Support "await a stable state" and "provide a stable state"

Source/core/html/StableState.cpp

+// Copyright 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.
+
+#include "config.h"
+#include "core/html/StableState.h"
+
+#include "wtf/HashSet.h"
+
+namespace WebCore {
+
+typedef HashSet<StableState::Awaiter*> StableStateAwaiters;
+
+static StableStateAwaiters& stableStateAwaiters()
+{
+    DEFINE_STATIC_LOCAL(StableStateAwaiters, stableStateAwaiters, ());

[acolwell GONE FROM CHROMIUM, 2014/02/07 19:09:31]

Do we really want this process global? Shouldn't this be thread local since I
believe we can have multiple documents in a single process? I must admit, I'm
not overly familiar with this part of the codebase so I apologize in advance if
these are silly questions.

[philipj_slow, 2014/02/09 17:10:17]

In the spec these concepts are not scoped per document, or anything at all. Of
course not all documents are reachable for all other documents, so there may be
ways to scope it that is indistinguishable in testing from a global StableState
instance.

That being said, I believe it is safe to have a global instance. These functions
should only be used from the main thread (asserts added) and because the
awaiters are run at the end of the tasks, the list of awaiters should be empty
before any other top-level browsing context ("tab") gets to see it.

+    return stableStateAwaiters;
+}
+
+StableState::Awaiter::~Awaiter()
+{
+    stableStateAwaiters().remove(this);
+}
+
+void StableState::await(Awaiter& awaiter)
+{
+    stableStateAwaiters().add(&awaiter);

[acolwell GONE FROM CHROMIUM, 2014/02/07 19:09:31]

nit: I wonder if we should keep a bool that keeps track of whether we are inside
a provide() call and ASSERT() on it here. It would be better to crash in the
actual invalid call instead of after the fact like the ASSERT() below.

[philipj_slow, 2014/02/09 17:10:17]

Done.

+}
+
+void StableState::provide()
+{
+    if (!stableStateAwaiters().isEmpty()) {

[acolwell GONE FROM CHROMIUM, 2014/02/07 19:09:31]

nit: Reverse condition and early return.

[philipj_slow, 2014/02/09 17:10:17]

Done.

+        StableStateAwaiters awaiters;
+        awaiters.swap(stableStateAwaiters());
+        for (StableStateAwaiters::iterator it = awaiters.begin(); it != awaiters.end(); ++it)
+            (*it)->didAwaitStableState();
+        ASSERT(stableStateAwaiters().isEmpty());
+    }
+}
+
+} // namespace WebCore