Implement HTTP/2 priority tree structure.

net/spdy/spdy_priority_tree.h

+// Copyright (c) 2014 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_SPDY_PRIORITY_TREE_H_
+#define NET_SPDY_SPDY_PRIORITY_TREE_H_
+
+#include <cmath>
+#include <list>
+#include <map>
+#include <queue>
+#include <set>
+
+#include "base/basictypes.h"
+#include "base/containers/hash_tables.h"
+#include "base/logging.h"
+#include "base/memory/scoped_ptr.h"
+
+namespace net {
+
+// This data structure implements the HTTP2 prioritization data structure
+// defined in this document: http://go/http2-spec

[Ryan Hamilton, 2014/06/25 23:22:58]

Consider a public URL here.

[Johnny, 2014/06/26 16:09:37]

Done.

+//
+// Nodes can be added and removed, and dependencies between them defined.  Each
+// node can have at most one parent and at most one child (forming a list), but
+// there can be multiple lists, with each list root having its own priority.
+// Individual nodes can also be marked as ready to read/write, and then the
+// whole structure can be queried to pick the next node to read/write out of
+// those ready.
+//
+// The NodeId type must be a POD that supports comparison (most
+// likely, it will be a number).
+
+namespace test {
+template <typename NodeId>
+class SpdyPriorityTreePeer;
+}
+
+const int kRootNodeId = 0;
+const int kDefaultWeight = 16;
+const int kMinWeight = 1;
+const int kMaxWeight = 256;
+
+template <typename NodeId>
+class SpdyPriorityTree {
+  typedef std::vector<std::pair<NodeId, float> > PriorityNodeList;
+
+ public:
+  SpdyPriorityTree();
+  ~SpdyPriorityTree();
+
+  typedef std::list<NodeId> List;
+  struct Node {
+    Node();
+    ~Node();
+
+    NodeId id;
+    NodeId parent_id;
+    int weight;  // Weights can range between 1 and 256 (inclusive).
+    // The total weight of this node's direct descendants.
+    int total_child_weights;
+    // The total weight of direct descendants that are writeable
+    // (ready to write and not blocked). This value does not necessarily
+    // reflect the current state of the tree; instead, we lazily update it
+    // on calls to PropagateNodeState(node.id).
+    int total_writeable_child_weights;
+    List* child_list;  // node ID's of children, if any
+    bool blocked;  // Is the associated stream write-blocked?
+    bool ready;  // Does the stream have data ready for writing?
+    float priority;  // The fraction of resources to dedicate to this node.
+  };
+
+  // Orders in descending order of priority.
+  struct NodePriorityComparator {
+    bool operator ()(const std::pair<NodeId, float>& lhs,
+                     const std::pair<NodeId, float>& rhs);
+  };
+
+  friend class test::SpdyPriorityTreePeer<NodeId>;
+
+  // Return the number of nodes currently in the tree.
+  int num_nodes() const;

[Ryan Hamilton, 2014/06/25 23:22:58]

Should this be size_t?

[Johnny, 2014/06/26 16:09:37]

I also think it ought to be, but am leaving as-is to avoid future merge
conflicts in signed-ness of unit tests.

This class will probably see further changes before it's integrated into
Chromium.

