic2 = 0;
+    mMagic3 = 0;
+    mMagic4 = nullptr;
+#  endif
+  };
+
+  const char* mCreationSite;  // For logging
+  Mutex mMutex MOZ_UNANNOTATED;
+  ResolveOrRejectValue mValue;
+  bool mUseSynchronousTaskDispatch = false;
+  bool mUseDirectTaskDispatch = false;
+  uint32_t mPriority = nsIRunnablePriority::PRIORITY_NORMAL;
+#  ifdef PROMISE_DEBUG
+  uint32_t mMagic1 = sMagic;
+#  endif
+  // Try shows we never have more than 3 elements when IsExclusive is false.
+  // So '3' is a good value to avoid heap allocation in most cases.
+  AutoTArray<RefPtr<ThenValueBase>, IsExclusive ? 1 : 3> mThenValues;
+#  ifdef PROMISE_DEBUG
+  uint32_t mMagic2 = sMagic;
+#  endif
+  nsTArray<RefPtr<Private>> mChainedPromises;
+#  ifdef PROMISE_DEBUG
+  uint32_t mMagic3 = sMagic;
+#  endif
+  bool mHaveRequest;
+  const bool mIsCompletionPromise;
+#  ifdef PROMISE_DEBUG
+  void* mMagic4;
+#  endif
+};
+
+template <typename ResolveValueT, typename RejectValueT, bool IsExclusive>
+class MozPromise<ResolveValueT, RejectValueT, IsExclusive>::Private
+    : public MozPromise<ResolveValueT, RejectValueT, IsExclusive> {
+ public:
+  explicit Private(const char* aCreationSite, bool aIsCompletionPromise = false)
+      : MozPromise(aCreationSite, aIsCompletionPromise) {}
+
+  template <typename ResolveValueT_>
+  void Resolve(ResolveValueT_&& aResolveValue, const char* aResolveSite) {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    PROMISE_LOG("%s resolving MozPromise (%p created at %s)", aResolveSite,
+                this, mCreationSite);
+    if (!IsPending()) {
+      PROMISE_LOG(
+          "%s ignored already resolved or rejected MozPromise (%p created at "
+          "%s)",
+          aResolveSite, this, mCreationSite);
+      return;
+    }
+    mValue.SetResolve(std::forward<ResolveValueT_>(aResolveValue));
+    DispatchAll();
+  }
+
+  template <typename RejectValueT_>
+  void Reject(RejectValueT_&& aRejectValue, const char* aRejectSite) {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    PROMISE_LOG("%s rejecting MozPromise (%p created at %s)", aRejectSite, this,
+                mCreationSite);
+    if (!IsPending()) {
+      PROMISE_LOG(
+          "%s ignored already resolved or rejected MozPromise (%p created at "
+          "%s)",
+          aRejectSite, this, mCreationSite);
+      return;
+    }
+    mValue.SetReject(std::forward<RejectValueT_>(aRejectValue));
+    DispatchAll();
+  }
+
+  template <typename ResolveOrRejectValue_>
+  void ResolveOrReject(ResolveOrRejectValue_&& aValue, const char* aSite) {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    PROMISE_LOG("%s resolveOrRejecting MozPromise (%p created at %s)", aSite,
+                this, mCreationSite);
+    if (!IsPending()) {
+      PROMISE_LOG(
+          "%s ignored already resolved or rejected MozPromise (%p created at "
+          "%s)",
+          aSite, this, mCreationSite);
+      return;
+    }
+    mValue = std::forward<ResolveOrRejectValue_>(aValue);
+    DispatchAll();
+  }
+
+  // If the caller and target are both on the same thread, run the the resolve
+  // or reject callback synchronously. Otherwise, the task will be dispatched
+  // via the target Dispatch method.
