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Use and prefer os.posix_spawn() when available #54

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Use and prefer os.posix_spawn() when available #54

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8b44a5d
Use and prefer os.posix_spawn() when available
cagney Apr 11, 2019
892d8ef
Reduce scope of lock to just os.openpty() and set O_CLOEXEC
cagney Apr 12, 2019
a6501d6
Use setpgroup=0 to put the child in its own process group.
cagney Apr 12, 2019
9d0c278
Require POSIX_SPAWN_SETSID extension.
cagney Apr 12, 2019
fce5af0
Delete stray print() lines.
cagney Apr 12, 2019
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270 changes: 176 additions & 94 deletions ptyprocess/ptyprocess.py
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Original file line number Diff line number Diff line change
Expand Up @@ -10,19 +10,34 @@
import sys
import termios
import time
import threading

try:
import builtins # Python 3
except ImportError:
import __builtin__ as builtins # Python 2

# Constants
from pty import (STDIN_FILENO, CHILD)
from pty import (STDIN_FILENO, CHILD, STDOUT_FILENO, STDERR_FILENO)

from .util import which, PtyProcessError

_platform = sys.platform.lower()

# Is posix_spawn() robust? Require both os.posix_spawn() and the
# POSIX_SPAWN_SETSID extension (the latter, via setsid(),
# disassociates the child from the parent's session creating a new
# session and group and should be called when the system has things
# like job control) (there doesn't seem to be a runtime test for the
# latter, see bpo-36619). On systems that don't have job control,
# POSIX_SPAWN_SETPGROUP is probably sufficient but detecting that case
# is even harder.
_have_posix_spawn = (
hasattr(os, 'posix_spawn') and
_platform.startswith("linux"))
if _have_posix_spawn:
_posix_spawn_lock = threading.Lock()

# Solaris uses internal __fork_pty(). All others use pty.fork().
_is_solaris = (
_platform.startswith('solaris') or
Expand Down Expand Up @@ -217,124 +232,191 @@ def spawn(
command = command_with_path
argv[0] = command

# [issue #119] To prevent the case where exec fails and the user is
# stuck interacting with a python child process instead of whatever
# was expected, we implement the solution from
# http://stackoverflow.com/a/3703179 to pass the exception to the
# parent process

# [issue #119] 1. Before forking, open a pipe in the parent process.
exec_err_pipe_read, exec_err_pipe_write = os.pipe()

if use_native_pty_fork:
pid, fd = pty.fork()
else:
# Use internal fork_pty, for Solaris
pid, fd = _fork_pty.fork_pty()

# Some platforms must call setwinsize() and setecho() from the
# child process, and others from the master process. We do both,
# allowing IOError for either.

if pid == CHILD:
# set window size
if _have_posix_spawn:
with _posix_spawn_lock:
# Try to ensure that the tty/pty have O_CLOEXEC set
# (aka non-inheritable) so that a parallel call to
# this code won't end up with an open PTY/TTY.
# Unfortunately os.openpty() (openpty(3)) never sets
# O_CLOEXEC and pty.open() (Issue 36603) only
# sometimes sets O_CLOEXEC.
#
# Is this a bug? Arguably yes. However it isn't easy
# to fix - unless the native openpty(3) atomically
# opens the pty/tty with O_CLOEXEC there's always
# going to be a race. This lock mitigates the case
# where fork()/exec() is using this path (or other
# code paths such as subprocess that brute force a
# close of all FDs).
fd, tty = os.openpty()
os.set_inheritable(fd, False)
os.set_inheritable(tty, False)
# Try to set window size on TTY per below; but is this
# needed?
try:
_setwinsize(STDIN_FILENO, *dimensions)
_setwinsize(tty, *dimensions)
except IOError as err:
if err.args[0] not in (errno.EINVAL, errno.ENOTTY):
raise

