Remove Telemetry's Chrome tracing dependency on having a BrowserBackend.

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
index d566320db5cc47ec9ae509350c79e5bbc5182165..f8aa1024753292c1386777842d8ce8caa0834266 100644
--- a/tools/telemetry/telemetry/core/platform/tracing_controller_backend.py
+++ b/tools/telemetry/telemetry/core/platform/tracing_controller_backend.py
@@ -12,6 +12,10 @@ class TracingControllerBackend(object):
     self._platform_backend = platform_backend
     self._current_trace_options = None
     self._current_category_filter = None
+    # A map from 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.
+    self._devtools_clients_map = {}
 
   def Start(self, trace_options, category_filter, timeout):
     if self.is_tracing_running:
@@ -22,27 +26,36 @@ class TracingControllerBackend(object):
     assert isinstance(trace_options,
                       tracing_options.TracingOptions)
 
-    self._AssertOneBrowserBackend()
-
     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(
-          trace_options, category_filter.filter_string, timeout)
+      self._RemoveStaleDevToolsClient()
+      for _, devtools_client in self._devtools_clients_map.iteritems():
+        devtools_client.StartChromeTracing(
+            trace_options, category_filter.filter_string, timeout)
 
     if trace_options.enable_platform_display_trace:
       self._platform_backend.StartDisplayTracing()
 
+  def _RemoveStaleDevToolsClient(self):
+    """Removes DevTools clients that are no longer connectable."""
+    self._devtools_clients_map = {
+        port: client
+        for port, client in self._devtools_clients_map.iteritems()
+        if client.IsAlive()
+        }
+
   def Stop(self):
     assert self.is_tracing_running, 'Can only stop tracing when tracing.'
-    self._AssertOneBrowserBackend()
 
     trace_data_builder = trace_data_module.TraceDataBuilder()
     if self._current_trace_options.enable_chrome_trace:
-      browser_backend = self.running_browser_backends[0]
-      browser_backend.StopTracing(trace_data_builder)
+      for _, devtools_client in self._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)
 
     if self._current_trace_options.enable_platform_display_trace:
       surface_flinger_trace_data = self._platform_backend.StopDisplayTracing()
@@ -53,24 +66,18 @@ class TracingControllerBackend(object):
     self._current_category_filter = None
     return trace_data_builder.AsData()
 
-  def _AssertOneBrowserBackend(self):
-    # 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.
-    num_instances = len(self.running_browser_backends)
-    assert num_instances == 1, (
-        'Tracing only supports one browser instance (not %i).' % num_instances)
+  def RegisterDevToolsClient(self, devtools_client_backend):
+    assert not self.is_tracing_running, (
+        'Cannot add new DevTools client when tracing is running.')
+    remote_port = str(devtools_client_backend.remote_port)
+    self._devtools_clients_map[remote_port] = devtools_client_backend
 
   def IsChromeTracingSupported(self):
-    self._AssertOneBrowserBackend()
-    browser_backend = self.running_browser_backends[0]
-    return browser_backend.devtools_client.IsChromeTracingSupported()
+    self._RemoveStaleDevToolsClient()
+    for _, devtools_client in self._devtools_clients_map.iteritems():
+      if devtools_client.IsChromeTracingSupported():
+        return True
+    return False
 
   def IsDisplayTracingSupported(self):
     return self._platform_backend.IsDisplayTracingSupported()
@@ -79,10 +86,6 @@ class TracingControllerBackend(object):
   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):

[nednguyen, 2015/01/13 16:12:00]

Kill these methods?

     pass