+
+  // Return true if the tree contains a node with the given ID.
+  bool NodeExists(NodeId node_id) const;
+
+  // Add a new node with the given weight and parent. Non-exclusive nodes
+  // simply get added below the parent node. If exclusive = true, the node
+  // becomes the parent's sole child and the parent's previous children
+  // become the children of the new node.
+  // Returns true on success. Returns false if the node already exists
+  // in the tree, or if the parent node does not exist.
+  bool AddNode(NodeId node_id, NodeId parent_id, int weight, bool exclusive);
+
+  // Remove an existing node from the tree.  Returns true on success, or
+  // false if the node doesn't exist.
+  bool RemoveNode(NodeId node_id);
+
+  // Get the weight of the given node.
+  int GetWeight(NodeId node_id) const;
+
+  // Get the parent of the given node.  If the node doesn't exist, or is a root
+  // node (and thus has no parent), returns NodeId().
+  NodeId GetParent(NodeId node_id) const;
+
+  // Get the child list of the given node.  If the node doesn't exist, or has no
+  // child, returns NULL.
+  std::list<NodeId>* GetChildren(NodeId node_id) const;
+
+  // Set the priority of the given node.
+  bool SetWeight(NodeId node_id, int weight);
+
+  // Set the parent of the given node.  Returns true on success.
+  // Returns false and has no effect if the node and/or the parent doesn't
+  // exist. If the new parent is a descendant of the node (i.e. this would have
+  // created a cycle) then we rearrange the topology of the tree as described
+  // in the HTTP2 spec.
+  bool SetParent(NodeId node_id, NodeId parent_id, bool exclusive);
+
+  // Returns true if the node parent_id has child_id in its child_list.
+  bool HasChild(NodeId parent_id, NodeId child_id) const;
+
+  // Mark a node as blocked or unblocked. Return true on success, or false
+  // if unable to mark the specified node.
+  bool SetBlocked(NodeId node_id, bool blocked);
+
+  // Mark whether or not a node is ready to write; i.e. whether there is
+  // buffered data for the associated stream. Return true on success, or false
+  // if unable to mark the specified node.
+  bool SetReady(NodeId node_id, bool ready);
+
+  // Return true if all internal invariants hold (useful for unit tests).
+  // Unless there are bugs, this should always return true.
+  bool ValidateInvariantsForTests() const;
+
+  // Get the given node, or return NULL if it doesn't exist.
+  const Node* FindNode(NodeId node_id) const;
+
+  // Returns an ordered list of writeable nodes and their priorities.
+  // Priority is calculated as:
+  // parent's priority * (node's weight / sum of sibling weights)
+  PriorityNodeList GetPriorityList();
+
+ protected:
+  // Update the value of total_writeable_child_weights for the given node
+  // to reflect the current state of the tree.
+  void PropagateNodeState(NodeId node);
+
+ private:
+  typedef base::hash_map<NodeId, Node> NodeMap;
+
+  NodeMap all_nodes_;  // maps from node IDs to Node objects
+
+  DISALLOW_COPY_AND_ASSIGN(SpdyPriorityTree);
+};
+
+template <typename NodeId>
+SpdyPriorityTree<NodeId>::SpdyPriorityTree() {
+  Node* root_node = &all_nodes_[kRootNodeId];
+  root_node->id = kRootNodeId;
+  root_node->weight = kDefaultWeight;
+  root_node->parent_id = static_cast<NodeId>(kRootNodeId);
+  root_node->child_list = new std::list<NodeId>;
+  root_node->priority = 1.0;
+  root_node->ready = true;
+}
+
+template <typename NodeId>
+SpdyPriorityTree<NodeId>::~SpdyPriorityTree() {}
+
+template <typename NodeId>
+SpdyPriorityTree<NodeId>::Node::Node() :
+  parent_id(kRootNodeId),
+  weight(kDefaultWeight),
+  total_child_weights(0),
+  total_writeable_child_weights(0),
+  child_list(),
+  blocked(false),
+  ready(false),
+  priority(0) {
+}
+
+template <typename NodeId>
+SpdyPriorityTree<NodeId>::Node::~Node() {
+  delete child_list;
+}
+
+template <typename NodeId>
+bool SpdyPriorityTree<NodeId>::NodePriorityComparator::operator ()(
+    const std::pair<NodeId, float>& lhs,
+    const std::pair<NodeId, float>& rhs) {
+  return lhs.second > rhs.second;
+}
+