# disable echo if spawn argument echo was unset
# Try to disable echo if spawn argument echo was unset per
# below; but does this work?
if not echo:
try:
_setecho(STDIN_FILENO, False)
_setecho(tty, False)
except (IOError, termios.error) as err:
if err.args[0] not in (errno.EINVAL, errno.ENOTTY):
raise
# Create the child:
#
# - convert the tty into STDIN / STDOUT / STDERR
#
# - always specify ENV (use the default if needed)
#
# - use setsid=True to create a new session (and
# disassociate the child from the parent's session /
# terminal) and make the child the process group leader
#
# - assume that all files have inheritable (close-on-exec)
# correctly set.
file_actions=[
(os.POSIX_SPAWN_DUP2, tty, STDIN_FILENO),
(os.POSIX_SPAWN_DUP2, tty, STDOUT_FILENO),
(os.POSIX_SPAWN_DUP2, tty, STDERR_FILENO),
# not needed?
(os.POSIX_SPAWN_CLOSE, tty),
(os.POSIX_SPAWN_CLOSE, fd),
]
spawn_env = env or os.environ
pid = os.posix_spawn(command, argv, spawn_env,
file_actions=file_actions,
setsid=0)
# Child started; close the child's tty.
os.close(tty)
else:

# [issue #119] To prevent the case where exec fails and
# the user is stuck interacting with a python child
# process instead of whatever was expected, we implement
# the solution from http://stackoverflow.com/a/3703179 to
# pass the exception to the parent process

# [issue #119] 1. Before forking, open a pipe in the
# parent process.
exec_err_pipe_read, exec_err_pipe_write = os.pipe()

# XXX: only use fork_pty()?
if use_native_pty_fork:
pid, fd = pty.fork()
else:
# Use internal fork_pty, for Solaris
pid, fd = _fork_pty.fork_pty()

# Some platforms must call setwinsize() and setecho() from
# the child process, and others from the master
# process. We do both, allowing IOError for either.

if pid == CHILD:
# set window size
try:
_setwinsize(STDIN_FILENO, *dimensions)
except IOError as err:
if err.args[0] not in (errno.EINVAL, errno.ENOTTY):
raise

# disable echo if spawn argument echo was unset
if not echo:
try:
_setecho(STDIN_FILENO, False)
except (IOError, termios.error) as err:
if err.args[0] not in (errno.EINVAL, errno.ENOTTY):
raise

# [issue #119] 3. The child closes the reading end and
# sets the close-on-exec flag for the writing end.
os.close(exec_err_pipe_read)
fcntl.fcntl(exec_err_pipe_write, fcntl.F_SETFD, fcntl.FD_CLOEXEC)

# Do not allow child to inherit open file descriptors from parent,
# with the exception of the exec_err_pipe_write of the pipe
# and pass_fds.

# Impose ceiling on max_fd: AIX bugfix for users with
# unlimited nofiles where resource.RLIMIT_NOFILE is
# 2^63-1 and os.closerange() occasionally raises out
# of range error
max_fd = min(1048576, resource.getrlimit(resource.RLIMIT_NOFILE)[0])
spass_fds = sorted(set(pass_fds) | {exec_err_pipe_write})
for pair in zip([2] + spass_fds, spass_fds + [max_fd]):
os.closerange(pair[0]+1, pair[1])

if cwd is not None:
os.chdir(cwd)

if preexec_fn is not None:
try:
preexec_fn()
except Exception as e:
ename = type(e).__name__
tosend = '{}:0:{}'.format(ename, str(e))
if PY3:
tosend = tosend.encode('utf-8')

os.write(exec_err_pipe_write, tosend)
os.close(exec_err_pipe_write)
os._exit(1)

# [issue #119] 3. The child closes the reading end and sets the
# close-on-exec flag for the writing end.
os.close(exec_err_pipe_read)
fcntl.fcntl(exec_err_pipe_write, fcntl.F_SETFD, fcntl.FD_CLOEXEC)

