~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.

Generally, code that wants to perform some cleanup at the end of an action will

look like this::

    from bzrlib.cleanups import run_cleanup

    try:

        do_something()

    finally:

        run_cleanup(cleanup_something)

Any errors from `cleanup_something` will be logged, but not raised.

Importantly, any errors from do_something will be propagated.

There is also convenience function for running multiple, independent cleanups

in sequence: run_cleanups.  e.g.::

    try:

        do_something()

    finally:

        run_cleanups([cleanup_func_a, cleanup_func_b], ...)

Developers can use the `-Dcleanup` debug flag to cause cleanup errors to be

reported in the UI as well as logged.

Note the tradeoff that run_cleanup/run_cleanups makes: errors from

`do_something` will not be obscured by errors from `cleanup_something`, but

errors from `cleanup_something` will never reach the user, even if there is no

error from `do_something`.  So run_cleanup is good to use when a failure of

internal housekeeping (e.g. failure to finish a progress bar) is unimportant to

a user.

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

then use::

    do_with_cleanups(do_something, cleanups)

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.

"""

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_cleanup_reporting_errors(func, *args, **kwargs):

    try:

        func(*args, **kwargs)

    except KeyboardInterrupt:

        raise

    except Exception, exc:

        trace.mutter('Cleanup failed:')

        trace.log_exception_quietly()

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

        return False

    return True

def run_cleanups(funcs, on_error='log'):

    """Run a series of cleanup functions.

    :param errors: One of 'log', 'warn first', 'warn all'

    """

    seen_error = False

    for func in funcs:

        if on_error == 'log' or (on_error == 'warn first' and seen_error):

            seen_error |= run_cleanup(func)

        else:

            seen_error |= run_cleanup_reporting_errors(func)

def do_with_cleanups(func, cleanup_funcs):

    """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()

        finally:

            for cleanup in cleanup_funcs:

                cleanup()

    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()

    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 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()

                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)

        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