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diff --git a/netwerk/base/BackgroundFileSaver.h b/netwerk/base/BackgroundFileSaver.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=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/. */
+
+/**
+ * This file defines two implementations of the nsIBackgroundFileSaver
+ * interface.  See the "test_backgroundfilesaver.js" file for usage examples.
+ */
+
+#ifndef BackgroundFileSaver_h__
+#define BackgroundFileSaver_h__
+
+#include "ScopedNSSTypes.h"
+#include "mozilla/Mutex.h"
+#include "nsCOMArray.h"
+#include "nsCOMPtr.h"
+#include "nsIAsyncOutputStream.h"
+#include "nsIBackgroundFileSaver.h"
+#include "nsIStreamListener.h"
+#include "nsStreamUtils.h"
+#include "nsString.h"
+
+class nsIAsyncInputStream;
+class nsISerialEventTarget;
+
+namespace mozilla {
+namespace net {
+
+class DigestOutputStream;
+
+////////////////////////////////////////////////////////////////////////////////
+//// BackgroundFileSaver
+
+class BackgroundFileSaver : public nsIBackgroundFileSaver {
+ public:
+  NS_DECL_NSIBACKGROUNDFILESAVER
+
+  BackgroundFileSaver();
+
+  /**
+   * Initializes the pipe and the worker thread on XPCOM construction.
+   *
+   * This is called automatically by the XPCOM infrastructure, and if this
+   * fails, the factory will delete this object without returning a reference.
+   */
+  nsresult Init();
+
+  /**
+   * Number of worker threads that are currently running.
+   */
+  static uint32_t sThreadCount;
+
+  /**
+   * Maximum number of worker threads reached during the current download
+   * session, used for telemetry.
+   *
+   * When there are no more worker threads running, we consider the download
+   * session finished, and this counter is reset.
+   */
+  static uint32_t sTelemetryMaxThreadCount;
+
+ protected:
+  virtual ~BackgroundFileSaver();
+
+  /**
+   * Thread that constructed this object.
+   */
+  nsCOMPtr<nsIEventTarget> mControlEventTarget;
+
+  /**
+   * Thread to which the actual input/output is delegated.
+   */
+  nsCOMPtr<nsISerialEventTarget> mBackgroundET;
+
+  /**
+   * Stream that receives data from derived classes.  The received data will be
+   * available to the worker thread through mPipeInputStream. This is an
+   * instance of nsPipeOutputStream, not BackgroundFileSaverOutputStream.
+   */
+  nsCOMPtr<nsIAsyncOutputStream> mPipeOutputStream;
+
+  /**
+   * Used during initialization, determines if the pipe is created with an
+   * infinite buffer.  An infinite buffer is required if the derived class
+   * implements nsIStreamListener, because this interface requires all the
+   * provided data to be consumed synchronously.
+   */
+  virtual bool HasInfiniteBuffer() = 0;
+
+  /**
+   * Used by derived classes if they need to be called back while copying.
+   */
+  virtual nsAsyncCopyProgressFun GetProgressCallback() = 0;
+
+  /**
+   * Stream used by the worker thread to read the data to be saved.
+   */
+  nsCOMPtr<nsIAsyncInputStream> mPipeInputStream;
+
+ private:
+  friend class NotifyTargetChangeRunnable;
+
+  /**
+   * Matches the nsIBackgroundFileSaver::observer property.
+   *
+   * @remarks This is a strong reference so that JavaScript callers don't need
+   *          to worry about keeping another reference to the observer.
+   */
+  nsCOMPtr<nsIBackgroundFileSaverObserver> mObserver;
+
+  //////////////////////////////////////////////////////////////////////////////
+  //// Shared state between control and worker threads
+
+  /**
+   * Protects the shared state between control and worker threads.  This mutex
+   * is always locked for a very short time, never during input/output.
+   */
+  mozilla::Mutex mLock;
+
+  /**
+   * True if the worker thread is already waiting to process a change in state.
+   */
+  bool mWorkerThreadAttentionRequested;
+
+  /**
+   * True if the operation should finish as soon as possibile.
+   */
+  bool mFinishRequested;
+
+  /**
+   * True if the operation completed, with either success or failure.