+  void UseSynchronousTaskDispatch(const char* aSite) {
+    static_assert(
+        IsExclusive,
+        "Synchronous dispatch can only be used with exclusive promises");
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    PROMISE_LOG("%s UseSynchronousTaskDispatch MozPromise (%p created at %s)",
+                aSite, this, mCreationSite);
+    MOZ_ASSERT(IsPending(),
+               "A Promise must not have been already resolved or rejected to "
+               "set dispatch state");
+    mUseSynchronousTaskDispatch = true;
+  }
+
+  // If the caller and target are both on the same thread, run the
+  // resolve/reject callback off the direct task queue instead. This avoids a
+  // full trip to the back of the event queue for each additional asynchronous
+  // step when using MozPromise, and is similar (but not identical to) the
+  // microtask semantics of JS promises.
+  void UseDirectTaskDispatch(const char* aSite) {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    PROMISE_LOG("%s UseDirectTaskDispatch MozPromise (%p created at %s)", aSite,
+                this, mCreationSite);
+    MOZ_ASSERT(IsPending(),
+               "A Promise must not have been already resolved or rejected to "
+               "set dispatch state");
+    MOZ_ASSERT(!mUseSynchronousTaskDispatch,
+               "Promise already set for synchronous dispatch");
+    mUseDirectTaskDispatch = true;
+  }
+
+  // If the resolve/reject will be handled on a thread supporting priorities,
+  // one may want to tweak the priority of the task by passing a
+  // nsIRunnablePriority::PRIORITY_* to SetTaskPriority.
+  void SetTaskPriority(uint32_t aPriority, const char* aSite) {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    PROMISE_LOG("%s TaskPriority MozPromise (%p created at %s)", aSite, this,
+                mCreationSite);
+    MOZ_ASSERT(IsPending(),
+               "A Promise must not have been already resolved or rejected to "
+               "set dispatch state");
+    MOZ_ASSERT(!mUseSynchronousTaskDispatch,
+               "Promise already set for synchronous dispatch");
+    MOZ_ASSERT(!mUseDirectTaskDispatch,
+               "Promise already set for direct dispatch");
+    mPriority = aPriority;
+  }
+};
+
+// A generic promise type that does the trick for simple use cases.
+typedef MozPromise<bool, nsresult, /* IsExclusive = */ true> GenericPromise;
+
+// A generic, non-exclusive promise type that does the trick for simple use
+// cases.
+typedef MozPromise<bool, nsresult, /* IsExclusive = */ false>
+    GenericNonExclusivePromise;
+
+/*
+ * Class to encapsulate a promise for a particular role. Use this as the member
+ * variable for a class whose method returns a promise.
+ */
+template <typename PromiseType, typename ImplType>
+class MozPromiseHolderBase {
+ public:
+  MozPromiseHolderBase() = default;
+
+  MozPromiseHolderBase(MozPromiseHolderBase&& aOther) = default;
+  MozPromiseHolderBase& operator=(MozPromiseHolderBase&& aOther) = default;
+
+  ~MozPromiseHolderBase() { MOZ_ASSERT(!mPromise); }
+
+  already_AddRefed<PromiseType> Ensure(const char* aMethodName) {
+    static_cast<ImplType*>(this)->Check();
+    if (!mPromise) {
+      mPromise = new (typename PromiseType::Private)(aMethodName);
+    }
+    RefPtr<PromiseType> p = mPromise.get();
+    return p.forget();
+  }
+
+  bool IsEmpty() const {
+    static_cast<const ImplType*>(this)->Check();
+    return !mPromise;
+  }
+
+  already_AddRefed<typename PromiseType::Private> Steal() {
+    static_cast<ImplType*>(this)->Check();
+    return mPromise.forget();
+  }
+
+  template <typename ResolveValueType_>
+  void Resolve(ResolveValueType_&& aResolveValue, const char* aMethodName) {
+    static_assert(std::is_convertible_v<ResolveValueType_,
