~bzr-pqm/bzr/bzr.dev

# Copyright (C) 2009, 2010 Canonical Ltd

#

# This program is free software; you can redistribute it and/or modify

# it under the terms of the GNU General Public License as published by

# the Free Software Foundation; either version 2 of the License, or

# (at your option) any later version.

#

# 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

# 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

"""Helpers for managing cleanup functions and the errors they might raise.

The usual way to run cleanup code in Python is::

    try:

        do_something()

    finally:

        cleanup_something()

However if both `do_something` and `cleanup_something` raise an exception

Python will forget the original exception and propagate the one from

cleanup_something.  Unfortunately, this is almost always much less useful than

the original exception.

If you want to be certain that the first, and only the first, error is raised,

then use::

    operation = OperationWithCleanups(do_something)

    operation.add_cleanup(cleanup_something)

    operation.run_simple()

This is more inconvenient (because you need to make every try block a

function), but will ensure that the first error encountered is the one raised,

while also ensuring all cleanups are run.  See OperationWithCleanups for more

details.

"""

from __future__ import absolute_import

from collections import deque

import sys

from bzrlib import (

    debug,

    trace,

    )

def _log_cleanup_error(exc):

    trace.mutter('Cleanup failed:')

    trace.log_exception_quietly()

    if 'cleanup' in debug.debug_flags:

        trace.warning('bzr: warning: Cleanup failed: %s', exc)

def _run_cleanup(func, *args, **kwargs):

    """Run func(*args, **kwargs), logging but not propagating any error it

    raises.

    :returns: True if func raised no errors, else False.

    """

    try:

        func(*args, **kwargs)

    except KeyboardInterrupt:

        raise

    except Exception, exc:

        _log_cleanup_error(exc)

        return False

    return True

def _run_cleanups(funcs):

    """Run a series of cleanup functions."""

    for func, args, kwargs in funcs:

        _run_cleanup(func, *args, **kwargs)

class ObjectWithCleanups(object):

    """A mixin for objects that hold a cleanup list.

    Subclass or client code can call add_cleanup and then later `cleanup_now`.

    """

    def __init__(self):

        self.cleanups = deque()

    def add_cleanup(self, cleanup_func, *args, **kwargs):

        """Add a cleanup to run.

        Cleanups may be added at any time.  

        Cleanups will be executed in LIFO order.

        """

        self.cleanups.appendleft((cleanup_func, args, kwargs))

    def cleanup_now(self):

        _run_cleanups(self.cleanups)

        self.cleanups.clear()

class OperationWithCleanups(ObjectWithCleanups):

    """A way to run some code with a dynamic cleanup list.

    This provides a way to add cleanups while the function-with-cleanups is

    running.

    Typical use::

        operation = OperationWithCleanups(some_func)

        operation.run(args...)

    where `some_func` is::

        def some_func(operation, args, ...):

            do_something()

            operation.add_cleanup(something)

            # etc

    Note that the first argument passed to `some_func` will be the

    OperationWithCleanups object.  To invoke `some_func` without that, use

    `run_simple` instead of `run`.

    """

    def __init__(self, func):

        super(OperationWithCleanups, self).__init__()

        self.func = func

    def run(self, *args, **kwargs):

        return _do_with_cleanups(

            self.cleanups, self.func, self, *args, **kwargs)

    def run_simple(self, *args, **kwargs):

        return _do_with_cleanups(

            self.cleanups, self.func, *args, **kwargs)

def _do_with_cleanups(cleanup_funcs, func, *args, **kwargs):

    """Run `func`, then call all the cleanup_funcs.

    All the cleanup_funcs are guaranteed to be run.  The first exception raised

    by func or any of the cleanup_funcs is the one that will be propagted by

    this function (subsequent errors are caught and logged).

    Conceptually similar to::

        try:

            return func(*args, **kwargs)

        finally:

            for cleanup, cargs, ckwargs in cleanup_funcs:

                cleanup(*cargs, **ckwargs)

    It avoids several problems with using try/finally directly:

     * an exception from func will not be obscured by a subsequent exception

       from a cleanup.

     * an exception from a cleanup will not prevent other cleanups from

       running (but the first exception encountered is still the one

       propagated).

    Unike `_run_cleanup`, `_do_with_cleanups` can propagate an exception from a

    cleanup, but only if there is no exception from func.

    """

    # As correct as Python 2.4 allows.

    try:

        result = func(*args, **kwargs)

    except:

        # We have an exception from func already, so suppress cleanup errors.

        _run_cleanups(cleanup_funcs)

        raise

    else:

        # No exception from func, so allow the first exception from

        # cleanup_funcs to propagate if one occurs (but only after running all

        # of them).

        exc_info = None

        for cleanup, c_args, c_kwargs in cleanup_funcs:

            # XXX: Hmm, if KeyboardInterrupt arrives at exactly this line, we

            # won't run all cleanups... perhaps we should temporarily install a

            # SIGINT handler?

            if exc_info is None:

                try:

                    cleanup(*c_args, **c_kwargs)

                except:

                    # This is the first cleanup to fail, so remember its

                    # details.

                    exc_info = sys.exc_info()

            else:

                # We already have an exception to propagate, so log any errors

                # but don't propagate them.

                _run_cleanup(cleanup, *c_args, **kwargs)

        if exc_info is not None:

            try:

                raise exc_info[0], exc_info[1], exc_info[2]

            finally:

                del exc_info

        # No error, so we can return the result

        return result