+template <typename NodeId>
+int SpdyPriorityTree<NodeId>::num_nodes() const {
+  return all_nodes_.size();
+}
+
+template <typename NodeId>
+bool SpdyPriorityTree<NodeId>::NodeExists(NodeId node_id) const {
+  return all_nodes_.count(node_id) != 0;
+}
+
+template <typename NodeId>
+bool SpdyPriorityTree<NodeId>::AddNode(NodeId node_id,
+                                       NodeId parent_id,
+                                       int weight,
+                                       bool exclusive) {
+  if (NodeExists(node_id) || !NodeExists(parent_id)) {
+    return false;
+  }
+  if (weight < kMinWeight || weight > kMaxWeight) {
+    return false;
+  }
+  Node* parent = &all_nodes_[parent_id];
+  Node* new_node = &all_nodes_[node_id];
+  new_node->id = node_id;
+  new_node->weight = weight;
+  new_node->parent_id = parent_id;
+  if (exclusive) {
+    // Move the parent's current children below the new node.
+    new_node->child_list = parent->child_list;
+    new_node->total_child_weights = parent->total_child_weights;
+    // Update each child's parent_id.
+    for (typename List::iterator it = new_node->child_list->begin();
+         it != new_node->child_list->end(); ++it) {
+      Node* child = &all_nodes_[*it];
+      child->parent_id = node_id;
+    }
+    // Clear parent's old child data.
+    parent->child_list = new std::list<NodeId>;
+    parent->total_child_weights = 0;
+  } else {
+    new_node->child_list = new std::list<NodeId>;
+  }
+  // Add new node to parent.
+  parent->child_list->push_back(node_id);
+  parent->total_child_weights += weight;
+  return true;
+}
+
+template <typename NodeId>
+bool SpdyPriorityTree<NodeId>::RemoveNode(NodeId node_id) {
+  if (node_id == static_cast<NodeId>(kRootNodeId) || !NodeExists(node_id)) {
+    return false;
+  }
+  const Node& node = all_nodes_[node_id];
+
+  DCHECK(NodeExists(node.parent_id));
+  Node* parent = &all_nodes_[node.parent_id];
+  // Remove the node id from parent's child list.
+  parent->child_list->remove(node_id);
+  parent->total_child_weights -= node.weight;
+
+  // Move the node's children to the parent's child list.
+  if (node.child_list != NULL) {
+    // Update each child's parent_id and weight.
+    for (typename List::iterator it = node.child_list->begin();
+         it != node.child_list->end(); ++it) {
+      Node* child = &all_nodes_[*it];
+      child->parent_id = node.parent_id;
+      // Divide the removed node's weight among its children, rounding to the
+      // nearest valid weight.
+      float float_weight =  node.weight * static_cast<float>(child->weight) /
+                            static_cast<float>(node.total_child_weights);
+      int new_weight = std::floor(float_weight + 0.5);
+      if (new_weight == 0) {
+        new_weight = 1;
+      }
+      child->weight = new_weight;
+      parent->total_child_weights += child->weight;
+    }
+    parent->child_list->splice(parent->child_list->end(), *node.child_list);
+  }
+
+  // Delete the node.
+  all_nodes_.erase(node_id);
+  return true;
+}
+
+template <typename NodeId>
+int SpdyPriorityTree<NodeId>::GetWeight(NodeId node_id) const {
+  const Node* node = FindNode(node_id);
+  if (node != NULL) {
+    return node->weight;
+  }
+  return 0;
+}
+
+template <typename NodeId>
+NodeId SpdyPriorityTree<NodeId>::GetParent(NodeId node_id) const {
+  const Node* node = FindNode(node_id);
+  if (node != NULL && node->id != static_cast<NodeId>(kRootNodeId)) {
+    return node->parent_id;
+  }
+  return static_cast<NodeId>(kRootNodeId);
+}
+
+template <typename NodeId>
+std::list<NodeId>* SpdyPriorityTree<NodeId>::GetChildren(NodeId node_id) const {
+  const Node* node = FindNode(node_id);
+  if (node != NULL) {
+    return node->child_list;
+  }
+  return NULL;
+}
+
+template <typename NodeId>
+bool SpdyPriorityTree<NodeId>::SetWeight(
+    NodeId node_id, int weight) {
+  if (!NodeExists(node_id)) {
+    return false;
+  }
+  if (weight < kMinWeight || weight > kMaxWeight) {
+    return false;
+  }
+
+  Node* node = &all_nodes_[node_id];
+  Node* parent = &all_nodes_[node->parent_id];
+
