telemetry: Make trace profiler work with trace-based benchmarks.

tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py

Index: tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py b/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
index 4d0c68a8a3edb617314407f60d0a8624407e4a14..d0c696c2aeaf693cdd934959084df6bee1608f39 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
@@ -8,7 +8,6 @@ import logging
 import socket
 import threading
 
-
 from telemetry.core.backends.chrome import trace_result
 from telemetry.core.backends.chrome import websocket
 from telemetry.core.backends.chrome import websocket_browser_connection
@@ -70,9 +69,19 @@ class TracingBackend(object):
     self._conn = websocket_browser_connection.WebSocketBrowserConnection(
         devtools_port)
     self._thread = None
+    self._nesting = 0
     self._tracing_data = []
 
-  def BeginTracing(self, custom_categories=None, timeout=10):
+  def _IsTracing(self):
+    return self._thread != None
+
+  def StartTracing(self, custom_categories=None, timeout=10):
+    """ Starts tracing on the first nested call and returns True. Returns False
+        and does nothing on subsequent nested calls.
+    """
+    self._nesting += 1
+    if self._IsTracing():
+      return False
     self._CheckNotificationSupported()
     req = {'method': 'Tracing.start'}
     if custom_categories:
@@ -82,22 +91,37 @@ class TracingBackend(object):
     # data, until Tracing.end is called.
     self._thread = threading.Thread(target=self._TracingReader)
     self._thread.start()
+    return True
+
+  def StopTracing(self):
+    """ Stops tracing on the innermost (!) nested call, because we cannot get
+        results otherwise. Resets _tracing_data on the outermost nested call.
+        Returns the result of the trace, as TraceResult object.
+    """
+    self._nesting -= 1

[tonyg, 2013/09/12 00:49:55]

Let's assert that _nesting doesn't go negative

[ernstm, 2013/09/12 18:06:07]

Done.

+    if self._IsTracing():
+      req = {'method': 'Tracing.end'}
+      self._conn.SendRequest(req)
+      self._thread.join()
+      self._thread = None
+    if self._nesting == 0:
+      return self._GetTraceResultAndReset()
+    else:
+      return self._GetTraceResult()
 
-  def EndTracing(self):
-    req = {'method': 'Tracing.end'}
-    self._conn.SendRequest(req)
-    self._thread.join()
-    self._thread = None
-
-  def GetTraceResultAndReset(self):
-    assert not self._thread
+  def _GetTraceResult(self):
+    assert not self._IsTracing()
     if self._tracing_data and type(self._tracing_data[0]) in [str, unicode]:
       result_impl = TraceResultImpl(self._tracing_data)
     else:
       result_impl = RawTraceResultImpl(self._tracing_data)
-    self._tracing_data = []
     return trace_result.TraceResult(result_impl)
 
+  def _GetTraceResultAndReset(self):
+    result = self._GetTraceResult()
+    self._tracing_data = []
+    return result
+
   def _TracingReader(self):
     while self._conn.socket:
       try: