compute_build_timestamp.py

#!/usr/bin/env python
# Copyright 2018 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.
"""Returns a timestamp that approximates the build date.

build_type impacts the timestamp generated:
- default: the build date is set to the most recent first Sunday of a month at
  5:00am. The reason is that it is a time where invalidating the build cache
  shouldn't have major reprecussions (due to lower load).
- official: the build date is set to the current date at 5:00am, or the day
  before if the current time is before 5:00am.
Either way, it is guaranteed to be in the past and always in UTC.
"""

# The requirements for the timestamp:
# (1) for the purposes of continuous integration, longer duration
#     between cache invalidation is better, but >=1mo is preferable.
# (2) for security purposes, timebombs would ideally be as close to
#     the actual time of the build as possible. It must be in the past.
# (3) HSTS certificate pinning is valid for 70 days. To make CI builds enforce
#     HTST pinning, <=1mo is preferable.
#
# On Windows, the timestamp is also written in the PE/COFF file header of
# executables of dlls.  That timestamp and the executable's file size are
# the only two pieces of information that identify a given executable on
# the symbol server, so rarely changing timestamps can cause conflicts there
# as well. We only upload symbols for official builds to the symbol server.

import argparse
import calendar
import datetime
import doctest
import sys


def GetFirstSundayOfMonth(year, month):
  """Returns the first sunday of the given month of the given year.

  >>> GetFirstSundayOfMonth(2016, 2)
  7
  >>> GetFirstSundayOfMonth(2016, 3)
  6
  >>> GetFirstSundayOfMonth(2000, 1)
  2
  """
  weeks = calendar.Calendar().monthdays2calendar(year, month)
  # Return the first day in the first week that is a Sunday.
  return [date_day[0] for date_day in weeks[0] if date_day[1] == 6][0]


def GetBuildDate(build_type, utc_now):
  """Gets the approximate build date given the specific build type.

  >>> GetBuildDate('default', datetime.datetime(2016, 2, 6, 1, 2, 3))
  datetime.datetime(2016, 1, 3, 1, 2, 3)
  >>> GetBuildDate('default', datetime.datetime(2016, 2, 7, 5))
  datetime.datetime(2016, 2, 7, 5, 0)
  >>> GetBuildDate('default', datetime.datetime(2016, 2, 8, 5))
  datetime.datetime(2016, 2, 7, 5, 0)
  >>> GetBuildDate('official', datetime.datetime(2016, 2, 8, 5))
  datetime.datetime(2016, 2, 8, 5, 0)
  """
  day = utc_now.day
  month = utc_now.month
  year = utc_now.year
  if build_type != 'official':
    first_sunday = GetFirstSundayOfMonth(year, month)
    # If our build is after the first Sunday, we've already refreshed our build
    # cache on a quiet day, so just use that day.
    # Otherwise, take the first Sunday of the previous month.
    if day >= first_sunday:
      day = first_sunday
    else:
      month -= 1
      if month == 0:
        month = 12
        year -= 1
      day = GetFirstSundayOfMonth(year, month)
  return datetime.datetime(
      year, month, day, utc_now.hour, utc_now.minute, utc_now.second)


def main():
  if doctest.testmod()[0]:
    return 1
  argument_parser = argparse.ArgumentParser()
  argument_parser.add_argument(
      'build_type', help='The type of build', choices=('official', 'default'))
  args = argument_parser.parse_args()

  now = datetime.datetime.utcnow()
  if now.hour < 5:
    # The time is locked at 5:00 am in UTC to cause the build cache
    # invalidation to not happen exactly at midnight. Use the same calculation
    # as the day before.
    # See //base/build_time.cc.
    now = now - datetime.timedelta(days=1)
  now = datetime.datetime(now.year, now.month, now.day, 5, 0, 0)
  build_date = GetBuildDate(args.build_type, now)
  print int(calendar.timegm(build_date.utctimetuple()))
  return 0


if __name__ == '__main__':
  sys.exit(main())