old_contents: #! /usr/bin/env nix-shell
  #! nix-shell -p common-updater-scripts python3
  #! nix-shell -i python
  
  import csv
  import json
  import re
  import shlex
  import subprocess
  from os.path import abspath, dirname, splitext
  from urllib.request import urlopen
  
  # CrOS version numbers look like this:
  # [<chrome-major-version>.]<tip-build>.<branch-build>.<branch-branch-build>
  #
  # As far as I can tell, branches are where internal Google
  # modifications are added to turn Chromium OS into Chrome OS, and
  # branch branches are used for fixes for specific devices.  So for
  # Chromium OS they will always be 0.  This is a best guess, and is not
  # documented.
  with urlopen('https://chromiumdash.appspot.com/cros/download_serving_builds_csv?deviceCategory=ChromeOS') as resp:
      reader = csv.reader(map(bytes.decode, resp))
      header = reader.__next__()
      cr_stable_index = header.index('cr_stable')
      cros_stable_index = header.index('cros_stable')
      chrome_version = []
      platform_version = []
  
      for line in reader:
          this_chrome_version = list(map(int, line[cr_stable_index].split('.')))
          this_platform_version = list(map(int, line[cros_stable_index].split('.')))
          chrome_version = max(chrome_version, this_chrome_version)
          platform_version = max(platform_version, this_platform_version)
  
  chrome_major_version = chrome_version[0]
  chromeos_tip_build = platform_version[0]
  release_branch = f'release-R{chrome_major_version}-{chromeos_tip_build}.B-chromeos'
  
  # Determine the git revision.
  with urlopen(f'https://chromium.googlesource.com/chromiumos/platform/crosvm/+/refs/heads/{release_branch}?format=JSON') as resp:
      resp.readline() # Remove )]}' header
      rev = json.load(resp)['commit']
  
  # Determine the patch version by counting the commits that have been
  # added to the release branch since it forked off the chromeos branch.
  with urlopen(f'https://chromium.googlesource.com/chromiumos/platform/crosvm/+log/refs/heads/chromeos..{rev}?format=JSON') as resp:
      resp.readline() # Remove )]}' header
      branch_commits = json.load(resp)['log']
      version = f'{chrome_major_version}.{len(branch_commits)}'
  
  # Update the version, git revision, and hash in crosvm's default.nix.
  subprocess.run(['update-source-version', 'crosvm', f'--rev={rev}', version])
  
  # Find the path to crosvm's default.nix, so Cargo.lock can be written
  # into the same directory.
  argv = ['nix-instantiate', '--eval', '--json', '-A', 'crosvm.meta.position']
  position = json.loads(subprocess.check_output(argv).decode('utf-8'))
  filename = re.match(r'[^:]*', position)[0]
  
  # Generate a Cargo.lock
  run = ['.',
         dirname(abspath(__file__)) + '/generate-cargo.sh',
         dirname(filename) + '/Cargo.lock']
  expr = '(import ./. {}).crosvm.overrideAttrs (_: { dontCargoSetupPostUnpack = true; })'
  subprocess.run(['nix-shell', '-E', expr, '--run', shlex.join(run)])
  
new_contents: #! /usr/bin/env nix-shell
  #! nix-shell -p common-updater-scripts python3
  #! nix-shell -i python
  
  import csv
  import json
  import re
  import shlex
  import subprocess
  from os.path import abspath, dirname, splitext
  from urllib.request import urlopen
  
  # CrOS version numbers look like this:
  # [<chrome-major-version>.]<tip-build>.<branch-build>.<branch-branch-build>
  #
  # As far as I can tell, branches are where internal Google
  # modifications are added to turn Chromium OS into Chrome OS, and
  # branch branches are used for fixes for specific devices.  So for
  # Chromium OS they will always be 0.  This is a best guess, and is not
  # documented.
  with urlopen('https://chromiumdash.appspot.com/cros/download_serving_builds_csv?deviceCategory=ChromeOS') as resp:
      reader = csv.reader(map(bytes.decode, resp))
      header = reader.__next__()
      cr_stable_index = header.index('cr_stable')
      cros_stable_index = header.index('cros_stable')
      chrome_version = []
      platform_version = []
  
      for line in reader:
          this_chrome_version = list(map(int, line[cr_stable_index].split('.')))
          this_platform_version = list(map(int, line[cros_stable_index].split('.')))
          chrome_version = max(chrome_version, this_chrome_version)
          platform_version = max(platform_version, this_platform_version)
  
  chrome_major_version = chrome_version[0]
  chromeos_tip_build = platform_version[0]
  release_branch = f'release-R{chrome_major_version}-{chromeos_tip_build}.B'
  
  # Determine the git revision.
  with urlopen(f'https://chromium.googlesource.com/chromiumos/platform/crosvm/+/refs/heads/{release_branch}?format=JSON') as resp:
      resp.readline() # Remove )]}' header
      rev = json.load(resp)['commit']
  
  # Determine the patch version by counting the commits that have been
  # added to the release branch since it forked off the chromeos branch.
  with urlopen(f'https://chromium.googlesource.com/chromiumos/platform/crosvm/+log/refs/heads/chromeos..{rev}?format=JSON') as resp:
      resp.readline() # Remove )]}' header
      branch_commits = json.load(resp)['log']
      version = f'{chrome_major_version}.{len(branch_commits)}'
  
  # Update the version, git revision, and hash in crosvm's default.nix.
  subprocess.run(['update-source-version', 'crosvm', f'--rev={rev}', version])
  
  # Find the path to crosvm's default.nix, so Cargo.lock can be written
  # into the same directory.
  argv = ['nix-instantiate', '--eval', '--json', '-A', 'crosvm.meta.position']
  position = json.loads(subprocess.check_output(argv).decode('utf-8'))
  filename = re.match(r'[^:]*', position)[0]
  
  # Generate a Cargo.lock
  run = ['.',
         dirname(abspath(__file__)) + '/generate-cargo.sh',
         dirname(filename) + '/Cargo.lock']
  expr = '(import ./. {}).crosvm.overrideAttrs (_: { dontCargoSetupPostUnpack = true; })'
  subprocess.run(['nix-shell', '-E', expr, '--run', shlex.join(run)])
  
subject: update release branch format