Introduce ParallelDownloadJob.

content/browser/download/parallel_download_job.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CONTENT_BROWSER_DOWNLOAD_PARALLEL_DOWNLOAD_JOB_H_
+#define CONTENT_BROWSER_DOWNLOAD_PARALLEL_DOWNLOAD_JOB_H_
+
+#include <memory>
+#include <vector>
+
+#include "base/macros.h"
+#include "content/browser/download/download_job_impl.h"
+#include "content/browser/download/download_worker.h"
+#include "content/common/content_export.h"
+
+namespace content {
+
+// DownloadJob that can create concurrent range requests to fetch different
+// parts of the file.
+// The original request is hold in base class DownloadUrlJob.
+class CONTENT_EXPORT ParallelDownloadJob : public DownloadJobImpl {
+ public:
+  typedef std::vector<std::unique_ptr<DownloadWorker>> WorkerList;
+
+  ParallelDownloadJob(
+      DownloadItemImpl* download_item,
+      std::unique_ptr<DownloadRequestHandleInterface> request_handle);
+  ~ParallelDownloadJob() override;
+
+  // DownloadUrlJob implementation.
+  void Cancel(bool user_cancel) override;
+  void Pause() override;
+  void Resume() override;
+
+ private:
+  friend class ParallelDownloadJobTest;
+
+  // Build multiple http requests for a new download,
+  // the rest of the bytes starting from |bytes_received| will be equally
+  // distributed to each connection, including the original connection.
+  // the last connection may take additional bytes.
+  void ForkRequestsForNewDownload(int64_t bytes_received, int64_t total_bytes);
+
+  // Create one range request, virtual for testing.
+  virtual void CreateRequest(int64_t offset, int64_t length);
+
+  // Number of concurrent requests, include the original request.
+  // Actual number of requests may be less than this, due to |min_length_|.
+  int request_num_;
+
+  // Minimum length for each range request. Requests with only a few bytes are
+  // not preferred.
+  int64_t min_length_;

[qinmin, 2017/02/22 19:51:23]

why is this a member variable? it feels like this is a constant, do you want to
configure this by finch?
Even so, if there are holes in the file, we might still request something less
than min_length_

[xingliu, 2017/02/22 21:22:41]

Done. Thanks for this suggestion.

Yeah, this one should be only used for new download and doesn't make too much
sense to do it here then. Removed this variable and only check the whole file
size in download_job_factory.cc.

+
+  // Subsequent tasks to send range requests.
+  WorkerList workers_;
+
+  DISALLOW_COPY_AND_ASSIGN(ParallelDownloadJob);
+};
+
+}  //  namespace content
+
+#endif  // CONTENT_BROWSER_DOWNLOAD_PARALLEL_DOWNLOAD_JOB_H_