Source code for llnl.util.lock

# Copyright 2013-2022 Lawrence Livermore National Security, LLC and other
# Spack Project Developers. See the top-level COPYRIGHT file for details.
#
# SPDX-License-Identifier: (Apache-2.0 OR MIT)

import errno
import os
import socket
import sys
import time
from datetime import datetime
from typing import Dict, Tuple  # novm

import llnl.util.tty as tty

import spack.util.string

if sys.platform != 'win32':
    import fcntl


__all__ = [
    'Lock',
    'LockDowngradeError',
    'LockUpgradeError',
    'LockTransaction',
    'WriteTransaction',
    'ReadTransaction',
    'LockError',
    'LockTimeoutError',
    'LockPermissionError',
    'LockROFileError',
    'CantCreateLockError'
]


#: A useful replacement for functions that should return True when not provided
#: for example.
true_fn = lambda: True


class OpenFile(object):
    """Record for keeping track of open lockfiles (with reference counting).

    There's really only one ``OpenFile`` per inode, per process, but we record the
    filehandle here as it's the thing we end up using in python code.  You can get
    the file descriptor from the file handle if needed -- or we could make this track
    file descriptors as well in the future.
    """
    def __init__(self, fh):
        self.fh = fh
        self.refs = 0


class OpenFileTracker(object):
    """Track open lockfiles, to minimize number of open file descriptors.

    The ``fcntl`` locks that Spack uses are associated with an inode and a process.
    This is convenient, because if a process exits, it releases its locks.
    Unfortunately, this also means that if you close a file, *all* locks associated
    with that file's inode are released, regardless of whether the process has any
    other open file descriptors on it.

    Because of this, we need to track open lock files so that we only close them when
    a process no longer needs them.  We do this by tracking each lockfile by its
    inode and process id.  This has several nice properties:

    1. Tracking by pid ensures that, if we fork, we don't inadvertently track the parent
       process's lockfiles. ``fcntl`` locks are not inherited across forks, so we'll
       just track new lockfiles in the child.
    2. Tracking by inode ensures that referencs are counted per inode, and that we don't
       inadvertently close a file whose inode still has open locks.
    3. Tracking by both pid and inode ensures that we only open lockfiles the minimum
       number of times necessary for the locks we have.

    Note: as mentioned elsewhere, these locks aren't thread safe -- they're designed to
    work in Python and assume the GIL.
    """

    def __init__(self):
        """Create a new ``OpenFileTracker``."""
        self._descriptors = {}  # type: Dict[Tuple[int, int], OpenFile]

    def get_fh(self, path):
        """Get a filehandle for a lockfile.

        This routine will open writable files for read/write even if you're asking
        for a shared (read-only) lock. This is so that we can upgrade to an exclusive
        (write) lock later if requested.

        Arguments:
          path (str): path to lock file we want a filehandle for
        """
        # Open writable files as 'r+' so we can upgrade to write later
        os_mode, fh_mode = (os.O_RDWR | os.O_CREAT), 'r+'

        pid = os.getpid()
        open_file = None  # OpenFile object, if there is one
        stat = None       # stat result for the lockfile, if it exists

        try:
            # see whether we've seen this inode/pid before
            stat = os.stat(path)
            key = (stat.st_ino, pid)
            open_file = self._descriptors.get(key)

        except OSError as e:
            if e.errno != errno.ENOENT:  # only handle file not found
                raise

            # path does not exist -- fail if we won't be able to create it
            parent = os.path.dirname(path) or '.'
            if not os.access(parent, os.W_OK):
                raise CantCreateLockError(path)

        # if there was no already open file, we'll need to open one
        if not open_file:
            if stat and not os.access(path, os.W_OK):
                # we know path exists but not if it's writable. If it's read-only,
                # only open the file for reading (and fail if we're trying to get
                # an exclusive (write) lock on it)
                os_mode, fh_mode = os.O_RDONLY, 'r'

            fd = os.open(path, os_mode)
            fh = os.fdopen(fd, fh_mode)
            open_file = OpenFile(fh)

