~bzr-pqm/bzr/bzr.dev

# Copyright (C) 2009 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(lambda operation: do_something())

    operation.add_cleanup(cleanup_something)

    operation.run()

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 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 OperationWithCleanups(object):

    """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.

    """

    def __init__(self, func):

        self.func = func

        self.cleanups = deque()

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

        """Add a cleanup to run.

        Cleanups may be added at any time before or during the execution of

        self.func.  Cleanups will be executed in LIFO order.

        """

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

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

        return _do_with_cleanups(

            self.cleanups, self.func, self, *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:

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

        # No error, so we can return the result

        return result