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diff --git a/dom/webauthn/WebAuthnManager.h b/dom/webauthn/WebAuthnManager.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/. */
+
+#ifndef mozilla_dom_WebAuthnManager_h
+#define mozilla_dom_WebAuthnManager_h
+
+#include "mozilla/Maybe.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/RandomNum.h"
+#include "mozilla/dom/AbortSignal.h"
+#include "mozilla/dom/PWebAuthnTransaction.h"
+#include "mozilla/dom/WebAuthnManagerBase.h"
+
+/*
+ * Content process manager for the WebAuthn protocol. Created on calls to the
+ * WebAuthentication DOM object, this manager handles establishing IPC channels
+ * for WebAuthn transactions, as well as keeping track of JS Promise objects
+ * representing transactions in flight.
+ *
+ * The WebAuthn spec (https://www.w3.org/TR/webauthn/) allows for two different
+ * types of transactions: registration and signing. When either of these is
+ * requested via the DOM API, the following steps are executed in the
+ * WebAuthnManager:
+ *
+ * - Validation of the request. Return a failed promise to js if request does
+ *   not have correct parameters.
+ *
+ * - If request is valid, open a new IPC channel for running the transaction. If
+ *   another transaction is already running in this content process, cancel it.
+ *   Return a pending promise to js.
+ *
+ * - Send transaction information to parent process (by running the Start*
+ *   functions of WebAuthnManager). Assuming another transaction is currently in
+ *   flight in another content process, parent will handle canceling it.
+ *
+ * - On return of successful transaction information from parent process, turn
+ *   information into DOM object format required by spec, and resolve promise
+ *   (by running the Finish* functions of WebAuthnManager). On cancellation
+ *   request from parent, reject promise with corresponding error code. Either
+ *   outcome will also close the IPC channel.
+ *
+ */
+
+namespace mozilla::dom {
+
+class Credential;
+
+class WebAuthnTransaction {
+ public:
+  explicit WebAuthnTransaction(const RefPtr<Promise>& aPromise)
+      : mPromise(aPromise), mId(NextId()), mVisibilityChanged(false) {
+    MOZ_ASSERT(mId > 0);
+  }
+
+  // JS Promise representing the transaction status.
+  RefPtr<Promise> mPromise;
+
+  // Unique transaction id.
+  uint64_t mId;
+
+  // Whether or not visibility has changed for the window during this
+  // transaction
+  bool mVisibilityChanged;
+
+ private:
+  // Generates a probabilistically unique ID for the new transaction. IDs are 53
+  // bits, as they are used in javascript. We use a random value if possible,
+  // otherwise a counter.
+  static uint64_t NextId() {
+    static uint64_t id = 0;
+    Maybe<uint64_t> rand = mozilla::RandomUint64();
+    return rand.valueOr(++id) & UINT64_C(0x1fffffffffffff);  // 2^53 - 1
+  }
+};
+
+class WebAuthnManager final : public WebAuthnManagerBase, public AbortFollower {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(WebAuthnManager, WebAuthnManagerBase)
+
+  explicit WebAuthnManager(nsPIDOMWindowInner* aParent)
+      : WebAuthnManagerBase(aParent) {}
+
+  already_AddRefed<Promise> MakeCredential(
+      const PublicKeyCredentialCreationOptions& aOptions,
+      const Optional<OwningNonNull<AbortSignal>>& aSignal, ErrorResult& aError);
+
+  already_AddRefed<Promise> GetAssertion(
+      const PublicKeyCredentialRequestOptions& aOptions,
+      const Optional<OwningNonNull<AbortSignal>>& aSignal, ErrorResult& aError);
+
+  already_AddRefed<Promise> Store(const Credential& aCredential,
+                                  ErrorResult& aError);
+
+  // WebAuthnManagerBase
+
+  void FinishMakeCredential(
+      const uint64_t& aTransactionId,
+      const WebAuthnMakeCredentialResult& aResult) override;
+
+  void FinishGetAssertion(const uint64_t& aTransactionId,
+                          const WebAuthnGetAssertionResult& aResult) override;
+
+  void RequestAborted(const uint64_t& aTransactionId,
+                      const nsresult& aError) override;
+
+  // AbortFollower
+
+  void RunAbortAlgorithm() override;
+
+ protected:
+  // Cancels the current transaction (by sending a Cancel message to the
+  // parent) and rejects it by calling RejectTransaction().
+  void CancelTransaction(const nsresult& aError);
+  // Upon a visibility change, makes note of it in the current transaction.
+  void HandleVisibilityChange() override;
+
+ private:
+  virtual ~WebAuthnManager();
+
+  // Rejects the current transaction and calls ClearTransaction().
+  void RejectTransaction(const nsresult& aError);
+
+  // Clears all information we have about the current transaction.
+  void ClearTransaction();
+
+  // The current transaction, if any.
+  Maybe<WebAuthnTransaction> mTransaction;
+};
+
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    WebAuthnTransaction& aTransaction, const char* aName, uint32_t aFlags = 0) {
+  ImplCycleCollectionTraverse(aCallback, aTransaction.mPromise, aName, aFlags);
+}
+
+inline void ImplCycleCollectionUnlink(WebAuthnTransaction& aTransaction) {
+  ImplCycleCollectionUnlink(aTransaction.mPromise);
+}
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_WebAuthnManager_h