            # if we just created the file, we'll need to get its inode here
            if not stat:
                inode = os.fstat(fd).st_ino
                key = (inode, pid)

            self._descriptors[key] = open_file

        open_file.refs += 1
        return open_file.fh

    def release_fh(self, path):
        """Release a filehandle, only closing it if there are no more references."""
        try:
            inode = os.stat(path).st_ino
        except OSError as e:
            if e.errno != errno.ENOENT:  # only handle file not found
                raise
            inode = None  # this will not be in self._descriptors

        key = (inode, os.getpid())
        open_file = self._descriptors.get(key)
        assert open_file, "Attempted to close non-existing lock path: %s" % path

        open_file.refs -= 1
        if not open_file.refs:
            del self._descriptors[key]
            open_file.fh.close()


#: Open file descriptors for locks in this process. Used to prevent one process
#: from opening the sam file many times for different byte range locks
file_tracker = OpenFileTracker()


def _attempts_str(wait_time, nattempts):
    # Don't print anything if we succeeded on the first try
    if nattempts <= 1:
        return ''

    attempts = spack.util.string.plural(nattempts, 'attempt')
    return ' after {0:0.2f}s and {1}'.format(wait_time, attempts)


class LockType(object):
    READ = 0
    WRITE = 1

    @staticmethod
    def to_str(tid):
        ret = "READ"
        if tid == LockType.WRITE:
            ret = "WRITE"
        return ret

    @staticmethod
    def to_module(tid):
        lock = fcntl.LOCK_SH
        if tid == LockType.WRITE:
            lock = fcntl.LOCK_EX
        return lock

    @staticmethod
    def is_valid(op):
        return op == LockType.READ \
            or op == LockType.WRITE


