Set limit for field trial parameters length.

Windows has a limit of 32767 on a command line. When testing with field
trails, the command to launch chrome.exe may exceed the limit if there're
too many field trial configs passed as parameters. This was causing
the chrome perf waterfall to completely fail on Windows.

This CL added a presubmit check to see whether the configs under Windows
will exceed the limit of 31000.

Bug: chromium:1045530
Change-Id: I9d181783f1bdad58eeba802583c052bfd0321c6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2021468
Commit-Queue: Wenbin Zhang <wenbinzhang@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735981}

testing/variations/PRESUBMIT.py

@@ -10,6 +10,10 @@ for more details on the presubmit API built into depot_tools.
 import copy
 import json
 import sys
+import os
+
+sys.path.insert(0, os.path.abspath('../../tools/variations'))
+import fieldtrial_util
 
 from collections import OrderedDict
 
@@ -18,6 +22,16 @@ VALID_EXPERIMENT_KEYS = [
     '//0', '//1', '//2', '//3', '//4', '//5', '//6', '//7', '//8', '//9'
 ]
 
+FIELDTRIAL_CONFIG_FILE_NAME = 'fieldtrial_testing_config.json'
+
+FIELDTRIAL_CONFIG_TOO_LONG_ERROR_MSG = \
+  "Contents of %s result in command-line flags that exceed Windows' 32767 "\
+  "character limit. To add a new entry, please remove existing obsolete "\
+  "entries. To check if an entry is obsolete, do a code search for its "\
+  "'enable_features' and 'disable_features' strings and verify there are "\
+  "no results other than the files related to the testing config. "\
+  "See crbug.com/1045530 for more details." % FIELDTRIAL_CONFIG_FILE_NAME
+
 
 def PrettyPrint(contents):
   """Pretty prints a fieldtrial configuration.
@@ -106,6 +120,9 @@ def ValidateData(json_data, file_path, message_type):
     return _CreateMalformedConfigMessage(message_type, file_path,
                                          message_format, *args)
 
+  if not _IsFieldTrialSizeBelowLimitOnWindows(file_path):
+    return [message_type(FIELDTRIAL_CONFIG_TOO_LONG_ERROR_MSG)]
+
   if not isinstance(json_data, dict):
     return _CreateMessage('Expecting dict')
   for (study, experiment_configs) in json_data.iteritems():
@@ -228,15 +245,45 @@ def CheckPretty(contents, file_path, message_type):
   return []
 
 
+def _IsFieldTrialSizeBelowLimitOnWindows(file_path):
+  """Checks whether the fieldtrial parameters exceeded the Windows cmd limit.
+
+  When launching chrome, the fieldtrial related parameters take more than
+  30,000 characters. Windows has a limit of 32767 characters on command
+  line and thus it raises parameter error when the parameters are too long.
+  Before we have a valid fix, we need to limit the number of fieldtrias in
+  fieldtrial_testing_config.json to 31,000, from the fact that the
+  non-fieldtrial parameters take roughly 1450 characters.
+  See crbug.com/1045530 for more details.
+
+  Args:
+    file_path: the absolute path to the json file with the fieldtrial configs.
+  """
+  args = fieldtrial_util.GenerateArgs(file_path, 'windows')
+  total_length = 0
+  for arg in args:
+    total_length += len(arg)
+  return total_length < 31000
+
+
 def CommonChecks(input_api, output_api):
   affected_files = input_api.AffectedFiles(
       include_deletes=False,
       file_filter=lambda x: x.LocalPath().endswith('.json'))
   for f in affected_files:
+    if not f.LocalPath().endswith(FIELDTRIAL_CONFIG_FILE_NAME):
+      return [
+          output_api.PresubmitError(
+              '%s is the only json file expected in this folder. If new jsons '
+              'are added, please update the presubmit process with proper '
+              'validation. ' % FIELDTRIAL_CONFIG_FILE_NAME
+          )
+      ]
     contents = input_api.ReadFile(f)
     try:
       json_data = input_api.json.loads(contents)
-      result = ValidateData(json_data, f.LocalPath(), output_api.PresubmitError)
+      result = ValidateData(
+          json_data, f.AbsoluteLocalPath(), output_api.PresubmitError)
       if len(result):
         return result
       result = CheckPretty(contents, f.LocalPath(), output_api.PresubmitError)