[Telemetry] Add tracing_agent API.

tools/telemetry/telemetry/core/platform/tracing_agent/chrome_tracing_agent.py

Index: tools/telemetry/telemetry/core/platform/tracing_agent/chrome_tracing_agent.py
diff --git a/tools/telemetry/telemetry/core/platform/tracing_agent/chrome_tracing_agent.py b/tools/telemetry/telemetry/core/platform/tracing_agent/chrome_tracing_agent.py
new file mode 100644
index 0000000000000000000000000000000000000000..b3bb05d7eff57831d166e7ccf1bae438edbda67e
--- /dev/null
+++ b/tools/telemetry/telemetry/core/platform/tracing_agent/chrome_tracing_agent.py
@@ -0,0 +1,98 @@
+# 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.
+
+from telemetry.core.platform import tracing_agent
+
+
+class ChromeTracingStartedError(Exception):
+  pass
+
+
+class ChromeTracingAgent(tracing_agent.TracingAgent):
+  # A singleton map from platform backends to maps of uniquely-identifying
+  # remote port (which may be the same as local port) to DevToolsClientBackend.
+  # There is no guarantee that the devtools agent is still alive.
+  _platform_backends_to_devtools_clients_maps = {}
+  _is_tracing_running_for_platform_backend = {}
+  _is_tracing_running_for_platform_backend.setdefault(False)
+
+  def __init__(self, platform_backend):
+    super(ChromeTracingAgent, self).__init__(platform_backend)
+
+  @classmethod
+  def _RemoveStaleDevToolsClient(cls, platform_backend):
+    """Removes DevTools clients that are no longer connectable."""
+    devtools_clients_map = cls._platform_backends_to_devtools_clients_maps.get(
+        platform_backend, {})
+    devtools_clients_map = {
+        port: client
+        for port, client in devtools_clients_map.iteritems()
+        if client.IsAlive()
+        }
+    cls._platform_backends_to_devtools_clients_maps[platform_backend] = (
+        devtools_clients_map)
+
+  @classmethod
+  def RegisterDevToolsClient(cls, devtools_client_backend, platform_backend):
+    is_tracing_running = cls._is_tracing_running_for_platform_backend.get(
+        platform_backend)
+    if is_tracing_running:
+      raise ChromeTracingStartedError(
+          'Cannot add new DevTools client when tracing is running on '
+          'platform backend %s.' % platform_backend)
+    remote_port = str(devtools_client_backend.remote_port)
+    if platform_backend not in cls._platform_backends_to_devtools_clients_maps:
+      cls._platform_backends_to_devtools_clients_maps[platform_backend] = {}
+    devtools_clients_map = (
+      cls._platform_backends_to_devtools_clients_maps[platform_backend])
+    devtools_clients_map[remote_port] = devtools_client_backend
+
+  @classmethod
+  def IsSupported(cls, platform_backend):
+    cls._RemoveStaleDevToolsClient(platform_backend)
+    devtools_clients_map = cls._platform_backends_to_devtools_clients_maps.get(
+        platform_backend, {})
+    for _, devtools_client in devtools_clients_map.iteritems():
+      if devtools_client.IsChromeTracingSupported():
+        return True
+    return False
+
+  @property
+  def _is_active(self):
+    return self._is_tracing_running_for_platform_backend.get(
+        self._platform_backend)
+
+  @_is_active.setter
+  def _is_active(self, value):
+    self._is_tracing_running_for_platform_backend[self._platform_backend] = (
+        value)
+
+  def Start(self, trace_options, category_filter, timeout):
+    if not trace_options.enable_chrome_trace:
+      return False
+
+    if self._is_active:
+      raise ChromeTracingStartedError(
+          'Tracing is already running on platform backend %s.'
+          % self._platform_backend)
+    self._RemoveStaleDevToolsClient(self._platform_backend)
+    devtools_clients_map = self._platform_backends_to_devtools_clients_maps.get(
+        self._platform_backend, {})
+    if not devtools_clients_map:
+      return False
+    for _, devtools_client in devtools_clients_map.iteritems():
+      devtools_client.StartChromeTracing(
+          trace_options, category_filter.filter_string, timeout)
+    self._is_active = True
+    return True
+
+  def Stop(self, trace_data_builder):
+    devtools_clients_map = (
+      self._platform_backends_to_devtools_clients_maps[self._platform_backend])
+    for _, devtools_client in devtools_clients_map.iteritems():
+      # We do not check for stale DevTools client, so that we get an
+      # exception if there is a stale client. This is because we
+      # will potentially lose data if there is a stale client.
+      devtools_client.StopChromeTracing(trace_data_builder)
+    self._is_active = False