+                                        typename PromiseType::ResolveValueType>,
+                  "Resolve() argument must be implicitly convertible to "
+                  "MozPromise's ResolveValueT");
+
+    static_cast<ImplType*>(this)->Check();
+    MOZ_ASSERT(mPromise);
+    mPromise->Resolve(std::forward<ResolveValueType_>(aResolveValue),
+                      aMethodName);
+    mPromise = nullptr;
+  }
+
+  template <typename ResolveValueType_>
+  void ResolveIfExists(ResolveValueType_&& aResolveValue,
+                       const char* aMethodName) {
+    if (!IsEmpty()) {
+      Resolve(std::forward<ResolveValueType_>(aResolveValue), aMethodName);
+    }
+  }
+
+  template <typename RejectValueType_>
+  void Reject(RejectValueType_&& aRejectValue, const char* aMethodName) {
+    static_assert(std::is_convertible_v<RejectValueType_,
+                                        typename PromiseType::RejectValueType>,
+                  "Reject() argument must be implicitly convertible to "
+                  "MozPromise's RejectValueT");
+
+    static_cast<ImplType*>(this)->Check();
+    MOZ_ASSERT(mPromise);
+    mPromise->Reject(std::forward<RejectValueType_>(aRejectValue), aMethodName);
+    mPromise = nullptr;
+  }
+
+  template <typename RejectValueType_>
+  void RejectIfExists(RejectValueType_&& aRejectValue,
+                      const char* aMethodName) {
+    if (!IsEmpty()) {
+      Reject(std::forward<RejectValueType_>(aRejectValue), aMethodName);
+    }
+  }
+
+  template <typename ResolveOrRejectValueType_>
+  void ResolveOrReject(ResolveOrRejectValueType_&& aValue,
+                       const char* aMethodName) {
+    static_cast<ImplType*>(this)->Check();
+    MOZ_ASSERT(mPromise);
+    mPromise->ResolveOrReject(std::forward<ResolveOrRejectValueType_>(aValue),
+                              aMethodName);
+    mPromise = nullptr;
+  }
+
+  template <typename ResolveOrRejectValueType_>
+  void ResolveOrRejectIfExists(ResolveOrRejectValueType_&& aValue,
+                               const char* aMethodName) {
+    if (!IsEmpty()) {
+      ResolveOrReject(std::forward<ResolveOrRejectValueType_>(aValue),
+                      aMethodName);
+    }
+  }
+
+  void UseSynchronousTaskDispatch(const char* aSite) {
+    MOZ_ASSERT(mPromise);
+    mPromise->UseSynchronousTaskDispatch(aSite);
+  }
+
+  void UseDirectTaskDispatch(const char* aSite) {
+    MOZ_ASSERT(mPromise);
+    mPromise->UseDirectTaskDispatch(aSite);
+  }
+
+  void SetTaskPriority(uint32_t aPriority, const char* aSite) {
+    MOZ_ASSERT(mPromise);
+    mPromise->SetTaskPriority(aPriority, aSite);
+  }
+
+ private:
+  RefPtr<typename PromiseType::Private> mPromise;
+};
+
+template <typename PromiseType>
+class MozPromiseHolder
+    : public MozPromiseHolderBase<PromiseType, MozPromiseHolder<PromiseType>> {
+ public:
+  using MozPromiseHolderBase<
+      PromiseType, MozPromiseHolder<PromiseType>>::MozPromiseHolderBase;
+  static constexpr void Check(){};
+};
+
+template <typename PromiseType>
+class MozMonitoredPromiseHolder
+    : public MozPromiseHolderBase<PromiseType,
+                                  MozMonitoredPromiseHolder<PromiseType>> {
+ public:
+  // Provide a Monitor that should always be held when accessing this instance.
+  explicit MozMonitoredPromiseHolder(Monitor* const aMonitor)
+      : mMonitor(aMonitor) {
+    MOZ_ASSERT(aMonitor);
+  }
+
+  MozMonitoredPromiseHolder(MozMonitoredPromiseHolder&& aOther) = delete;
+  MozMonitoredPromiseHolder& operator=(MozMonitoredPromiseHolder&& aOther) =
+      delete;
+
+  void Check() const { mMonitor->AssertCurrentThreadOwns(); }
+
+ private:
+  Monitor* const mMonitor;
+};
+
+/*
+ * Class to encapsulate a MozPromise::Request reference. Use this as the member
+ * variable for a class waiting on a MozPromise.
