Introduce ParallelDownloadJob.

content/browser/download/download_job_factory.cc

+// 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.
+
+#include "content/browser/download/download_job_factory.h"
+
+#include <memory>
+
+#include "base/memory/ptr_util.h"
+#include "content/browser/download/download_item_impl.h"
+#include "content/browser/download/download_job.h"
+#include "content/browser/download/download_job_impl.h"
+#include "content/browser/download/parallel_download_job.h"
+#include "content/public/common/content_features.h"
+
+namespace content {
+
+namespace {
+
+// Default minimum file size in bytes to enable a parallel download, 2048KB.
+const int64_t kMinFileSizeParallelDownload = 2097152;

[qinmin, 2017/02/27 21:54:46]

nit: make this finch configurable, or add a TODO for that. We need to study the
UMA on how long it tooks to download a file of a particular size. If it is too
short, user won't notice any changes in download speed.

[xingliu, 2017/02/27 23:53:26]

Done.

+
+bool ShouldUseParallelDownload(const DownloadCreateInfo& create_info) {
+  // 1. Accept-Ranges, Content-Length and strong validators response headers.
+  // 2. Feature |kParallelDownloading| enabled.
+  // 3. Content-Length is no less than |kMinFileSizeParallelDownload|.
+  // 3. (Undetermined) Http/1.1 protocol.
+  // 4. (Undetermined) Not under http proxy, e.g. data saver.
+
+  // Etag and last modified are stored into DownloadCreateInfo in
+  // DownloadRequestCore only if the response header complies to the strong
+  // validator rule.
+  bool has_strong_validator =
+      !create_info.etag.empty() || !create_info.last_modified.empty();
+
+  return has_strong_validator && create_info.accept_range &&
+         create_info.total_bytes >= kMinFileSizeParallelDownload &&
+         base::FeatureList::IsEnabled(features::kParallelDownloading);
+}
+
+}  // namespace
+
+std::unique_ptr<DownloadJob> DownloadJobFactory::CreateJob(
+    DownloadItemImpl* download_item,
+    std::unique_ptr<DownloadRequestHandleInterface> req_handle,
+    const DownloadCreateInfo& create_info) {
+  // Build parallel download job.
+  if (ShouldUseParallelDownload(create_info)) {
+    return base::MakeUnique<ParallelDownloadJob>(download_item,
+                                                 std::move(req_handle));
+  }
+
+  // An ordinary download job.
+  return base::MakeUnique<DownloadJobImpl>(download_item,
+                                           std::move(req_handle));
+}
+
+}  // namespace