net/spdy/http2_priority_dependencies.h

// Copyright 2016 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 NET_SPDY_HTTP2_PRIORITY_DEPENDENCIES_H_
#define NET_SPDY_HTTP2_PRIORITY_DEPENDENCIES_H_

#include <list>
#include <map>
#include <utility>
#include <vector>

#include "net/base/net_export.h"
#include "net/third_party/spdy/core/spdy_protocol.h"

namespace net {

// A helper class encapsulating the state and logic to set the priority fields
// for HTTP/2 streams based on their spdy::SpdyPriority and the ordering of
// creation and deletion of the streams. This implentation includes a gross hack
// in which the HTTP/2 weight is set to a transformation of the
// spdy::SpdyPriority value in order to support servers which do not honor
// HTTP/2 stream dependencies and instead treat the weight value like a SPDY/3
// priority.
// TODO(rch): Eliminate this gross hack when servers no longer act like this.
class NET_EXPORT_PRIVATE Http2PriorityDependencies {
 public:
  Http2PriorityDependencies();
  ~Http2PriorityDependencies();

  // Called when a stream is created. This is used for both client-initiated
  // and server-initiated (pushed) streams.
  // On return, |*parent_stream_id| is set to the stream id that should become
  // the parent of this stream, |*exclusive| is set to whether that dependency
  // should be exclusive, and |*weight| is set to the relative weight for the
  // created stream given this priority.
  void OnStreamCreation(spdy::SpdyStreamId id,
                        spdy::SpdyPriority priority,
                        spdy::SpdyStreamId* parent_stream_id,
                        int* weight,
                        bool* exclusive);

  // Called when a stream is destroyed.
  void OnStreamDestruction(spdy::SpdyStreamId id);

  struct DependencyUpdate {
    spdy::SpdyStreamId id;
    spdy::SpdyStreamId parent_stream_id;
    int weight;
    bool exclusive;
  };

  // Called when a stream's priority has changed. Returns a list of
  // dependency updates that should be sent to the server to describe
  // the requested priority change. The updates should be sent in the
  // given order.
  std::vector<DependencyUpdate> OnStreamUpdate(spdy::SpdyStreamId id,
                                               spdy::SpdyPriority new_priority);

  // Returns the estimate of dynamically allocated memory in bytes.
  size_t EstimateMemoryUsage() const;

 private:
  // The requirements for the internal data structure for this class are:
  //     a) Constant time insertion of entries at the end of the list,
  //     b) Fast removal of any entry based on its id.
  //     c) Constant time lookup of the entry at the end of the list.
  // std::list would satisfy (a) & (c), but some form of map is
  // needed for (b).  The priority must be included in the map
  // entries so that deletion can determine which list in id_priority_lists_
  // to erase from.
  using IdList = std::list<std::pair<spdy::SpdyStreamId, spdy::SpdyPriority>>;
  using EntryMap = std::map<spdy::SpdyStreamId, IdList::iterator>;

  IdList id_priority_lists_[spdy::kV3LowestPriority + 1];

  // Tracks the location of an id anywhere in the above vector of lists.
  // Iterators to list elements remain valid until those particular elements
  // are erased.
  EntryMap entry_by_stream_id_;

  // Finds the lowest-priority stream that has a priority >= |priority|.
  // Returns false if there are no such streams.
  // Otherwise, returns true and sets |*bound|.
  bool PriorityLowerBound(spdy::SpdyPriority priority, IdList::iterator* bound);

  // Finds the stream just above |id| in the total order.
  // Returns false if there are no streams with a higher priority.
  // Otherwise, returns true and sets |*parent|.
  bool ParentOfStream(spdy::SpdyStreamId id, IdList::iterator* parent);

  // Finds the stream just below |id| in the total order.
  // Returns false if there are no streams with a lower priority.
  // Otherwise, returns true and sets |*child|.
  bool ChildOfStream(spdy::SpdyStreamId id, IdList::iterator* child);
};

}  // namespace net

#endif  // NET_SPDY_HTTP2_PRIORITY_DEPENDENCIES_H_