+ */
+template <typename PromiseType>
+class MozPromiseRequestHolder {
+ public:
+  MozPromiseRequestHolder() = default;
+  ~MozPromiseRequestHolder() { MOZ_ASSERT(!mRequest); }
+
+  void Track(already_AddRefed<typename PromiseType::Request> aRequest) {
+    MOZ_DIAGNOSTIC_ASSERT(!Exists());
+    mRequest = aRequest;
+  }
+
+  void Complete() {
+    MOZ_DIAGNOSTIC_ASSERT(Exists());
+    mRequest = nullptr;
+  }
+
+  // Disconnects and forgets an outstanding promise. The resolve/reject methods
+  // will never be called.
+  void Disconnect() {
+    MOZ_ASSERT(Exists());
+    mRequest->Disconnect();
+    mRequest = nullptr;
+  }
+
+  void DisconnectIfExists() {
+    if (Exists()) {
+      Disconnect();
+    }
+  }
+
+  bool Exists() const { return !!mRequest; }
+
+ private:
+  RefPtr<typename PromiseType::Request> mRequest;
+};
+
+// Asynchronous Potentially-Cross-Thread Method Calls.
+//
+// This machinery allows callers to schedule a promise-returning function
+// (a method and object, or a function object like a lambda) to be invoked
+// asynchronously on a given thread, while at the same time receiving a
+// promise upon which to invoke Then() immediately. InvokeAsync dispatches a
+// task to invoke the function on the proper thread and also chain the
+// resulting promise to the one that the caller received, so that resolve/
+// reject values are forwarded through.
+
+namespace detail {
+
+// Non-templated base class to allow us to use MOZ_COUNT_{C,D}TOR, which cause
+// assertions when used on templated types.
+class MethodCallBase {
+ public:
+  MOZ_COUNTED_DEFAULT_CTOR(MethodCallBase)
+  MOZ_COUNTED_DTOR_VIRTUAL(MethodCallBase)
+};
+
+template <typename PromiseType, typename MethodType, typename ThisType,
+          typename... Storages>
+class MethodCall : public MethodCallBase {
+ public:
+  template <typename... Args>
+  MethodCall(MethodType aMethod, ThisType* aThisVal, Args&&... aArgs)
+      : mMethod(aMethod),
+        mThisVal(aThisVal),
+        mArgs(std::forward<Args>(aArgs)...) {
+    static_assert(sizeof...(Storages) == sizeof...(Args),
+                  "Storages and Args should have equal sizes");
+  }
+
+  RefPtr<PromiseType> Invoke() { return mArgs.apply(mThisVal.get(), mMethod); }
+
+ private:
+  MethodType mMethod;
+  RefPtr<ThisType> mThisVal;
+  RunnableMethodArguments<Storages...> mArgs;
+};
+
+template <typename PromiseType, typename MethodType, typename ThisType,
+          typename... Storages>
+class ProxyRunnable : public CancelableRunnable {
+ public:
+  ProxyRunnable(
+      typename PromiseType::Private* aProxyPromise,
+      MethodCall<PromiseType, MethodType, ThisType, Storages...>* aMethodCall)
+      : CancelableRunnable("detail::ProxyRunnable"),
+        mProxyPromise(aProxyPromise),
+        mMethodCall(aMethodCall) {}
