Test maximum shebang length

shebang.py

import os
import stat
import tempfile
import subprocess

shebang_prefix = '#!/usr/bin/env DUMMY=x'
shebang_suffix = ' python\n'
shebang_length_min = len(shebang_prefix) + len(shebang_suffix)

TEST_FILE_HANDLE, TEST_FILE = tempfile.mkstemp('.py')

def executable(shebang_length):
    assert shebang_length >= shebang_length_min
    num_extra_chars = shebang_length - shebang_length_min
    extra_chars = 'x' * num_extra_chars
    shebang = shebang_prefix + extra_chars + shebang_suffix
    return shebang + 'import sys; sys.exit(7)\n'

def shebang_works(shebang_length):
    with open(TEST_FILE, 'w') as f:
        f.write(executable(shebang_length))

    # Make executable
    st = os.stat(TEST_FILE)
    os.chmod(TEST_FILE, st.st_mode | stat.S_IEXEC)
    os.close(TEST_FILE_HANDLE)

    try:
        subprocess.check_output([TEST_FILE])
    except subprocess.CalledProcessError as e:
        if e.returncode == 7:
            return True
    except OSError as err:
        if err.args == (8, 'Exec format error'):
            # This OSError means shebang is bad
            return False
        else:
            # Not sure what this one means
            raise err
    return False

def main():
    # shebang works when (<=) bound_lower and does not work when (>=) bound_upper
    bound_lower = length_test = 32
    bound_upper = None

    print('finding upper bound')
    while bound_upper is None:
        if shebang_works(length_test):
            bound_lower = length_test
            length_test *= 2
        else:
            bound_upper = length_test

    print('searching cutoff')
    while bound_upper - bound_lower > 1:
        length_test = bound_lower + (bound_upper - bound_lower) // 2
        if shebang_works(length_test):
            bound_lower = length_test
        else:
            bound_upper = length_test

    print('maximum shebang length (including newline)')
    print(bound_lower)

if __name__ == '__main__':
    main()