# Do not allow child to inherit open file descriptors from parent,
# with the exception of the exec_err_pipe_write of the pipe
# and pass_fds.
# Impose ceiling on max_fd: AIX bugfix for users with unlimited
# nofiles where resource.RLIMIT_NOFILE is 2^63-1 and os.closerange()
# occasionally raises out of range error
max_fd = min(1048576, resource.getrlimit(resource.RLIMIT_NOFILE)[0])
spass_fds = sorted(set(pass_fds) | {exec_err_pipe_write})
for pair in zip([2] + spass_fds, spass_fds + [max_fd]):
os.closerange(pair[0]+1, pair[1])

if cwd is not None:
os.chdir(cwd)

if preexec_fn is not None:
try:
preexec_fn()
except Exception as e:
ename = type(e).__name__
tosend = '{}:0:{}'.format(ename, str(e))
if env is None:
os.execv(command, argv)
else:
os.execvpe(command, argv, env)
except OSError as err:
# [issue #119] 5. If exec fails, the child writes
# the error code back to the parent using the
# pipe, then exits.
tosend = 'OSError:{}:{}'.format(err.errno, str(err))
if PY3:
tosend = tosend.encode('utf-8')
os.write(exec_err_pipe_write, tosend)
os.close(exec_err_pipe_write)
os._exit(os.EX_OSERR)

# [issue #119] 2. After forking, the parent closes the
# writing end of the pipe and reads from the reading end.
os.close(exec_err_pipe_write)
exec_err_data = os.read(exec_err_pipe_read, 4096)
os.close(exec_err_pipe_read)

os.write(exec_err_pipe_write, tosend)
os.close(exec_err_pipe_write)
os._exit(1)

try:
if env is None:
os.execv(command, argv)
# [issue #119] 6. The parent reads eof (a zero-length
# read) if the child successfully performed exec, since
# close-on-exec made successful exec close the writing end
# of the pipe. Or, if exec failed, the parent reads the
# error code and can proceed accordingly. Either way, the
# parent blocks until the child calls exec.
if len(exec_err_data) != 0:
try:
errclass, errno_s, errmsg = exec_err_data.split(b':', 2)
exctype = getattr(builtins, errclass.decode('ascii'), Exception)

exception = exctype(errmsg.decode('utf-8', 'replace'))
if exctype is OSError:
exception.errno = int(errno_s)
except:
raise Exception('Subprocess failed, got bad error data: %r'
% exec_err_data)
else:
os.execvpe(command, argv, env)
except OSError as err:
# [issue #119] 5. If exec fails, the child writes the error
# code back to the parent using the pipe, then exits.
tosend = 'OSError:{}:{}'.format(err.errno, str(err))
if PY3:
tosend = tosend.encode('utf-8')
os.write(exec_err_pipe_write, tosend)
os.close(exec_err_pipe_write)
os._exit(os.EX_OSERR)
raise exception

# Parent
inst = cls(pid, fd)

# Set some informational attributes
inst.argv = argv
if env is not None:
inst.env = env
if cwd is not None:
inst.launch_dir = cwd

# [issue #119] 2. After forking, the parent closes the writing end
# of the pipe and reads from the reading end.
os.close(exec_err_pipe_write)
exec_err_data = os.read(exec_err_pipe_read, 4096)
os.close(exec_err_pipe_read)

# [issue #119] 6. The parent reads eof (a zero-length read) if the
# child successfully performed exec, since close-on-exec made
# successful exec close the writing end of the pipe. Or, if exec
# failed, the parent reads the error code and can proceed
# accordingly. Either way, the parent blocks until the child calls
# exec.
if len(exec_err_data) != 0:
try:
errclass, errno_s, errmsg = exec_err_data.split(b':', 2)
exctype = getattr(builtins, errclass.decode('ascii'), Exception)

exception = exctype(errmsg.decode('utf-8', 'replace'))
if exctype is OSError:
exception.errno = int(errno_s)
except:
raise Exception('Subprocess failed, got bad error data: %r'
% exec_err_data)
else:
raise exception

try:
inst.setwinsize(*dimensions)
except IOError as err:
Expand Down