+
+  NS_IMETHOD Run() override {
+    RefPtr<PromiseType> p = mMethodCall->Invoke();
+    mMethodCall = nullptr;
+    p->ChainTo(mProxyPromise.forget(), "<Proxy Promise>");
+    return NS_OK;
+  }
+
+  nsresult Cancel() override { return Run(); }
+
+ private:
+  RefPtr<typename PromiseType::Private> mProxyPromise;
+  UniquePtr<MethodCall<PromiseType, MethodType, ThisType, Storages...>>
+      mMethodCall;
+};
+
+template <typename... Storages, typename PromiseType, typename ThisType,
+          typename... ArgTypes, typename... ActualArgTypes>
+static RefPtr<PromiseType> InvokeAsyncImpl(
+    nsISerialEventTarget* aTarget, ThisType* aThisVal, const char* aCallerName,
+    RefPtr<PromiseType> (ThisType::*aMethod)(ArgTypes...),
+    ActualArgTypes&&... aArgs) {
+  MOZ_ASSERT(aTarget);
+
+  typedef RefPtr<PromiseType> (ThisType::*MethodType)(ArgTypes...);
+  typedef detail::MethodCall<PromiseType, MethodType, ThisType, Storages...>
+      MethodCallType;
+  typedef detail::ProxyRunnable<PromiseType, MethodType, ThisType, Storages...>
+      ProxyRunnableType;
+
+  MethodCallType* methodCall = new MethodCallType(
+      aMethod, aThisVal, std::forward<ActualArgTypes>(aArgs)...);
+  RefPtr<typename PromiseType::Private> p =
+      new (typename PromiseType::Private)(aCallerName);
+  RefPtr<ProxyRunnableType> r = new ProxyRunnableType(p, methodCall);
+  aTarget->Dispatch(r.forget());
+  return p;
+}
+
+constexpr bool Any() { return false; }
+
+template <typename T1>
+constexpr bool Any(T1 a) {
+  return static_cast<bool>(a);
+}
+
+template <typename T1, typename... Ts>
+constexpr bool Any(T1 a, Ts... aOthers) {
+  return a || Any(aOthers...);
+}
+
+}  // namespace detail
+
+// InvokeAsync with explicitly-specified storages.
+// See ParameterStorage in nsThreadUtils.h for help.
+template <typename... Storages, typename PromiseType, typename ThisType,
+          typename... ArgTypes, typename... ActualArgTypes,
+          std::enable_if_t<sizeof...(Storages) != 0, int> = 0>
+static RefPtr<PromiseType> InvokeAsync(
+    nsISerialEventTarget* aTarget, ThisType* aThisVal, const char* aCallerName,
+    RefPtr<PromiseType> (ThisType::*aMethod)(ArgTypes...),
+    ActualArgTypes&&... aArgs) {
+  static_assert(
+      sizeof...(Storages) == sizeof...(ArgTypes),
+      "Provided Storages and method's ArgTypes should have equal sizes");
+  static_assert(sizeof...(Storages) == sizeof...(ActualArgTypes),
+                "Provided Storages and ActualArgTypes should have equal sizes");
+  return detail::InvokeAsyncImpl<Storages...>(
+      aTarget, aThisVal, aCallerName, aMethod,
+      std::forward<ActualArgTypes>(aArgs)...);
+}
+
+// InvokeAsync with no explicitly-specified storages, will copy arguments and
+// then move them out of the runnable into the target method parameters.