+   */
+  bool mComplete;
+
+  /**
+   * Holds the current file saver status.  This is a success status while the
+   * object is working correctly, and remains such if the operation completes
+   * successfully.  This becomes an error status when an error occurs on the
+   * worker thread, or when the operation is canceled.
+   */
+  nsresult mStatus;
+
+  /**
+   * True if we should append data to the initial target file, instead of
+   * overwriting it.
+   */
+  bool mAppend;
+
+  /**
+   * This is set by the first SetTarget call on the control thread, and contains
+   * the target file name that will be used by the worker thread, as soon as it
+   * is possible to update mActualTarget and open the file.  This is null if no
+   * target was ever assigned to this object.
+   */
+  nsCOMPtr<nsIFile> mInitialTarget;
+
+  /**
+   * This is set by the first SetTarget call on the control thread, and
+   * indicates whether mInitialTarget should be kept as partially completed,
+   * rather than deleted, if the operation fails or is canceled.
+   */
+  bool mInitialTargetKeepPartial;
+
+  /**
+   * This is set by subsequent SetTarget calls on the control thread, and
+   * contains the new target file name to which the worker thread will move the
+   * target file, as soon as it can be done.  This is null if SetTarget was
+   * called only once, or no target was ever assigned to this object.
+   *
+   * The target file can be renamed multiple times, though only the most recent
+   * rename is guaranteed to be processed by the worker thread.
+   */
+  nsCOMPtr<nsIFile> mRenamedTarget;
+
+  /**
+   * This is set by subsequent SetTarget calls on the control thread, and
+   * indicates whether mRenamedTarget should be kept as partially completed,
+   * rather than deleted, if the operation fails or is canceled.
+   */
+  bool mRenamedTargetKeepPartial;
+
+  /**
+   * While NS_AsyncCopy is in progress, allows canceling it.  Null otherwise.
+   * This is read by both threads but only written by the worker thread.
+   */
+  nsCOMPtr<nsISupports> mAsyncCopyContext;
+
+  /**
+   * The SHA 256 hash in raw bytes of the downloaded file. This is written
+   * by the worker thread but can be read on the main thread.
+   */
+  nsCString mSha256;
+
+  /**
+   * Whether or not to compute the hash. Must be set on the main thread before
+   * setTarget is called.
+   */
+  bool mSha256Enabled;
+
+  /**
+   * Store the signature info.
+   */
+  nsTArray<nsTArray<nsTArray<uint8_t>>> mSignatureInfo;
+
+  /**
+   * Whether or not to extract the signature. Must be set on the main thread
+   * before setTarget is called.
+   */
+  bool mSignatureInfoEnabled;
+
+  //////////////////////////////////////////////////////////////////////////////
+  //// State handled exclusively by the worker thread
+
+  /**
+   * Current target file associated to the input and output streams.
+   */
+  nsCOMPtr<nsIFile> mActualTarget;
+
+  /**
+   * Indicates whether mActualTarget should be kept as partially completed,
+   * rather than deleted, if the operation fails or is canceled.
+   */
+  bool mActualTargetKeepPartial;
+
+  /**
+   * Used to calculate the file hash. This keeps state across file renames and
+   * is lazily initialized in ProcessStateChange.
+   */
+  Maybe<Digest> mDigest;
+
+  //////////////////////////////////////////////////////////////////////////////
+  //// Private methods
+
+  /**
+   * Called when NS_AsyncCopy completes.
+   *
+   * @param aClosure
+   *        Populated with a raw pointer to the BackgroundFileSaver object.
+   * @param aStatus
+   *        Success or failure status specified when the copy was interrupted.
+   */
+  static void AsyncCopyCallback(void* aClosure, nsresult aStatus);
+
+  /**
+   * Called on the control thread after state changes, to ensure that the worker
+   * thread will process the state change appropriately.
+   *
+   * @param aShouldInterruptCopy
+   *        If true, the current NS_AsyncCopy, if any, is canceled.
+   */
+  nsresult GetWorkerThreadAttention(bool aShouldInterruptCopy);
+
+  /**
+   * Event called on the worker thread to begin processing a state change.
+   */
+  nsresult ProcessAttention();
+
+  /**
+   * Called by ProcessAttention to execute the operations corresponding to the
+   * state change.  If this results in an error, ProcessAttention will force the
+   * entire operation to be aborted.
