[Chromoting] GetHostAttributes with tests

remoting/host/host_attributes.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 REMOTING_HOST_HOST_ATTRIBUTES_H_
+#define REMOTING_HOST_HOST_ATTRIBUTES_H_
+
+#include <string>
+
+namespace remoting {
+
+struct DynamicAttribute {

[Sergey Ulanov, 2016/11/07 20:11:33]

I don't think we need this to be in the header.

[Hzj_jie, 2016/11/08 01:29:54]

Done.

+  const char* name;
+  bool(* value)();

[Sergey Ulanov, 2016/11/07 20:11:33]

this is not a good name for a function. maybe get_value_func?

[Hzj_jie, 2016/11/08 01:29:54]

Done.

+};
+
+// This structure requires to be POD, so use EvaluatedValue() instead of |value|
+// or |attribute| variables directly.
+// This class is thread-safe, but |attribute|.value() may be evaluated several
+// times concurrently if EvaluatedValue() function is called concurrently.
+struct StaticAttribute {
+  static StaticAttribute Create(DynamicAttribute&& attribute) {

[Hzj_jie, 2016/11/05 05:29:28]

MSVC does not allow to change a constexpr object, it simply crashes. So this
function is not constexpr.

+    // std::move is not constexpr in c++ 11.
+    // TODO(zijiehe): Add std::move once we move to c++ 14.
+    return StaticAttribute { attribute, false, false };
+  }
+
+  const char* name() const;
+  bool EvaluatedValue();
+
+  DynamicAttribute attribute;

[Hzj_jie, 2016/11/05 05:29:28]

I do not think it's a good idea to evaluate the StaticAttribute in static object
direct, it will impact startup time if the evaluation is costly. So a
StaticAttribute uses a DynamicAttribute, and caches its result in value field.

[Sergey Ulanov, 2016/11/07 20:11:33]

We are not allowed to have any static initializers at all. But I don't
understand why static values have to be evaluated dynamically at all. They
should have static values set during compile time, e.g.

struct StaticAttribute {
  const char* name;
  bool value;
};

StaticAttribute kStaticAttributes = {
  {"IsDebug", 
#if defined(NDEBUG)
   false
#else
   true
#endif
  },
  ...
}

Or for simplicity all attributes could be "dynamic" if they have to be evaluated
in runtime.

[Hzj_jie, 2016/11/08 01:29:54]

See my comments in host_attributes_win.h

+  bool evaluated;
+  bool value;
+};
+
+// Returns a comma-separated string to represent host attributes. The result may
+// vary if any system configurations change. So consumers should not cache the
+// result.
+// This function is thread-safe.
+std::string GetHostAttributes();
+
+}  // namespace remoting
+
+#endif  // REMOTING_HOST_HOST_ATTRIBUTES_H_