+  parent->total_child_weights += (weight - node->weight);
+  node->weight = weight;
+
+  return true;
+}
+
+
+template <typename NodeId>
+bool SpdyPriorityTree<NodeId>::SetParent(
+    NodeId node_id, NodeId parent_id, bool exclusive) {
+  if (!NodeExists(node_id) || !NodeExists(parent_id)) {
+    return false;
+  }
+  if (node_id == parent_id) return false;
+
+  Node* node = &all_nodes_[node_id];
+  Node* new_parent = &all_nodes_[parent_id];
+  // If the new parent is already the node's parent, we're done.
+  if (node->parent_id == parent_id) {
+    return true;
+  }
+
+  // Next, check to see if the new parent is currently a descendant
+  // of the node.
+  Node* last = new_parent;
+  NodeId last_id = parent_id;
+  bool cycle_exists = false;
+  while (last->parent_id != static_cast<NodeId>(kRootNodeId)) {
+    if (last->parent_id == node_id) {
+      cycle_exists = true;
+      break;
+    }
+    last_id = last->parent_id;
+    DCHECK(NodeExists(last_id));
+    last = &all_nodes_[last_id];
+  }
+
+  if (cycle_exists) {
+    // The new parent moves to the level of the current node.
+    SetParent(parent_id, node->parent_id, false);
+  }
+
+  // Remove node from old parent's child list.
+  const NodeId old_parent_id = node->parent_id;
+  DCHECK(NodeExists(old_parent_id));
+  Node* old_parent = &all_nodes_[old_parent_id];
+  old_parent->child_list->remove(node_id);
+  old_parent->total_child_weights -= node->weight;
+
+  // Make the change.
+  node->parent_id = parent_id;
+  new_parent->child_list->push_back(node_id);
+  new_parent->total_child_weights += node->weight;
+  return true;
+}
+
+template <typename NodeId>
+bool SpdyPriorityTree<NodeId>::SetBlocked(NodeId node_id, bool blocked) {
+  if (!NodeExists(node_id)) {
+    return false;
+  }
+
+  Node* node = &all_nodes_[node_id];
+  node->blocked = blocked;
+  return true;
+}
+
+template <typename NodeId>
+bool SpdyPriorityTree<NodeId>::SetReady(NodeId node_id, bool ready) {
+  if (!NodeExists(node_id)) {
+    return false;
+  }
+  Node* node = &all_nodes_[node_id];
+  node->ready = ready;
+  return true;
+}
+
+template <typename NodeId>
+void SpdyPriorityTree<NodeId>::PropagateNodeState(NodeId node_id) {
+  // Reset total_writeable_child_weights to its maximum value.
+  Node* node = &all_nodes_[node_id];
+  node->total_writeable_child_weights = node->total_child_weights;
+  for (typename List::iterator it = node->child_list->begin();
+       it != node->child_list->end(); ++it) {
+    PropagateNodeState(*it);
+  }
+  if (node->total_writeable_child_weights == 0 &&
+      (node->blocked || !node->ready)) {
+    // Tell the parent that this entire subtree is unwriteable.
+    Node* parent = &all_nodes_[node->parent_id];
+    parent->total_writeable_child_weights -= node->weight;
+  }
+}
+
+template <typename NodeId>
+const typename SpdyPriorityTree<NodeId>::Node*
+SpdyPriorityTree<NodeId>::FindNode(NodeId node_id) const {
+  typename NodeMap::const_iterator iter = all_nodes_.find(node_id);
+  if (iter == all_nodes_.end()) {
+    return NULL;
+  }
+  return &iter->second;
+}
+
+template <typename NodeId>
+bool SpdyPriorityTree<NodeId>::HasChild(NodeId parent_id,
+                                        NodeId child_id) const {
+  const Node* parent = FindNode(parent_id);
+  return parent->child_list->end() !=
+         std::find(parent->child_list->begin(),
+                   parent->child_list->end(),
+                   child_id);
+}
+
+template <typename NodeId>
+std::vector<std::pair<NodeId, float> >
+SpdyPriorityTree<NodeId>::GetPriorityList() {
+  typedef std::pair<NodeId, float> PriorityNode;
+  typedef std::vector<PriorityNode> PriorityList;
+  PriorityList priority_list;
+
+  // Update total_writeable_child_weights to reflect the current
+  // state of the tree.
+  PropagateNodeState(kRootNodeId);
+
+  List queue;
+  const Node* root_node = FindNode(kRootNodeId);
+  DCHECK(root_node->priority == 1.0);
+  // Start by examining our top-level nodes.
+  for (typename List::iterator it = root_node->child_list->begin();
+       it != root_node->child_list->end(); ++it) {
+    queue.push_back(*it);