+template <typename... Storages, typename PromiseType, typename ThisType,
+          typename... ArgTypes, typename... ActualArgTypes,
+          std::enable_if_t<sizeof...(Storages) == 0, int> = 0>
+static RefPtr<PromiseType> InvokeAsync(
+    nsISerialEventTarget* aTarget, ThisType* aThisVal, const char* aCallerName,
+    RefPtr<PromiseType> (ThisType::*aMethod)(ArgTypes...),
+    ActualArgTypes&&... aArgs) {
+  static_assert(
+      !detail::Any(
+          std::is_pointer_v<std::remove_reference_t<ActualArgTypes>>...),
+      "Cannot pass pointer types through InvokeAsync, Storages must be "
+      "provided");
+  static_assert(sizeof...(ArgTypes) == sizeof...(ActualArgTypes),
+                "Method's ArgTypes and ActualArgTypes should have equal sizes");
+  return detail::InvokeAsyncImpl<
+      StoreCopyPassByRRef<std::decay_t<ActualArgTypes>>...>(
+      aTarget, aThisVal, aCallerName, aMethod,
+      std::forward<ActualArgTypes>(aArgs)...);
+}
+
+namespace detail {
+
+template <typename Function, typename PromiseType>
+class ProxyFunctionRunnable : public CancelableRunnable {
+  using FunctionStorage = std::decay_t<Function>;
+
+ public:
+  template <typename F>
+  ProxyFunctionRunnable(typename PromiseType::Private* aProxyPromise,
+                        F&& aFunction)
+      : CancelableRunnable("detail::ProxyFunctionRunnable"),
+        mProxyPromise(aProxyPromise),
+        mFunction(new FunctionStorage(std::forward<F>(aFunction))) {}
+
+  NS_IMETHOD Run() override {
+    RefPtr<PromiseType> p = (*mFunction)();
+    mFunction = nullptr;
+    p->ChainTo(mProxyPromise.forget(), "<Proxy Promise>");
+    return NS_OK;
+  }
+
+  nsresult Cancel() override { return Run(); }
+
+ private:
+  RefPtr<typename PromiseType::Private> mProxyPromise;
+  UniquePtr<FunctionStorage> mFunction;
+};
+
+// Note: The following struct and function are not for public consumption (yet?)
+// as we would prefer all calls to pass on-the-spot lambdas (or at least moved
+// function objects). They could be moved outside of detail if really needed.
+
+// We prefer getting function objects by non-lvalue-ref (to avoid copying them
+// and their captures). This struct is a tag that allows the use of objects
+// through lvalue-refs where necessary.
+struct AllowInvokeAsyncFunctionLVRef {};
+
+// Invoke a function object (e.g., lambda or std/mozilla::function)
+// asynchronously; note that the object will be copied if provided by
+// lvalue-ref. Return a promise that the function should eventually resolve or
+// reject.
+template <typename Function>
+static auto InvokeAsync(nsISerialEventTarget* aTarget, const char* aCallerName,
+                        AllowInvokeAsyncFunctionLVRef, Function&& aFunction)
+    -> decltype(aFunction()) {
+  static_assert(
+      IsRefcountedSmartPointer<decltype(aFunction())>::value &&
+          IsMozPromise<
+              typename RemoveSmartPointer<decltype(aFunction())>::Type>::value,
+      "Function object must return RefPtr<MozPromise>");
+  MOZ_ASSERT(aTarget);
+  typedef typename RemoveSmartPointer<decltype(aFunction())>::Type PromiseType;
+  typedef detail::ProxyFunctionRunnable<Function, PromiseType>
+      ProxyRunnableType;
+
+  auto p = MakeRefPtr<typename PromiseType::Private>(aCallerName);
+  auto r = MakeRefPtr<ProxyRunnableType>(p, std::forward<Function>(aFunction));
+  aTarget->Dispatch(r.forget());
+  return p;
+}
+
+}  // namespace detail
+
+// Invoke a function object (e.g., lambda) asynchronously.
+// Return a promise that the function should eventually resolve or reject.
+template <typename Function>
+static auto InvokeAsync(nsISerialEventTarget* aTarget, const char* aCallerName,
+                        Function&& aFunction) -> decltype(aFunction()) {
+  static_assert(!std::is_lvalue_reference_v<Function>,
+                "Function object must not be passed by lvalue-ref (to avoid "
+                "unplanned copies); Consider move()ing the object.");
+  return detail::InvokeAsync(aTarget, aCallerName,
+                             detail::AllowInvokeAsyncFunctionLVRef(),
+                             std::forward<Function>(aFunction));
+}
+
+#  undef PROMISE_LOG
+#  undef PROMISE_ASSERT
+#  undef PROMISE_DEBUG
+
+}  // namespace mozilla
+
+#endif