#!/usr/bin/env python """ process.py Trishank Karthik Kuppusamy Lukas Puehringer September 25, 2018 See LICENSE for licensing information. Provide a common interface for Python's subprocess module to: - namespace subprocess constants (DEVNULL, PIPE) and - provide a custom `subprocess.run` wrapper - provide a special `run_duplicate_streams` function """ import io import logging import os import shlex import subprocess # nosec import sys import tempfile import time from securesystemslib import formats, settings DEVNULL = subprocess.DEVNULL PIPE = subprocess.PIPE log = logging.getLogger(__name__) def _default_timeout(): """Helper to use securesystemslib.settings.SUBPROCESS_TIMEOUT as default argument, and still be able to modify it after the function definitions are evaluated.""" return settings.SUBPROCESS_TIMEOUT def run(cmd, check=True, timeout=_default_timeout(), **kwargs): """ Provide wrapper for `subprocess.run` (see https://github.com/python/cpython/blob/3.5/Lib/subprocess.py#L352-L399) where: * `timeout` has a default (securesystemslib.settings.SUBPROCESS_TIMEOUT), * `check` is `True` by default, * there is only one positional argument, i.e. `cmd` that can be either a str (will be split with shlex) or a list of str and * instead of raising a ValueError if both `input` and `stdin` are passed, `stdin` is ignored. cmd: The command and its arguments. (list of str, or str) Splits a string specifying a command and its argument into a list of substrings, if necessary. check: (default True) "If check is true, and the process exits with a non-zero exit code, a CalledProcessError exception will be raised. Attributes of that exception hold the arguments, the exit code, and stdout and stderr if they were captured." timeout: (default see securesystemslib.settings.SUBPROCESS_TIMEOUT) "The timeout argument is passed to Popen.communicate(). If the timeout expires, the child process will be killed and waited for. The TimeoutExpired exception will be re-raised after the child process has terminated." **kwargs: See subprocess.run and Frequently Used Arguments to Popen constructor for available kwargs. https://docs.python.org/3.5/library/subprocess.html#subprocess.run https://docs.python.org/3.5/library/subprocess.html#frequently-used-arguments securesystemslib.exceptions.FormatError: If the `cmd` is a list and does not match securesystemslib.formats.LIST_OF_ANY_STRING_SCHEMA. OSError: If the given command is not present or non-executable. subprocess.TimeoutExpired: If the process does not terminate after timeout seconds. Default is `settings.SUBPROCESS_TIMEOUT` The side effects of executing the given command in this environment. A subprocess.CompletedProcess instance. """ # Make list of command passed as string for convenience if isinstance(cmd, str): cmd = shlex.split(cmd) else: formats.LIST_OF_ANY_STRING_SCHEMA.check_match(cmd) # NOTE: The CPython implementation would raise a ValueError here, we just # don't pass on `stdin` if the user passes `input` and `stdin` # https://github.com/python/cpython/blob/3.5/Lib/subprocess.py#L378-L381 if kwargs.get("input") is not None and "stdin" in kwargs: log.debug( # pylint: disable=logging-not-lazy "stdin and input arguments may not both be used. " "Ignoring passed stdin: " + str(kwargs["stdin"]) ) del kwargs["stdin"] return subprocess.run(cmd, check=check, timeout=timeout, **kwargs) # nosec def run_duplicate_streams(cmd, timeout=_default_timeout()): """ Provide a function that executes a command in a subprocess and, upon termination, returns its exit code and the contents of what was printed to its standard streams. * Might behave unexpectedly with interactive commands. * Might not duplicate output in real time, if the command buffers it (see e.g. `print("foo")` vs. `print("foo", flush=True)` in Python 3). cmd: The command and its arguments. (list of str, or str) Splits a string specifying a command and its argument into a list of substrings, if necessary. timeout: (default see settings.SUBPROCESS_TIMEOUT) If the timeout expires, the child process will be killed and waited for and then subprocess.TimeoutExpired will be raised. securesystemslib.exceptions.FormatError: If the `cmd` is a list and does not match securesystemslib.formats.LIST_OF_ANY_STRING_SCHEMA. OSError: If the given command is not present or non-executable. subprocess.TimeoutExpired: If the process does not terminate after timeout seconds. Default is `settings.SUBPROCESS_TIMEOUT` The side effects of executing the given command in this environment. A tuple of command's exit code, standard output and standard error contents. """ if isinstance(cmd, str): cmd = shlex.split(cmd) else: formats.LIST_OF_ANY_STRING_SCHEMA.check_match(cmd) # Use temporary files as targets for child process standard stream redirects # They seem to work better (i.e. do not hang) than pipes, when using # interactive commands like `vi`. stdout_fd, stdout_name = tempfile.mkstemp() stderr_fd, stderr_name = tempfile.mkstemp() try: with io.open( # pylint: disable=unspecified-encoding stdout_name, "r" ) as stdout_reader, os.fdopen( # pylint: disable=unspecified-encoding stdout_fd, "w" ) as stdout_writer, io.open( # pylint: disable=unspecified-encoding stderr_name, "r" ) as stderr_reader, os.fdopen( stderr_fd, "w" ) as stderr_writer: # Store stream results in mutable dict to update it inside nested helper _std = {"out": "", "err": ""} # pylint: disable=invalid-name def _duplicate_streams(): """Helper to read from child process standard streams, write their contents to parent process standard streams, and build up return values for outer function. """ # Read until EOF but at most `io.DEFAULT_BUFFER_SIZE` bytes per call. # Reading and writing in reasonably sized chunks prevents us from # subverting a timeout, due to being busy for too long or indefinitely. stdout_part = stdout_reader.read(io.DEFAULT_BUFFER_SIZE) stderr_part = stderr_reader.read(io.DEFAULT_BUFFER_SIZE) sys.stdout.write(stdout_part) sys.stderr.write(stderr_part) sys.stdout.flush() sys.stderr.flush() _std["out"] += stdout_part _std["err"] += stderr_part # Start child process, writing its standard streams to temporary files proc = subprocess.Popen( # pylint: disable=consider-using-with # nosec cmd, stdout=stdout_writer, stderr=stderr_writer, universal_newlines=True, ) proc_start_time = time.time() # Duplicate streams until the process exits (or times out) while proc.poll() is None: # Time out as Python's `subprocess` would do it if ( timeout is not None and time.time() > proc_start_time + timeout ): proc.kill() proc.wait() raise subprocess.TimeoutExpired(cmd, timeout) _duplicate_streams() # Read/write once more to grab everything that the process wrote between # our last read in the loop and exiting, i.e. breaking the loop. _duplicate_streams() finally: # The work is done or was interrupted, the temp files can be removed os.remove(stdout_name) os.remove(stderr_name) # Return process exit code and captured streams return proc.poll(), _std["out"], _std["err"]