1# Copyright (C) 2007, 2008 Canonical Ltd
2#
3# This program is free software; you can redistribute it and/or modify
4# it under the terms of the GNU General Public License as published by
5# the Free Software Foundation; either version 2 of the License, or
6# (at your option) any later version.
7#
8# This program is distributed in the hope that it will be useful,
9# but WITHOUT ANY WARRANTY; without even the implied warranty of
10# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
11# GNU General Public License for more details.
12#
13# You should have received a copy of the GNU General Public License
14# along with this program; if not, write to the Free Software
15# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
16
17"""Counted lock class"""
18
19from . import (
20    errors,
21    )
22
23
24class CountedLock(object):
25    """Decorator around a lock that makes it reentrant.
26
27    This can be used with any object that provides a basic Lock interface,
28    including LockDirs and OS file locks.
29
30    :ivar _token: While a write lock is held, this is the token
31        for it.
32    """
33
34    def __init__(self, real_lock):
35        self._real_lock = real_lock
36        self._lock_mode = None
37        self._lock_count = 0
38
39    def __repr__(self):
40        return "%s(%r)" % (self.__class__.__name__,
41                           self._real_lock)
42
43    def break_lock(self):
44        self._real_lock.break_lock()
45        self._lock_mode = None
46        self._lock_count = 0
47
48    def get_physical_lock_status(self):
49        """Return physical lock status.
50
51        Returns true if a lock is held on the transport. If no lock is held, or
52        the underlying locking mechanism does not support querying lock
53        status, false is returned.
54        """
55        try:
56            return self._real_lock.peek() is not None
57        except NotImplementedError:
58            return False
59
60    def is_locked(self):
61        return self._lock_mode is not None
62
63    def lock_read(self):
64        """Acquire the lock in read mode.
65
66        If the lock is already held in either read or write mode this
67        increments the count and succeeds.  If the lock is not already held,
68        it is taken in read mode.
69        """
70        if self._lock_mode:
71            self._lock_count += 1
72        else:
73            self._real_lock.lock_read()
74            self._lock_count = 1
75            self._lock_mode = 'r'
76
77    def lock_write(self, token=None):
78        """Acquire the lock in write mode.
79
80        If the lock was originally acquired in read mode this will fail.
81
82        :param token: If given and the lock is already held,
83            then validate that we already hold the real
84            lock with this token.
85
86        :returns: The token from the underlying lock.
87        """
88        if self._lock_count == 0:
89            self._token = self._real_lock.lock_write(token=token)
90            self._lock_mode = 'w'
91            self._lock_count += 1
92            return self._token
93        elif self._lock_mode != 'w':
94            raise errors.ReadOnlyError(self)
95        else:
96            self._real_lock.validate_token(token)
97            self._lock_count += 1
98            return self._token
99
100    def unlock(self):
101        if self._lock_count == 0:
102            raise errors.LockNotHeld(self)
103        elif self._lock_count == 1:
104            # these are decremented first; if we fail to unlock the most
105            # reasonable assumption is that we still don't have the lock
106            # anymore
107            self._lock_mode = None
108            self._lock_count -= 1
109            self._real_lock.unlock()
110        else:
111            self._lock_count -= 1
112