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:
+  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(bool resume_request) override;
+
+ private:
+  friend class ParallelDownloadJobTest;
+
+  typedef std::vector<std::unique_ptr<DownloadWorker>> WorkerList;
+
+  // 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 for a new download, include the original
+  // request.
+  int request_num_;
+
+  // Subsequent tasks to send range requests.
+  WorkerList workers_;
+
+  DISALLOW_COPY_AND_ASSIGN(ParallelDownloadJob);
+};
+
+}  //  namespace content
+
+#endif  // CONTENT_BROWSER_DOWNLOAD_PARALLEL_DOWNLOAD_JOB_H_