Ensure disk cache init to make PCV1_COLD results stable

telemetry/telemetry/page/cache_temperature.py

 # 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.
 
 """
 Cache temperature specifies how the browser cache should be configured before
 the page run.
 
 See design doc for details:
 https://docs.google.com/document/u/1/d/12D7tkhZi887g9d0U2askU9JypU_wYiEI7Lw0bfwxUgA
 """
 
 import logging
 from telemetry.core import util
 
 # Default Cache Temperature. The page doesn't care which browser cache state
 # it is run on.
 ANY = 'any'
 # Emulates PageCycler V1 cold runs. Clears system DNS cache, browser DiskCache,
 # net/ predictor cache, and net/ host resolver cache.
 PCV1_COLD = 'pcv1-cold'
 # Emulates PageCycler V1 warm runs. Ensures that the page was visited at least
 # once just before the run.
 PCV1_WARM = 'pcv1-warm'
 
+class MarkTelemetryInternal(object):
+
+  def __init__(self, browser, identifier):
+    self.browser = browser
+    self.identifier = identifier
+
+  def __enter__(self):
+    self.browser.tabs[0].ExecuteJavaScript(
+        """console.time('telemetry.{0}.warmCache.start');""".format(
+          self.identifier))
+    self.browser.tabs[0].ExecuteJavaScript(
+        """console.timeEnd('telemetry.{0}.warmCache.start');""".format(
+          self.identifier))
+    return self
+
+  def __exit__(self, exception_type, exception_value, traceback):
+    if exception_type:
+      return True
+
+    self.browser.tabs[0].ExecuteJavaScript(
+        """console.time('telemetry.{0}.warmCache.end');""".format(
+          self.identifier))
+    self.browser.tabs[0].ExecuteJavaScript(
+        """console.timeEnd('telemetry.{0}.warmCache.end');""".format(
+          self.identifier))
+    return True
 
 def EnsurePageCacheTemperature(page, browser, previous_page=None):
   temperature = page.cache_temperature
   logging.info('PageCacheTemperature: %s', temperature)
 
   if temperature == ANY:
     return
-  elif temperature == PCV1_COLD:
-    any_valid_tab = browser.tabs[0]
-    any_valid_tab.ClearCache(force=True)
+
+  if temperature == PCV1_COLD:
+    if previous_page is None:
+      with MarkTelemetryInternal(browser, 'ensure_diskcache'):
+        tab = browser.tabs[0]
+        tab.Navigate("http://does.not.exist")

[nednguyen, 2016/05/18 01:51:13]

Err I forgot to this. Can you add some comment explaining why we do this? Future
maintainers will thank us :-)

+        tab.WaitForDocumentReadyStateToBeComplete()
+
+    any_tab = browser.tabs[0]
+    any_tab.ClearCache(force=True)
   elif temperature == PCV1_WARM:
     if (previous_page is not None and
         previous_page.url == page.url and
         (previous_page.cache_temperature == PCV1_COLD or
             previous_page.cache_temperature == PCV1_WARM)):
       return
 
-    tab = browser.tabs[0]
-    tab.ExecuteJavaScript(
-        """console.time('telemetry.internal.warmCache.start');""")
-    tab.ExecuteJavaScript(
-        """console.timeEnd('telemetry.internal.warmCache.start');""")
-    tab.Navigate(page.url)
-    util.WaitFor(tab.HasReachedQuiescence, 60)
-    tab.WaitForDocumentReadyStateToBeComplete()
-    tab.Navigate("about:blank")
-    tab.WaitForDocumentReadyStateToBeComplete()
-    tab.ExecuteJavaScript(
-        """console.time('telemetry.internal.warmCache.end');""")
-    tab.ExecuteJavaScript(
-        """console.timeEnd('telemetry.internal.warmCache.end');""")
+    with MarkTelemetryInternal(browser, 'warmCache'):
+      tab = browser.tabs[0]
+      tab.Navigate(page.url)
+      util.WaitFor(tab.HasReachedQuiescence, 60)
+      tab.WaitForDocumentReadyStateToBeComplete()
+      tab.Navigate("about:blank")
+      tab.WaitForDocumentReadyStateToBeComplete()