[docs]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. Note that this is for managing contention over resources *between* processes and not for managing contention between threads in a process: the functions of this object are not thread-safe. A process also must not maintain multiple locks on the same file (or, more specifically, on overlapping byte ranges in the same file). """ def __init__(self, path, start=0, length=0, default_timeout=None, debug=False, desc=''): """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. Args: path (str): path to the lock start (int): optional byte offset at which the lock starts length (int): optional number of bytes to lock default_timeout (int): number of seconds to wait for lock attempts, where None means to wait indefinitely debug (bool): debug mode specific to locking desc (str): optional debug message lock description, which is helpful for distinguishing between different Spack locks. """ self.path = path self._file = None self._reads = 0 self._writes = 0 # byte range parameters self._start = start self._length = length # enable debug mode self.debug = debug # optional debug description self.desc = ' ({0})'.format(desc) if desc else '' # If the user doesn't set a default timeout, or if they choose # None, 0, etc. then lock attempts will not time out (unless the # user sets a timeout for each attempt) self.default_timeout = default_timeout or None # PID and host of lock holder (only used in debug mode) self.pid = self.old_pid = None self.host = self.old_host = None @staticmethod def _poll_interval_generator(_wait_times=None): """This implements a backoff scheme for polling a contended resource by suggesting a succession of wait times between polls. It suggests a poll interval of .1s until 2 seconds have passed, then a poll interval of .2s until 10 seconds have passed, and finally (for all requests after 10s) suggests a poll interval of .5s. This doesn't actually track elapsed time, it estimates the waiting time as though the caller always waits for the full length of time suggested by this function. """ num_requests = 0 stage1, stage2, stage3 = _wait_times or (1e-1, 2e-1, 5e-1) wait_time = stage1 while True: if num_requests >= 60: # 40 * .2 = 8 wait_time = stage3 elif num_requests >= 20: # 20 * .1 = 2 wait_time = stage2 num_requests += 1 yield wait_time def __repr__(self): """Formal representation of the lock.""" rep = '{0}('.format(self.__class__.__name__) for attr, value in self.__dict__.items(): rep += '{0}={1}, '.format(attr, value.__repr__()) return '{0})'.format(rep.strip(', ')) def __str__(self): """Readable string (with key fields) of the lock.""" location = '{0}[{1}:{2}]'.format(self.path, self._start, self._length) timeout = 'timeout={0}'.format(self.default_timeout) activity = '#reads={0}, #writes={1}'.format(self._reads, self._writes) return '({0}, {1}, {2})'.format(location, timeout, activity) def _lock(self, op, timeout=None): """This takes a lock using POSIX locks (``fcntl.lockf``). The lock is implemented as a spin lock using a nonblocking call to ``lockf()``. If the lock times out, it raises a ``LockError``. If the lock is successfully acquired, the total wait time and the number of attempts is returned. """ assert LockType.is_valid(op) op_str = LockType.to_str(op) self._log_acquiring('{0} LOCK'.format(op_str)) timeout = timeout or self.default_timeout # Create file and parent directories if they don't exist. if self._file is None: self._ensure_parent_directory() self._file = file_tracker.get_fh(self.path) if LockType.to_module(op) == fcntl.LOCK_EX and self._file.mode == 'r': # Attempt to upgrade to write lock w/a read-only file. # If the file were writable, we'd have opened it 'r+' raise LockROFileError(self.path) self._log_debug("{0} locking [{1}:{2}]: timeout {3} sec" .format(op_str.lower(), self._start, self._length, timeout)) poll_intervals = iter(Lock._poll_interval_generator()) start_time = time.time() num_attempts = 0 while (not timeout) or (time.time() - start_time) < timeout: num_attempts += 1 if self._poll_lock(op): total_wait_time = time.time() - start_time return total_wait_time, num_attempts time.sleep(next(poll_intervals)) # TBD: Is an extra attempt after timeout needed/appropriate? num_attempts += 1 if self._poll_lock(op): total_wait_time = time.time() - start_time return total_wait_time, num_attempts raise LockTimeoutError("Timed out waiting for a {0} lock." .format(op_str.lower())) def _poll_lock(self, op): """Attempt to acquire the lock in a non-blocking manner. Return whether the locking attempt succeeds """ module_op = LockType.to_module(op) try: # Try to get the lock (will raise if not available.) fcntl.lockf(self._file, module_op | fcntl.LOCK_NB, self._length, self._start, os.SEEK_SET) # help for debugging distributed locking if self.debug: # All locks read the owner PID and host self._read_log_debug_data() self._log_debug('{0} locked {1} [{2}:{3}] (owner={4})' .format(LockType.to_str(op), self.path, self._start, self._length, self.pid)) # Exclusive locks write their PID/host if module_op == fcntl.LOCK_EX: self._write_log_debug_data() return True except IOError as e: # EAGAIN and EACCES == locked by another process (so try again) if e.errno not in (errno.EAGAIN, errno.EACCES): raise return False def _ensure_parent_directory(self): parent = os.path.dirname(self.path) # relative paths to lockfiles in the current directory have no parent if not parent: return '.' try: os.makedirs(parent) 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 return parent def _read_log_debug_data(self): """Read PID and host data out of the file if it is there.""" self.old_pid = self.pid self.old_host = self.host line = self._file.read() if line: pid, host = line.strip().split(',') _, _, self.pid = pid.rpartition('=') _, _, self.host = host.rpartition('=') self.pid = int(self.pid) def _write_log_debug_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.gethostname() # 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) file_tracker.release_fh(self.path) self._file = None self._reads = 0 self._writes = 0
