Introduce TracingController and TracingControllerBackend skeleton

tools/telemetry/telemetry/core/platform/tracing_controller_backend.py

Index: tools/telemetry/telemetry/core/platform/tracing_controller_backend.py
diff --git a/tools/telemetry/telemetry/core/platform/tracing_controller_backend.py b/tools/telemetry/telemetry/core/platform/tracing_controller_backend.py
new file mode 100644
index 0000000000000000000000000000000000000000..fe7849f85713258fcb783a00a52f384c14dadfcb
--- /dev/null
+++ b/tools/telemetry/telemetry/core/platform/tracing_controller_backend.py
@@ -0,0 +1,75 @@
+# 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_category_filter
+from telemetry.core.platform import tracing_options
+
+
+class TracingControllerBackend(object):
+  def __init__(self, platform_backend):
+    self._platform_backend = platform_backend
+    self._current_trace_options = None
+    self._current_category_filter = None
+
+  def Start(self, trace_options, category_filter, timeout):
+    if self.is_tracing_running:
+      return False
+
+    assert isinstance(category_filter,
+                      tracing_category_filter.TracingCategoryFilter)
+    assert isinstance(trace_options,
+                      tracing_options.TracingOptions)
+
+    if len(self.running_browser_backends) != 1:
+      # Note: it is possible to implement tracing for both the case of 0 and >1.
+      # For >1, we just need to merge the trace files at StopTracing.
+      #
+      # For 0, we want to modify chrome's trace-startup to support leaving
+      # tracing on indefinitely. Then have the backend notify the platform
+      # and the tracing controller that it is starting a browser, have
+      # the controller add in the trace-startup command, and then when we get
+      # the Stop message or the DidStopBrowser(), issue the stop tracing command
+      # on the right backend.
+      raise NotImplementedError()
+
+    self._current_trace_options = trace_options
+    self._current_category_filter = category_filter
+
+    if trace_options.enable_chrome_trace:
+      browser_backend = self.running_browser_backends[0]
+      browser_backend.StartTracing(
+          category_filter.filter_string, timeout)
+
+  def Stop(self):
+    if not self.is_tracing_running:
+      raise Exception('Not tracing')
+    if len(self.running_browser_backends) != 1:
+      raise NotImplementedError()
+
+    result = None
+    if self._current_trace_options.enable_chrome_trace:
+      browser_backend = self.running_browser_backends[0]
+      result = browser_backend.StopTracing()
+
+    self._current_trace_options = None
+    self._current_category_filter = None
+    return result
+
+  def IsChromeTracingSupported(self, browser):
+    browser_backend = self._platform_backend.GetBackendForBrowser(browser)
+    return browser_backend.supports_tracing
+
+  @property
+  def is_tracing_running(self):
+    return self._current_trace_options != None
+
+  @property
+  def running_browser_backends(self):
+    return self._platform_backend.running_browser_backends
+
+  def DidStartBrowser(self, browser, browser_backend):
+    pass
+
+  def WillCloseBrowser(self, browser, browser_backend):
+    pass