Fix bug in net::RequestPriority -> HTTP/2 dependency conversion.

net/spdy/http2_priority_dependencies.cc

Index: net/spdy/http2_priority_dependencies.cc
diff --git a/net/spdy/http2_priority_dependencies.cc b/net/spdy/http2_priority_dependencies.cc
new file mode 100644
index 0000000000000000000000000000000000000000..1709509287f43037d221d5532c52638f80713f97
--- /dev/null
+++ b/net/spdy/http2_priority_dependencies.cc
@@ -0,0 +1,50 @@
+// 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(
+    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;
+    }
+  }
+
+  id_priority_lists_[priority].push_back(std::make_pair(id, priority));
+  IdList::iterator it = id_priority_lists_[priority].end();
+  entry_by_stream_id_[id] = --it;

[Bence, 2016/03/17 01:48:59]

Optional: would you consider
    --it;
    entry_by_stream_id_[id] = it;
so that the reader does not have to scratch their head about whether the prefix
form is pre-decrement or post-decrement?  (I have to confess it takes me some
time to recall this.)  Alas, I understand that the style guide does not require
this.

[Randy Smith (Not in Mondays), 2016/03/17 20:27:42]

I think of this as your code, so ok, but I tend to think of "--it" ==
pre-decrement and "it--" == post-decrement as solidly part of the language.

I will include a mini-rant here that I consider it crazy-cookoo that the
iterator implementation makes you do --it instead of "it - 1" :-{.

+}
+
+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;
+
+  IdList::iterator it = emit->second;
+  id_priority_lists_[it->second].erase(it);
+  entry_by_stream_id_.erase(emit);
+}
+
+}  // namespace net