Add Data Saver ChromeDriver test for MissingViaHeader4xx bypass behavior

tools/chrome_proxy/webdriver/bypass.py

 # Copyright 2017 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.
 
 import common
 from common import TestDriver
 from common import IntegrationTest
 
 
 class Bypass(IntegrationTest):
@@ skipping 84 matching lines @@
       for response in responses:
           self.assertNotHasChromeProxyViaHeader(response)
 
       # Load HTTP page and check that Data Saver is used.
       test_driver.LoadURL('http://check.googlezip.net/test.html')
       responses = test_driver.GetHTTPResponses()
       self.assertNotEqual(0, len(responses))
       for response in responses:
         self.assertHasChromeProxyViaHeader(response)
 
+  # Verify that when Chrome receives a 4xx response through a Data Reduction
+  # Proxy that doesn't set a proper via header, Chrome bypasses all proxies and
+  # retries the request over direct.
+  def testMissingViaHeader4xxBypass(self):
+    with TestDriver() as test_driver:
+      test_driver.AddChromeArg('--enable-spdy-proxy-auth')
+
+      # Set the primary Data Reduction Proxy to be the test server, which does
+      # not add any Via headers.
+      test_driver.AddChromeArg('--data-reduction-proxy-http-proxies='
+                               'https://chromeproxy-test.appspot.com;'
+                               'http://compress.googlezip.net')
+
+      # Load a page that will come back with a 4xx response code and without the
+      # proper via header. Chrome should bypass all proxies and retry the
+      # request.
+      test_driver.LoadURL(
+          'http://chromeproxy-test.appspot.com/default?respStatus=414')
+      responses = test_driver.GetHTTPResponses()
+      self.assertNotEqual(0, len(responses))
+      for response in responses:
+        self.assertNotHasChromeProxyViaHeader(response)
+        self.assertEqual(u'http/1.1', response.protocol)
+
+      # Check that the BlockTypePrimary histogram has a single entry in the
+      # MissingViaHeader4xx category (which is enum value 4), to make sure that
+      # the bypass was caused by the missing via header logic and not something
+      # else.
+      histogram = test_driver.GetHistogram(
+          "DataReductionProxy.BlockTypePrimary")
+      self.assertEqual(1, histogram['count'])
+      self.assertIn({'count': 1, 'high': 5, 'low': 4}, histogram['buckets'])
+
 
 if __name__ == '__main__':
   IntegrationTest.RunAllTests()