borg/src/borg/platform/base.py

import errno
import os

"""
platform base module
====================

Contains platform API implementations based on what Python itself provides. More specific
APIs are stubs in this module.

When functions in this module use platform APIs themselves they access the public
platform API: that way platform APIs provided by the platform-specific support module
are correctly composed into the base functionality.
"""

API_VERSION = '1.1_01'

fdatasync = getattr(os, 'fdatasync', os.fsync)


def acl_get(path, item, st, numeric_owner=False):
    """
    Saves ACL Entries

    If `numeric_owner` is True the user/group field is not preserved only uid/gid
    """


def acl_set(path, item, numeric_owner=False):
    """
    Restore ACL Entries

    If `numeric_owner` is True the stored uid/gid is used instead
    of the user/group names
    """


try:
    from os import lchflags

    def set_flags(path, bsd_flags, fd=None):
        lchflags(path, bsd_flags)
except ImportError:
    def set_flags(path, bsd_flags, fd=None):
        pass


def get_flags(path, st):
    """Return BSD-style file flags for path or stat without following symlinks."""
    return getattr(st, 'st_flags', 0)


def sync_dir(path):
    fd = os.open(path, os.O_RDONLY)
    try:
        os.fsync(fd)
    except OSError as os_error:
        # Some network filesystems don't support this and fail with EINVAL.
        # Other error codes (e.g. EIO) shouldn't be silenced.
        if os_error.errno != errno.EINVAL:
            raise
    finally:
        os.close(fd)


def safe_fadvise(fd, offset, len, advice):
    if hasattr(os, 'posix_fadvise'):
        try:
            os.posix_fadvise(fd, offset, len, advice)
        except OSError:
            # usually, posix_fadvise can't fail for us, but there seem to
            # be failures when running borg under docker on ARM, likely due
            # to a bug outside of borg.
            # also, there is a python wrapper bug, always giving errno = 0.
            # https://github.com/borgbackup/borg/issues/2095
            # as this call is not critical for correct function (just to
            # optimize cache usage), we ignore these errors.
            pass


class SyncFile:
    """
    A file class that is supposed to enable write ordering (one way or another) and data durability after close().

    The degree to which either is possible varies with operating system, file system and hardware.

    This fallback implements a naive and slow way of doing this. On some operating systems it can't actually
    guarantee any of the above, since fsync() doesn't guarantee it. Furthermore it may not be possible at all
    to satisfy the above guarantees on some hardware or operating systems. In these cases we hope that the thorough
    checksumming implemented catches any corrupted data due to misordered, delayed or partial writes.

    Note that POSIX doesn't specify *anything* about power failures (or similar failures). A system that
    routinely loses files or corrupts file on power loss is POSIX compliant.

    TODO: Use F_FULLSYNC on OSX.
    TODO: A Windows implementation should use CreateFile with FILE_FLAG_WRITE_THROUGH.
    """

    def __init__(self, path, binary=False):
        mode = 'xb' if binary else 'x'
        self.fd = open(path, mode)
        self.fileno = self.fd.fileno()

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.close()

    def write(self, data):
        self.fd.write(data)

    def sync(self):
        """
        Synchronize file contents. Everything written prior to sync() must become durable before anything written
        after sync().
        """
        from .. import platform
        self.fd.flush()
        platform.fdatasync(self.fileno)
        # tell the OS that it does not need to cache what we just wrote,
        # avoids spoiling the cache for the OS and other processes.
        safe_fadvise(self.fileno, 0, 0, os.POSIX_FADV_DONTNEED)

    def close(self):
        """sync() and close."""
        from .. import platform
        dirname = None
        try:
            dirname = os.path.dirname(self.fd.name)
            self.sync()
        finally:
            self.fd.close()
            if dirname:
                platform.sync_dir(dirname)


class SaveFile:
    """
    Update file contents atomically.

    Must be used as a context manager (defining the scope of the transaction).

    On a journaling file system the file contents are always updated
    atomically and won't become corrupted, even on power failures or
    crashes (for caveats see SyncFile).
    """

    SUFFIX = '.tmp'

    def __init__(self, path, binary=False):
        self.binary = binary
        self.path = path
        self.tmppath = self.path + self.SUFFIX

    def __enter__(self):
        from .. import platform
        try:
            os.unlink(self.tmppath)
        except FileNotFoundError:
            pass
        self.fd = platform.SyncFile(self.tmppath, self.binary)
        return self.fd

    def __exit__(self, exc_type, exc_val, exc_tb):
        from .. import platform
        self.fd.close()
        if exc_type is not None:
            os.unlink(self.tmppath)
            return
        os.replace(self.tmppath, self.path)
        platform.sync_dir(os.path.dirname(self.path))


def swidth(s):
    """terminal output width of string <s>

    For western scripts, this is just len(s), but for cjk glyphs, 2 cells are used.
    """
    return len(s)


def umount(mountpoint):
    """un-mount the FUSE filesystem mounted at <mountpoint>"""
    return 0  # dummy, see also posix module


def get_process_id():
    """
    Return identification tuple (hostname, pid, thread_id) for 'us'. If this is a FUSE process, then the PID will be
    that of the parent, not the forked FUSE child.
    """
    raise NotImplementedError


def process_alive(host, pid, thread):
    """
    Check if the (host, pid, thread_id) combination corresponds to a potentially alive process.
    """
    raise NotImplementedError


def local_pid_alive(pid):
    """Return whether *pid* is alive."""
    raise NotImplementedError