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//lib/python3.9/site-packages/dnf/lock.py

# lock.py
# DNF Locking Subsystem.
#
# Copyright (C) 2013-2016 Red Hat, Inc.
#
# This copyrighted material is made available to anyone wishing to use,
# modify, copy, or redistribute it subject to the terms and conditions of
# the GNU General Public License v.2, or (at your option) any later version.
# This program is distributed in the hope that it will be useful, but WITHOUT
# ANY WARRANTY expressed or implied, including the implied warranties of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General
# Public License for more details.  You should have received a copy of the
# GNU General Public License along with this program; if not, write to the
# Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
# 02110-1301, USA.  Any Red Hat trademarks that are incorporated in the
# source code or documentation are not subject to the GNU General Public
# License and may only be used or replicated with the express permission of
# Red Hat, Inc.
#

from __future__ import absolute_import
from __future__ import unicode_literals
from dnf.exceptions import ProcessLockError, ThreadLockError, LockError
from dnf.i18n import _
from dnf.yum import misc
import dnf.logging
import dnf.util
import errno
import fcntl
import hashlib
import logging
import os
import threading
import time

logger = logging.getLogger("dnf")

def _fit_lock_dir(dir_):
    if not dnf.util.am_i_root():
        # for regular users the best we currently do is not to clash with
        # another DNF process of the same user. Since dir_ is quite definitely
        # not writable for us, yet significant, use its hash:
        hexdir = hashlib.sha1(dir_.encode('utf-8')).hexdigest()
        dir_ = os.path.join(misc.getCacheDir(), 'locks', hexdir)
    return dir_

def build_download_lock(cachedir, exit_on_lock):
    return ProcessLock(os.path.join(_fit_lock_dir(cachedir), 'download_lock.pid'),
                       'cachedir', not exit_on_lock)

def build_metadata_lock(cachedir, exit_on_lock):
    return ProcessLock(os.path.join(_fit_lock_dir(cachedir), 'metadata_lock.pid'),
                       'metadata', not exit_on_lock)

def build_rpmdb_lock(persistdir, exit_on_lock):
    return ProcessLock(os.path.join(_fit_lock_dir(persistdir), 'rpmdb_lock.pid'),
                       'RPMDB', not exit_on_lock)

def build_log_lock(logdir, exit_on_lock):
    return ProcessLock(os.path.join(_fit_lock_dir(logdir), 'log_lock.pid'),
                       'log', not exit_on_lock)

class ProcessLock(object):
    def __init__(self, target, description, blocking=False):
        self.blocking = blocking
        self.count = 0
        self.description = description
        self.target = target
        self.thread_lock = threading.RLock()

def _lock_thread(self):
        if not self.thread_lock.acquire(blocking=False):
            msg = '%s already locked by a different thread' % self.description
            raise ThreadLockError(msg)
        self.count += 1

def _try_lock(self, pid):
        fd = os.open(self.target, os.O_CREAT | os.O_RDWR, 0o644)

try:
            try:
                fcntl.flock(fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
            except OSError as e:
                if e.errno == errno.EWOULDBLOCK:
                    return -1
                raise

old_pid = os.read(fd, 20)
            if len(old_pid) == 0:
                # empty file, write our pid
                os.write(fd, str(pid).encode('utf-8'))
                return pid

try:
                old_pid = int(old_pid)
            except ValueError:
                msg = _('Malformed lock file found: %s.\n'
                        'Ensure no other dnf/yum process is running and '
                        'remove the lock file manually or run '
                        'systemd-tmpfiles --remove dnf.conf.') % (self.target)
                raise LockError(msg)

if old_pid == pid:
                # already locked by this process
                return pid

if not os.access('/proc/%d/stat' % old_pid, os.F_OK):
                # locked by a dead process, write our pid
                os.lseek(fd, 0, os.SEEK_SET)
                os.ftruncate(fd, 0)
                os.write(fd, str(pid).encode('utf-8'))
                return pid

return old_pid

finally:
            os.close(fd)

def _unlock_thread(self):
        self.count -= 1
        self.thread_lock.release()

def __enter__(self):
        dnf.util.ensure_dir(os.path.dirname(self.target))
        self._lock_thread()
        prev_pid = -1
        my_pid = os.getpid()
        pid = self._try_lock(my_pid)
        while pid != my_pid:
            if pid != -1:
                if not self.blocking:
                    self._unlock_thread()
                    msg = '%s already locked by %d' % (self.description, pid)
                    raise ProcessLockError(msg, pid)
                if prev_pid != pid:
                    msg = _('Waiting for process with pid %d to finish.') % (pid)
                    logger.info(msg)
                    prev_pid = pid
            time.sleep(1)
            pid = self._try_lock(my_pid)

def __exit__(self, *exc_args):
        if self.count == 1:
            os.unlink(self.target)
        self._unlock_thread()