Introduce ParallelDownloadJob.

content/browser/download/parallel_download_job.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/parallel_download_job.h"
+
+#include "base/memory/ptr_util.h"
+#include "content/browser/download/download_url_task.h"
+#include "content/public/browser/browser_context.h"
+#include "content/public/browser/storage_partition.h"
+
+namespace content {
+
+namespace {
+
+// TODO(xingliu): Use finch parameters to configure constants.
+// Default number of requests in a parallel download, including the original
+// request.

[asanka, 2017/02/16 16:27:39]

It'll be quite unlikely that we'd find a number that works well for everyone. We
may need to consider dynamically ramping up or down depending on the overall
throughput we are seeing. That said, 2 is a good starting number.

+const int kParallelRequestNumber = 2;

[asanka, 2017/02/16 16:27:39]

kParallelRequestCount or somesuch. Parallel request number implies it's an
identifier.

[xingliu, 2017/02/20 18:59:11]

Done.

+
+// Default minimum size of bytes for each range request.
+// If bytes left are less than this, no additional requests will be created.
+const int64_t kMinRangeLength = 1024;
+
+}  // namespace
+
+ParallelDownloadJob::ParallelDownloadJob(
+    std::unique_ptr<DownloadRequestHandleInterface> request_handle)
+    : DownloadUrlJob(std::move(request_handle)),
+      request_num_(kParallelRequestNumber),
+      min_length_(kMinRangeLength) {}
+
+ParallelDownloadJob::~ParallelDownloadJob() = default;
+
+void ParallelDownloadJob::Cancel(bool user_cancel) {
+  DownloadUrlJob::Cancel(user_cancel);
+  for (auto& task : tasks_)
+    task->Cancel();
+}
+
+void ParallelDownloadJob::Pause() {
+  DownloadUrlJob::Pause();
+  for (auto& task : tasks_)
+    task->Pause();
+}
+
+void ParallelDownloadJob::Resume() {
+  DownloadUrlJob::Resume();
+  for (auto& task : tasks_)
+    task->Resume();
+}
+
+void ParallelDownloadJob::ForkParallelRequests(int64_t bytes_received,
+                                               int64_t total_bytes) {

[asanka, 2017/02/16 16:27:39]

This differs from the "list of slices" model that's landed for restoring
downloads from history. I.e. we should be able to deal with a download file that
has a bunch of subranges that have already been downloaded. This API assumes
that the portion to be downloaded is always at the end, which is not the case.

[xingliu, 2017/02/20 18:59:11]

Renamed the function name. Added some comment in the header.

This function is to arrange slices for a new download. |bytes_received| here is
from the original request. Also we didn't hook this to the history db to add new
records yet.

Resumption from history db will have another function unimplemented yet. 

+  if (!download_item_ || total_bytes <= 0 || bytes_received >= total_bytes ||
+      request_num_ <= 1) {
+    return;
+  }
+
+  int64_t bytes_left = total_bytes - bytes_received;
+  int64_t piece_size = std::max(bytes_left / request_num_, min_length_);

[asanka, 2017/02/16 16:27:39]

Let's use consistent terminology. Elsewhere we've used 'slice' to refer to a
part of a download file.

[xingliu, 2017/02/20 18:59:11]

Done.

+  DCHECK(piece_size > 0);
+
+  // We may produce less requests if (bytes_left / request_num_) is less than

[asanka, 2017/02/16 16:27:39]

s/less requests/fewer requests/

[xingliu, 2017/02/20 18:59:11]

Done.

+  // |min_length_|.
+  int num_requests = bytes_left / piece_size;
+  int64_t current_offset = bytes_received + piece_size;
+
+  for (int i = 0; i < num_requests - 1; ++i) {
+    int64_t length = (i == (num_requests - 2))
+                         ? piece_size + (bytes_left % piece_size)
+                         : piece_size;
+    CreateRequest(current_offset, length);
+    current_offset += piece_size;
+  }
+}
+
+void ParallelDownloadJob::CreateRequest(int64_t offset, int64_t length) {
+  std::unique_ptr<DownloadUrlTask> task = base::MakeUnique<DownloadUrlTask>();
+
+  DCHECK(download_item_);
+  StoragePartition* storage_partition =
+      BrowserContext::GetStoragePartitionForSite(
+          download_item_->GetBrowserContext(), download_item_->GetSiteUrl());
+
+  std::unique_ptr<DownloadUrlParameters> download_params(
+      new DownloadUrlParameters(download_item_->GetURL(),
+                                storage_partition->GetURLRequestContext()));
+  download_params->set_file_path(download_item_->GetFullPath());
+  download_params->set_last_modified(download_item_->GetLastModifiedTime());
+  download_params->set_etag(download_item_->GetETag());
+  download_params->set_offset(offset);
+  download_params->set_length(length);

[asanka, 2017/02/16 16:27:39]

Worth adding a comment here that the length parameter is the one thing that
causes this request to be treated as a slice.

[xingliu, 2017/02/20 18:59:11]

Done.

+
+  // Subsequent range requests have the same referrer URL as the original
+  // download request.
+  download_params->set_referrer(Referrer(download_item_->GetReferrerUrl(),
+                                         blink::WebReferrerPolicyAlways));
+  // Send the request.
+  task->SendRequest(std::move(download_params));
+  tasks_.push_back(std::move(task));
+}
+
+}  // namespace content