+   */
+  nsresult ProcessStateChange();
+
+  /**
+   * Returns true if completion conditions are met on the worker thread.  The
+   * first time this happens, posts the completion event to the control thread.
+   */
+  bool CheckCompletion();
+
+  /**
+   * Event called on the control thread to indicate that file contents will now
+   * be saved to the specified file.
+   */
+  nsresult NotifyTargetChange(nsIFile* aTarget);
+
+  /**
+   * Event called on the control thread to send the final notification.
+   */
+  nsresult NotifySaveComplete();
+
+  /**
+   * Verifies the signature of the binary at the specified file path and stores
+   * the signature data in mSignatureInfo. We extract only X.509 certificates,
+   * since that is what Google's Safebrowsing protocol specifies.
+   */
+  nsresult ExtractSignatureInfo(const nsAString& filePath);
+};
+
+////////////////////////////////////////////////////////////////////////////////
+//// BackgroundFileSaverOutputStream
+
+class BackgroundFileSaverOutputStream : public BackgroundFileSaver,
+                                        public nsIAsyncOutputStream,
+                                        public nsIOutputStreamCallback {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIOUTPUTSTREAM
+  NS_DECL_NSIASYNCOUTPUTSTREAM
+  NS_DECL_NSIOUTPUTSTREAMCALLBACK
+
+  BackgroundFileSaverOutputStream();
+
+ protected:
+  virtual bool HasInfiniteBuffer() override;
+  virtual nsAsyncCopyProgressFun GetProgressCallback() override;
+
+ private:
+  ~BackgroundFileSaverOutputStream() = default;
+
+  /**
+   * Original callback provided to our AsyncWait wrapper.
+   */
+  nsCOMPtr<nsIOutputStreamCallback> mAsyncWaitCallback;
+};
+
+////////////////////////////////////////////////////////////////////////////////
+//// BackgroundFileSaverStreamListener. This class is instantiated by
+// nsExternalHelperAppService, DownloadCore.jsm, and possibly others.
+
+class BackgroundFileSaverStreamListener final : public BackgroundFileSaver,
+                                                public nsIStreamListener {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIREQUESTOBSERVER
+  NS_DECL_NSISTREAMLISTENER
+
+  BackgroundFileSaverStreamListener();
+
+ protected:
+  virtual bool HasInfiniteBuffer() override;
+  virtual nsAsyncCopyProgressFun GetProgressCallback() override;
+
+ private:
+  ~BackgroundFileSaverStreamListener() = default;
+
+  /**
+   * Protects the state related to whether the request should be suspended.
+   */
+  mozilla::Mutex mSuspensionLock;
+
+  /**
+   * Whether we should suspend the request because we received too much data.
+   */
+  bool mReceivedTooMuchData;
+
+  /**
+   * Request for which we received too much data.  This is populated when
+   * mReceivedTooMuchData becomes true for the first time.
+   */
+  nsCOMPtr<nsIRequest> mRequest;
+
+  /**
+   * Whether mRequest is currently suspended.
+   */
+  bool mRequestSuspended;
+
+  /**
+   * Called while NS_AsyncCopy is copying data.
+   */
+  static void AsyncCopyProgressCallback(void* aClosure, uint32_t aCount);
+
+  /**
+   * Called on the control thread to suspend or resume the request.
+   */
+  nsresult NotifySuspendOrResume();
+};
+
+// A wrapper around nsIOutputStream, so that we can compute hashes on the
+// stream without copying and without polluting pristine NSS code with XPCOM
+// interfaces.
+class DigestOutputStream : public nsIOutputStream {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIOUTPUTSTREAM
+  // Constructor. Neither parameter may be null. The caller owns both.
+  DigestOutputStream(nsIOutputStream* aStream, Digest& aDigest);
+
+ private:
+  virtual ~DigestOutputStream() = default;
+
+  // Calls to write are passed to this stream.
+  nsCOMPtr<nsIOutputStream> mOutputStream;
+  // Digest used to compute the hash, owned by the caller.
+  Digest& mDigest;
+
+  // Don't accidentally copy construct.
+  DigestOutputStream(const DigestOutputStream& d) = delete;
+};
+
+}  // namespace net
+}  // namespace mozilla
+
+#endif