+  }
+  while (!queue.empty()) {
+    NodeId current_node_id = queue.front();
+    Node* current_node = &all_nodes_[current_node_id];
+    const Node* parent_node = FindNode(current_node->parent_id);
+    if (current_node->blocked || !current_node->ready) {
+      if (current_node->total_writeable_child_weights > 0) {
+        // This node isn't writeable, but it has writeable children.
+        // Calculate the total fraction of resources we can allot
+        // to this subtree.
+        current_node->priority = parent_node->priority *
+            (static_cast<float>(current_node->weight) /
+             static_cast<float>(parent_node->total_writeable_child_weights));
+        // Examine the children.
+        for (typename List::iterator it = current_node->child_list->begin();
+             it != current_node->child_list->end(); ++it) {
+          queue.push_back(*it);
+        }
+      } else {
+        // There's nothing to see in this subtree.
+        current_node->priority = 0;
+      }
+    } else {
+      // This node is writeable; calculate its priority.
+      current_node->priority = parent_node->priority *
+          (static_cast<float>(current_node->weight) /
+           static_cast<float>(parent_node->total_writeable_child_weights));
+      // Add this node to the priority list.
+      priority_list.push_back(PriorityNode(current_node_id,
+                                           current_node->priority));
+    }
+    // Remove this node from the queue.
+    queue.pop_front();
+  }
+
+  // Sort the nodes in descending order of priority.
+  std::sort(priority_list.begin(), priority_list.end(),
+            NodePriorityComparator());
+
+  return priority_list;
+}
+
+template <typename NodeId>
+bool SpdyPriorityTree<NodeId>::ValidateInvariantsForTests() const {
+  int total_nodes = 0;
+  int nodes_visited = 0;
+  // Iterate through all nodes in the map.
+  for (typename NodeMap::const_iterator iter = all_nodes_.begin();
+       iter != all_nodes_.end(); ++iter) {
+    ++total_nodes;
+    ++nodes_visited;
+    const Node& node = iter->second;
+    // All nodes except the root should have a parent, and should appear in
+    // the child_list of that parent.
+    if (node.id != static_cast<NodeId>(kRootNodeId) &&
+        (!NodeExists(node.parent_id) ||
+         !HasChild(node.parent_id, node.id))) {
+      DLOG(INFO) << "Parent node " << node.parent_id
+                 << " does not exist, or does not list node " << node.id
+                 << " as its child.";
+      return false;
+    }
+
+    if (!node.child_list->empty()) {
+      int total_child_weights = 0;
+      // Iterate through the node's children.
+      for (typename List::iterator it = node.child_list->begin();
+           it != node.child_list->end(); ++it) {
+        ++nodes_visited;
+        // Each node in the list should exist and should have this node
+        // set as its parent.
+        if (!NodeExists(*it) || node.id != GetParent(*it)) {
+          DLOG(INFO) << "Child node " << *it << " does not exist, "
+                     << "or does not list " << node.id << " as its parent.";
+          return false;
+        }
+        const Node* child = FindNode(*it);
+        total_child_weights += child->weight;
+      }
+      // Verify that total_child_weights is correct.
+      if (total_child_weights != node.total_child_weights) {
+        DLOG(INFO) << "Child weight totals do not agree. For node " << node.id
+                   << " total_child_weights has value "
+                   << node.total_child_weights
+                   << ", expected " << total_child_weights;
+        return false;
+      }
+    }
+  }
+
+  // Make sure num_nodes reflects the total number of nodes the map contains.
+  if (total_nodes != num_nodes()) {
+    DLOG(INFO) << "Map contains incorrect number of nodes.";
+    return false;
+  }
+  // Validate the validation function; we should have visited each node twice
+  // (except for the root)
+  DCHECK(nodes_visited == 2*num_nodes() - 1);
+  return true;
+}
+
+}  // namespace net
+
+#endif  // NET_SPDY_SPDY_PRIORITY_TREE_H_