[docs] def acquire_read(self, timeout=None): """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. """ timeout = timeout or self.default_timeout if self._reads == 0 and self._writes == 0: # can raise LockError. wait_time, nattempts = self._lock(LockType.READ, timeout=timeout) self._reads += 1 # Log if acquired, which includes counts when verbose self._log_acquired('READ LOCK', wait_time, nattempts) return True else: # Increment the read count for nested lock tracking self._reads += 1 return False
[docs] def acquire_write(self, timeout=None): """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. """ timeout = timeout or self.default_timeout if self._writes == 0: # can raise LockError. wait_time, nattempts = self._lock(LockType.WRITE, timeout=timeout) self._writes += 1 # Log if acquired, which includes counts when verbose self._log_acquired('WRITE LOCK', wait_time, nattempts) # return True only if we weren't nested in a read lock. # TODO: we may need to return two values: whether we got # the write lock, and whether this is acquiring a read OR # write lock for the first time. Now it returns the latter. return self._reads == 0 else: # Increment the write count for nested lock tracking self._writes += 1 return False
[docs] def is_write_locked(self): """Check if the file is write locked Return: (bool): ``True`` if the path is write locked, otherwise, ``False`` """ try: self.acquire_read() # If we have a read lock then no other process has a write lock. self.release_read() except LockTimeoutError: # Another process is holding a write lock on the file return True return False
[docs] def downgrade_write_to_read(self, timeout=None): """ Downgrade from an exclusive write lock to a shared read. Raises: LockDowngradeError: if this is an attempt at a nested transaction """ timeout = timeout or self.default_timeout if self._writes == 1 and self._reads == 0: self._log_downgrading() # can raise LockError. wait_time, nattempts = self._lock(LockType.READ, timeout=timeout) self._reads = 1 self._writes = 0 self._log_downgraded(wait_time, nattempts) else: raise LockDowngradeError(self.path)
[docs] def upgrade_read_to_write(self, timeout=None): """ Attempts to upgrade from a shared read lock to an exclusive write. Raises: LockUpgradeError: if this is an attempt at a nested transaction """ timeout = timeout or self.default_timeout if self._reads == 1 and self._writes == 0: self._log_upgrading() # can raise LockError. wait_time, nattempts = self._lock(LockType.WRITE, timeout=timeout) self._reads = 0 self._writes = 1 self._log_upgraded(wait_time, nattempts) else: raise LockUpgradeError(self.path)
[docs] def release_read(self, release_fn=None): """Releases a read lock. Arguments: release_fn (typing.Callable): function to call *before* the last recursive lock (read or write) is released. If the last recursive lock will be released, then this will call release_fn and return its result (if provided), or return True (if release_fn was not provided). Otherwise, we are still nested inside some other lock, so do not call the release_fn and, return False. Does limited correctness checking: if a read lock is released when none are held, this will raise an assertion error. """ assert self._reads > 0 locktype = 'READ LOCK' if self._reads == 1 and self._writes == 0: self._log_releasing(locktype) # we need to call release_fn before releasing the lock release_fn = release_fn or true_fn result = release_fn() self._unlock() # can raise LockError. self._reads = 0 self._log_released(locktype) return result else: self._reads -= 1 return False
[docs] def release_write(self, release_fn=None): """Releases a write lock. Arguments: release_fn (typing.Callable): function to call before the last recursive write is released. If the last recursive *write* lock will be released, then this will call release_fn and return its result (if provided), or return True (if release_fn was not provided). Otherwise, we are still nested inside some other write lock, so do not call the release_fn, and return False. Does limited correctness checking: if a read lock is released when none are held, this will raise an assertion error. """ assert self._writes > 0 release_fn = release_fn or true_fn locktype = 'WRITE LOCK' if self._writes == 1 and self._reads == 0: self._log_releasing(locktype) # we need to call release_fn before releasing the lock result = release_fn() self._unlock() # can raise LockError. self._writes = 0 self._log_released(locktype) return result else: self._writes -= 1 # when the last *write* is released, we call release_fn here # instead of immediately before releasing the lock. if self._writes == 0: return release_fn() else: return False
