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diff --git a/netwerk/base/nsIIncrementalDownload.idl b/netwerk/base/nsIIncrementalDownload.idl
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+++ b/netwerk/base/nsIIncrementalDownload.idl
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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/. */
+
+#include "nsIRequest.idl"
+
+interface nsIURI;
+interface nsIFile;
+interface nsIRequestObserver;
+
+/**
+ * An incremental download object attempts to fetch a file piecemeal over time
+ * in an effort to minimize network bandwidth usage.
+ *
+ * Canceling a background download does not cause the file on disk to be
+ * deleted.
+ */
+[scriptable, uuid(6687823f-56c4-461d-93a1-7f6cb7dfbfba)]
+interface nsIIncrementalDownload : nsIRequest
+{
+  /**
+   * Initialize the incremental download object.  If the destination file
+   * already exists, then only the remaining portion of the file will be
+   * fetched.
+   *
+   * NOTE: The downloader will create the destination file if it does not
+   * already exist.  It will create the file with the permissions 0600 if
+   * needed.  To affect the permissions of the file, consumers of this
+   * interface may create an empty file at the specified destination prior to
+   * starting the incremental download.
+   *
+   * NOTE: Since this class may create a temporary file at the specified
+   * destination, it is advisable for the consumer of this interface to specify
+   * a file name for the destination that would not tempt the user into
+   * double-clicking it.  For example, it might be wise to append a file
+   * extension like ".part" to the end of the destination to protect users from
+   * accidentally running "blah.exe" before it is a complete file.
+   *
+   * @param uri
+   *        The URI to fetch.
+   * @param destination
+   *        The location where the file is to be stored.
+   * @param chunkSize
+   *        The size of the chunks to fetch.  A non-positive value results in
+   *        the default chunk size being used.
+   * @param intervalInSeconds
+   *        The amount of time to wait between fetching chunks.  Pass a
+   *        negative to use the default interval, or 0 to fetch the remaining
+   *        part of the file in one chunk.
+   */
+  void init(in nsIURI uri, in nsIFile destination, in long chunkSize,
+            in long intervalInSeconds);
+
+  /**
+   * The URI being fetched.
+   */
+  readonly attribute nsIURI URI;
+
+  /**
+   * The URI being fetched after any redirects have been followed.  This
+   * attribute is set just prior to calling OnStartRequest on the observer
+   * passed to the start method.
+   */
+  readonly attribute nsIURI finalURI;
+
+  /**
+   * The file where the download is being written.
+   */
+  readonly attribute nsIFile destination;
+
+  /**
+   * The total number of bytes for the requested file.  This attribute is set
+   * just prior to calling OnStartRequest on the observer passed to the start
+   * method.
+   *
+   * This attribute has a value of -1 if the total size is unknown.
+   */
+  readonly attribute long long totalSize;
+
+  /**
+   * The current number of bytes downloaded so far.  This attribute is set just
+   * prior to calling OnStartRequest on the observer passed to the start
+   * method.
+   *
+   * This attribute has a value of -1 if the current size is unknown.
+   */
+  readonly attribute long long currentSize;
+
+  /**
+   * Start the incremental download.
+   *
+   * @param observer
+   *        An observer to be notified of various events.  OnStartRequest is
+   *        called when finalURI and totalSize have been determined or when an
+   *        error occurs.  OnStopRequest is called when the file is completely
+   *        downloaded or when an error occurs.  If this object implements
+   *        nsIProgressEventSink, then its OnProgress method will be called as
+   *        data is written to the destination file.  If this object implements
+   *        nsIInterfaceRequestor, then it will be assigned as the underlying
+   *        channel's notification callbacks, which allows it to provide a
+   *        nsIAuthPrompt implementation if needed by the channel, for example.
+   * @param ctxt
+   *        User defined object forwarded to the observer's methods.
+   */
+  void start(in nsIRequestObserver observer,
+             in nsISupports ctxt);
+};