path: root/lib/spack/llnl/util/lock.py

##############################################################################
# Copyright (c) 2013-2016, Lawrence Livermore National Security, LLC.
# Produced at the Lawrence Livermore National Laboratory.
#
# This file is part of Spack.
# Created by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
# LLNL-CODE-647188
#
# For details, see https://github.com/llnl/spack
# Please also see the LICENSE file for our notice and the LGPL.
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU Lesser General Public License (as
# published by the Free Software Foundation) version 2.1, February 1999.
#
# This program is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
# conditions of the GNU Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public
# License along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
##############################################################################
import os
import fcntl
import errno
import time
import socket

import llnl.util.tty as tty


__all__ = ['Lock', 'LockTransaction', 'WriteTransaction', 'ReadTransaction',
           'LockError']


# Default timeout in seconds, after which locks will raise exceptions.
_default_timeout = 60

# Sleep time per iteration in spin loop (in seconds)
_sleep_time = 1e-5


class Lock(object):
    """This is an implementation of a filesystem lock using Python's lockf.

    In Python, `lockf` actually calls `fcntl`, so this should work with
    any filesystem implementation that supports locking through the fcntl
    calls.  This includes distributed filesystems like Lustre (when flock
    is enabled) and recent NFS versions.
    """

    def __init__(self, path, start=0, length=0):
        """Construct a new lock on the file at ``path``.

        By default, the lock applies to the whole file.  Optionally,
        caller can specify a byte range beginning ``start`` bytes from
        the start of the file and extending ``length`` bytes from there.

        This exposes a subset of fcntl locking functionality.  It does
        not currently expose the ``whence`` parameter -- ``whence`` is
        always os.SEEK_SET and ``start`` is always evaluated from the
        beginning of the file.
        """
        self.path = path
        self._file = None
        self._reads = 0
        self._writes = 0

        # byte range parameters
        self._start = start
        self._length = length

        # PID and host of lock holder
        self.pid = self.old_pid = None
        self.host = self.old_host = None

    def _lock(self, op, timeout=_default_timeout):
        """This takes a lock using POSIX locks (``fnctl.lockf``).

        The lock is implemented as a spin lock using a nonblocking call
        to lockf().

        On acquiring an exclusive lock, the lock writes this process's
        pid and host to the lock file, in case the holding process needs
        to be killed later.

        If the lock times out, it raises a ``LockError``.
        """
        start_time = time.time()
        while (time.time() - start_time) < timeout:
            try:
                # If we could write the file, we'd have opened it 'r+'.
                # Raise an error when we attempt to upgrade to a write lock.
                if op == fcntl.LOCK_EX:
                    if self._file and self._file.mode == 'r':
                        raise LockError(
                            "Can't take exclusive lock on read-only file: %s"
                            % self.path)

                # Create file and parent directories if they don't exist.
                if self._file is None:
                    self._ensure_parent_directory()

                    # Prefer to open 'r+' to allow upgrading to write
                    # lock later if possible.  Open read-only if we can't
                    # write the lock file at all.
                    os_mode, fd_mode = (os.O_RDWR | os.O_CREAT), 'r+'
                    if os.path.exists(self.path) and not os.access(
                            self.path, os.W_OK):
                        os_mode, fd_mode = os.O_RDONLY, 'r'

                    fd = os.open(self.path, os_mode)
                    self._file = os.fdopen(fd, fd_mode)

                # Try to get the lock (will raise if not available.)
                fcntl.lockf(self._file, op | fcntl.LOCK_NB,
                            self._length, self._start, os.SEEK_SET)

                # All locks read the owner PID and host
                self._read_lock_data()

                # Exclusive locks write their PID/host
                if op == fcntl.LOCK_EX:
                    self._write_lock_data()

                return

            except IOError as error:
                if error.errno == errno.EAGAIN or error.errno == errno.EACCES:
                    pass
                else:
                    raise
            time.sleep(_sleep_time)

        raise LockError("Timed out waiting for lock.")

    def _ensure_parent_directory(self):
        parent = os.path.dirname(self.path)
        try:
            os.makedirs(parent)
            return True
        except OSError as e:
            # makedirs can fail when diretory already exists.
            if not (e.errno == errno.EEXIST and os.path.isdir(parent) or
                    e.errno == errno.EISDIR):
                raise

    def _read_lock_data(self):
        """Read PID and host data out of the file if it is there."""
        line = self._file.read()
        if line:
            pid, host = line.strip().split(',')
            _, _, self.pid = pid.rpartition('=')
            _, _, self.host = host.rpartition('=')

    def _write_lock_data(self):
        """Write PID and host data to the file, recording old values."""
        self.old_pid = self.pid
        self.old_host = self.host

        self.pid = os.getpid()
        self.host = socket.getfqdn()

        # write pid, host to disk to sync over FS
        self._file.seek(0)
        self._file.write("pid=%s,host=%s" % (self.pid, self.host))
        self._file.truncate()
        self._file.flush()
        os.fsync(self._file.fileno())

    def _unlock(self):
        """Releases a lock using POSIX locks (``fcntl.lockf``)

        Releases the lock regardless of mode. Note that read locks may
        be masquerading as write locks, but this removes either.

