http2: Update priorities of pushed streams

net/spdy/http2_priority_dependencies.cc

 // 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.
 
 #include "net/spdy/http2_priority_dependencies.h"
 
 namespace net {
 
 Http2PriorityDependencies::Http2PriorityDependencies() {}
 
 Http2PriorityDependencies::~Http2PriorityDependencies() {}
 
-void Http2PriorityDependencies::OnStreamSynSent(
+void Http2PriorityDependencies::OnStreamCreation(
     SpdyStreamId id,
     SpdyPriority priority,
     SpdyStreamId* dependent_stream_id,
     bool* exclusive) {
   DCHECK(entry_by_stream_id_.find(id) == entry_by_stream_id_.end());
 
   *dependent_stream_id = 0ul;
   *exclusive = true;
 
-  // Find the next highest entry in total order.
-  for (int i = priority; i >= kV3HighestPriority; --i) {
-    if (!id_priority_lists_[i].empty()) {
-      *dependent_stream_id = id_priority_lists_[i].back().first;
-      break;
-    }
+  // Dependent on the lowest-priority stream that has a priority >= |priority|.
+  IdList::iterator parent;
+  if (PriorityLowerBound(priority, &parent)) {
+    *dependent_stream_id = parent->first;
   }
 
   id_priority_lists_[priority].push_back(std::make_pair(id, priority));
   IdList::iterator it = id_priority_lists_[priority].end();
   --it;
   entry_by_stream_id_[id] = it;
 }
 
+bool Http2PriorityDependencies::PriorityLowerBound(SpdyPriority priority,
+                                                   IdList::iterator* bound) {
+  for (int i = priority; i >= kV3HighestPriority; --i) {
+    if (!id_priority_lists_[i].empty()) {
+      *bound = id_priority_lists_[i].end();
+      --(*bound);
+      return true;
+    }
+  }
+  return false;
+}
+
+bool Http2PriorityDependencies::ParentOfStream(SpdyStreamId id,
+                                               IdList::iterator* parent) {
+  EntryMap::iterator entry = entry_by_stream_id_.find(id);
+  DCHECK(entry != entry_by_stream_id_.end());
+
+  SpdyPriority priority = entry->second->second;
+  IdList::iterator curr = entry->second;
+  if (curr != id_priority_lists_[priority].begin()) {
+    *parent = curr;
+    --(*parent);
+    return true;
+  }
+
+  // id is at the head of its priority list, so its parent is the last

[Bence, 2017/01/05 17:14:45]

s/id/|id|/, here and in other comments below.  Optionally also longer variable
names, though the underscore they contain helps visually convey that they are
variable names.

[Tom Bergan, 2017/01/06 00:08:40]

Done.

+  // entry of the next-highest priority band.
+  if (priority == kV3HighestPriority) {
+    return false;
+  }
+  return PriorityLowerBound(priority - 1, parent);
+}
+
+bool Http2PriorityDependencies::ChildOfStream(SpdyStreamId id,
+                                              IdList::iterator* child) {
+  EntryMap::iterator entry = entry_by_stream_id_.find(id);
+  DCHECK(entry != entry_by_stream_id_.end());
+
+  SpdyPriority priority = entry->second->second;
+  *child = entry->second;
+  ++(*child);
+  if (*child != id_priority_lists_[priority].end()) {
+    return true;
+  }
+
+  // id is at the end of its priority list, so its child is the stream
+  // at the front of the next-lowest priority band.
+  for (int i = priority + 1; i <= kV3LowestPriority; ++i) {
+    if (!id_priority_lists_[i].empty()) {
+      *child = id_priority_lists_[i].begin();
+      return true;
+    }
+  }
+
+  return false;
+}
+
+std::vector<Http2PriorityDependencies::DependencyUpdate>
+Http2PriorityDependencies::OnStreamUpdate(SpdyStreamId id,
+                                          SpdyPriority new_priority) {
+  std::vector<DependencyUpdate> result;
+  result.reserve(2);
+
+  EntryMap::iterator curr_entry = entry_by_stream_id_.find(id);
+  SpdyPriority old_priority = curr_entry->second->second;
+  if (old_priority == new_priority) {
+    return result;
+  }
+
+  IdList::iterator old_parent;
+  bool old_has_parent = ParentOfStream(id, &old_parent);
+
+  IdList::iterator new_parent;
+  bool new_has_parent = PriorityLowerBound(new_priority, &new_parent);
+
+  // If we move id from MEDIUM to LOW, where HIGH = {other_id}, MEDIUM = {id},
+  // and LOW = {}, then PriorityLowerBound(LOW) is id. In this corner case, id
+  // does not change parents.
+  if (new_has_parent && new_parent->first == id) {
+    new_has_parent = old_has_parent;
+    new_parent = old_parent;
+  }
+
+  // If the parent has changed, we generate dependency updates.
+  if ((old_has_parent != new_has_parent) ||
+      (old_has_parent && old_parent->first != new_parent->first)) {
+    // If id has a child, then that child moves to be dependent on old_parent.
+    IdList::iterator old_child;
+    if (ChildOfStream(id, &old_child)) {
+      if (old_has_parent) {
+        result.push_back({old_child->first, old_parent->first, true});
+      } else {
+        result.push_back({old_child->first, 0, true});
+      }
+    }
+
+    // id moves to be dependent on new_parent
+    if (new_has_parent) {
+      result.push_back({id, new_parent->first, true});
+    } else {
+      result.push_back({id, 0, true});
+    }
+  }
+
+  // Move to the new priority.
+  EntryMap::iterator old = entry_by_stream_id_.find(id);
+  id_priority_lists_[old->second->second].erase(old->second);
+  id_priority_lists_[new_priority].push_back(std::make_pair(id, new_priority));
+  IdList::iterator it = id_priority_lists_[new_priority].end();
+  --it;
+  entry_by_stream_id_[id] = it;
+
+  return result;
+}
+
 void Http2PriorityDependencies::OnStreamDestruction(SpdyStreamId id) {
   EntryMap::iterator emit = entry_by_stream_id_.find(id);
-
-  // This routine may be called without a matching call to
-  // OnStreamSynSent above, in the case of server push.  In that case,
-  // it's a no-op.
-  if (emit == entry_by_stream_id_.end())
-    return;
+  DCHECK(emit != entry_by_stream_id_.end());
 
   IdList::iterator it = emit->second;
   id_priority_lists_[it->second].erase(it);
   entry_by_stream_id_.erase(emit);
 }
 
 }  // namespace net