[Chromoting] GetHostAttributes with tests

remoting/host/host_attributes.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 "remoting/host/host_attributes.h"
+
+#include <type_traits>
+
+#include "base/atomicops.h"
+#include "base/logging.h"
+#include "base/macros.h"
+#include "build/build_config.h"
+#if defined(OS_WIN)

[Sergey Ulanov, 2016/11/08 20:11:05]

nit: add empty line here

[Hzj_jie, 2016/11/09 02:37:52]

Done.

+#include "third_party/webrtc/modules/desktop_capture/win/screen_capturer_win_directx.h"
+#endif
+
+namespace remoting {
+
+namespace {
+static constexpr char kSeparator[] = ",";
+
+struct DynamicAttribute {
+  const char* name;
+  bool(* get_value_func)();
+};
+
+// 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) {
+    // 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;
+  bool evaluated;
+  bool value;
+};
+
+inline constexpr bool IsDebug() {
+#if defined(NDEBUG)
+  return false;
+#else
+  return true;
+#endif
+}
+
+// By using arraysize() macro in base/macros.h, it's illegal to have empty
+// arrays.
+//
+// error: no matching function for call to 'ArraySizeHelper'
+// note: candidate template ignored: substitution failure
+// [with T = const remoting::StaticAttribute, N = 0]:
+// zero-length arrays are not permitted in C++.
+//
+// So we need ReturnTrue() function, and "Host-Attributes"
+// DynamicAttribute.
+
+inline constexpr bool ReturnTrue() {
+  return true;
+}
+
+static StaticAttribute kStaticAttributes[] = {
+  StaticAttribute::Create( { "Debug-Build", &IsDebug } ),
+};
+
+static constexpr DynamicAttribute kDynamicAttributes[] = {
+  { "Host-Attributes", &ReturnTrue },
+#if defined(OS_WIN)
+  { "DirectX-Capturer", &webrtc::ScreenCapturerWinDirectx::IsSupported },
+  // TODO(zijiehe): Add DirectX version attributes. Blocked by change
+  // https://codereview.chromium.org/2468083002/.
+#endif
+};
+
+}  // namespace
+
+static_assert(std::is_pod<DynamicAttribute>::value,
+              "DynamicAttribute should be POD.");
+static_assert(std::is_pod<StaticAttribute>::value,
+              "StaticAttribute should be POD.");
+
+const char* StaticAttribute::name() const {
+  return attribute.name;
+}
+
+bool StaticAttribute::EvaluatedValue() {
+  if (evaluated) {
+    return value;
+  } else {
+    value = attribute.get_value_func();

[Sergey Ulanov, 2016/11/08 20:11:05]

There is a data race because value is not atomic. This problem is easy to avoid
by making all attributes dynamic.

[Hzj_jie, 2016/11/09 02:37:52]

Done.

+    base::subtle::MemoryBarrier();
+    evaluated = true;
+    return value;
+  }
+}
+
+std::string GetHostAttributes() {
+  std::string result;
+  // By using ranged for-loop, MSVC throws error C3316:
+  // 'const remoting::StaticAttribute [0]':
+  // an array of unknown size cannot be used in a range-based for statement.
+  for (size_t i = 0; i < arraysize(kStaticAttributes); i++) {
+    auto& static_attribute = kStaticAttributes[i];
+    DCHECK(std::string(static_attribute.name()).find(kSeparator) ==
+           std::string::npos);
+    if (static_attribute.EvaluatedValue()) {
+      if (!result.empty()) {
+        result.append(kSeparator);
+      }
+      result.append(static_attribute.name());
+    }
+  }
+
+  for (size_t i = 0; i < arraysize(kDynamicAttributes); i++) {
+    const auto& dynamic_attribute = kDynamicAttributes[i];
+    DCHECK(std::string(dynamic_attribute.name).find(kSeparator) ==
+           std::string::npos);
+    if (dynamic_attribute.get_value_func()) {
+      if (!result.empty()) {
+        result.append(kSeparator);
+      }
+      result.append(dynamic_attribute.name);
+    }
+  }
+
+  return result;
+}
+
+}  // namespace remoting