        """
        fcntl.lockf(self._file, fcntl.LOCK_UN,
                    self._length, self._start, os.SEEK_SET)
        self._file.close()
        self._file = None

    def acquire_read(self, timeout=_default_timeout):
        """Acquires a recursive, shared lock for reading.

        Read and write locks can be acquired and released in arbitrary
        order, but the POSIX lock is held until all local read and
        write locks are released.

        Returns True if it is the first acquire and actually acquires
        the POSIX lock, False if it is a nested transaction.

        """
        if self._reads == 0 and self._writes == 0:
            tty.debug('READ LOCK: {0.path}[{0._start}:{0._length}] [Acquiring]'
                      .format(self))
            self._lock(fcntl.LOCK_SH, timeout=timeout)   # can raise LockError.
            self._reads += 1
            return True
        else:
            self._reads += 1
            return False

    def acquire_write(self, timeout=_default_timeout):
        """Acquires a recursive, exclusive lock for writing.

        Read and write locks can be acquired and released in arbitrary
        order, but the POSIX lock is held until all local read and
        write locks are released.

        Returns True if it is the first acquire and actually acquires
        the POSIX lock, False if it is a nested transaction.

        """
        if self._writes == 0:
            tty.debug(
                'WRITE LOCK: {0.path}[{0._start}:{0._length}] [Acquiring]'
                .format(self))
            self._lock(fcntl.LOCK_EX, timeout=timeout)   # can raise LockError.
            self._writes += 1
            return True
        else:
            self._writes += 1
            return False

    def release_read(self):
        """Releases a read lock.

        Returns True if the last recursive lock was released, False if
        there are still outstanding locks.

        Does limited correctness checking: if a read lock is released
        when none are held, this will raise an assertion error.

        """
        assert self._reads > 0

        if self._reads == 1 and self._writes == 0:
            tty.debug('READ LOCK: {0.path}[{0._start}:{0._length}] [Released]'
                      .format(self))
            self._unlock()      # can raise LockError.
            self._reads -= 1
            return True
        else:
            self._reads -= 1
            return False

    def release_write(self):
        """Releases a write lock.

        Returns True if the last recursive lock was released, False if
        there are still outstanding locks.

        Does limited correctness checking: if a read lock is released
        when none are held, this will raise an assertion error.

        """
        assert self._writes > 0

        if self._writes == 1 and self._reads == 0:
            tty.debug('WRITE LOCK: {0.path}[{0._start}:{0._length}] [Released]'
                      .format(self))
            self._unlock()      # can raise LockError.
            self._writes -= 1
            return True
        else:
            self._writes -= 1
            return False


class LockTransaction(object):
    """Simple nested transaction context manager that uses a file lock.

    This class can trigger actions when the lock is acquired for the
    first time and released for the last.

    If the acquire_fn returns a value, it is used as the return value for
    __enter__, allowing it to be passed as the `as` argument of a `with`
    statement.

    If acquire_fn returns a context manager, *its* `__enter__` function will be
    called in `__enter__` after acquire_fn, and its `__exit__` funciton will be
    called before `release_fn` in `__exit__`, allowing you to nest a context
    manager to be used along with the lock.

    Timeout for lock is customizable.

    """

    def __init__(self, lock, acquire_fn=None, release_fn=None,
                 timeout=_default_timeout):
        self._lock = lock
        self._timeout = timeout
        self._acquire_fn = acquire_fn
        self._release_fn = release_fn
        self._as = None

    def __enter__(self):
        if self._enter() and self._acquire_fn:
            self._as = self._acquire_fn()
            if hasattr(self._as, '__enter__'):
                return self._as.__enter__()
            else:
                return self._as

    def __exit__(self, type, value, traceback):
        suppress = False
        if self._exit():
            if self._as and hasattr(self._as, '__exit__'):
                if self._as.__exit__(type, value, traceback):
                    suppress = True
            if self._release_fn:
                if self._release_fn(type, value, traceback):
                    suppress = True
        return suppress


class ReadTransaction(LockTransaction):

    def _enter(self):
        return self._lock.acquire_read(self._timeout)

    def _exit(self):
        return self._lock.release_read()


class WriteTransaction(LockTransaction):

    def _enter(self):
        return self._lock.acquire_write(self._timeout)

    def _exit(self):
        return self._lock.release_write()


class LockError(Exception):
    """Raised when an attempt to acquire a lock times out."""