[docs] def cleanup(self): if self._reads == 0 and self._writes == 0: os.unlink(self.path) else: raise LockError("Attempting to cleanup active lock.")
def _get_counts_desc(self): return '(reads {0}, writes {1})'.format(self._reads, self._writes) \ if tty.is_verbose() else '' def _log_acquired(self, locktype, wait_time, nattempts): attempts_part = _attempts_str(wait_time, nattempts) now = datetime.now() desc = 'Acquired at %s' % now.strftime("%H:%M:%S.%f") self._log_debug(self._status_msg(locktype, '{0}{1}' .format(desc, attempts_part))) def _log_acquiring(self, locktype): self._log_debug(self._status_msg(locktype, 'Acquiring'), level=3) def _log_debug(self, *args, **kwargs): """Output lock debug messages.""" kwargs['level'] = kwargs.get('level', 2) tty.debug(*args, **kwargs) def _log_downgraded(self, wait_time, nattempts): attempts_part = _attempts_str(wait_time, nattempts) now = datetime.now() desc = 'Downgraded at %s' % now.strftime("%H:%M:%S.%f") self._log_debug(self._status_msg('READ LOCK', '{0}{1}' .format(desc, attempts_part))) def _log_downgrading(self): self._log_debug(self._status_msg('WRITE LOCK', 'Downgrading'), level=3) def _log_released(self, locktype): now = datetime.now() desc = 'Released at %s' % now.strftime("%H:%M:%S.%f") self._log_debug(self._status_msg(locktype, desc)) def _log_releasing(self, locktype): self._log_debug(self._status_msg(locktype, 'Releasing'), level=3) def _log_upgraded(self, wait_time, nattempts): attempts_part = _attempts_str(wait_time, nattempts) now = datetime.now() desc = 'Upgraded at %s' % now.strftime("%H:%M:%S.%f") self._log_debug(self._status_msg('WRITE LOCK', '{0}{1}'. format(desc, attempts_part))) def _log_upgrading(self): self._log_debug(self._status_msg('READ LOCK', 'Upgrading'), level=3) def _status_msg(self, locktype, status): status_desc = '[{0}] {1}'.format(status, self._get_counts_desc()) return '{0}{1.desc}: {1.path}[{1._start}:{1._length}] {2}'.format( locktype, self, status_desc)
[docs]class LockTransaction(object): """Simple nested transaction context manager that uses a file lock. Arguments: lock (Lock): underlying lock for this transaction to be accquired on enter and released on exit acquire (typing.Callable or contextlib.contextmanager): function to be called after lock is acquired, or contextmanager to enter after acquire and leave before release. release (typing.Callable): function to be called before release. If ``acquire`` is a contextmanager, this will be called *after* exiting the nexted context and before the lock is released. timeout (float): number of seconds to set for the timeout when accquiring the lock (default no timeout) 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 after the lock is acquired, and its ``__exit__`` funciton will be called before ``release_fn`` in ``__exit__``, allowing you to nest a context manager inside this one. Timeout for lock is customizable. """ def __init__(self, lock, acquire=None, release=None, timeout=None): self._lock = lock self._timeout = timeout self._acquire_fn = acquire self._release_fn = release 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 def release_fn(): if self._release_fn is not None: return self._release_fn(type, value, traceback) if self._as and hasattr(self._as, '__exit__'): if self._as.__exit__(type, value, traceback): suppress = True if self._exit(release_fn): suppress = True return suppress
[docs]class ReadTransaction(LockTransaction): """LockTransaction context manager that does a read and releases it.""" def _enter(self): return self._lock.acquire_read(self._timeout) def _exit(self, release_fn): return self._lock.release_read(release_fn)
[docs]class WriteTransaction(LockTransaction): """LockTransaction context manager that does a write and releases it.""" def _enter(self): return self._lock.acquire_write(self._timeout) def _exit(self, release_fn): return self._lock.release_write(release_fn)
[docs]class LockError(Exception): """Raised for any errors related to locks."""
[docs]class LockDowngradeError(LockError): """Raised when unable to downgrade from a write to a read lock.""" def __init__(self, path): msg = "Cannot downgrade lock from write to read on file: %s" % path super(LockDowngradeError, self).__init__(msg)
class LockLimitError(LockError): """Raised when exceed maximum attempts to acquire a lock."""
[docs]class LockTimeoutError(LockError): """Raised when an attempt to acquire a lock times out."""
[docs]class LockUpgradeError(LockError): """Raised when unable to upgrade from a read to a write lock.""" def __init__(self, path): msg = "Cannot upgrade lock from read to write on file: %s" % path super(LockUpgradeError, self).__init__(msg)
[docs]class LockPermissionError(LockError): """Raised when there are permission issues with a lock."""
[docs]class LockROFileError(LockPermissionError): """Tried to take an exclusive lock on a read-only file.""" def __init__(self, path): msg = "Can't take write lock on read-only file: %s" % path super(LockROFileError, self).__init__(msg)
[docs]class CantCreateLockError(LockPermissionError): """Attempt to create a lock in an unwritable location.""" def __init__(self, path): msg = "cannot create lock '%s': " % path msg += "file does not exist and location is not writable" super(LockError, self).__init__(msg)