1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
|
from util import threading
import logging
log = logging.getLogger(__name__)
class LockError(Exception):
pass
class ReadWriteMutex(object):
"""A mutex which allows multiple readers, single writer.
:class:`.ReadWriteMutex` uses a Python ``threading.Condition``
to provide this functionality across threads within a process.
The Beaker package also contained a file-lock based version
of this concept, so that readers/writers could be synchronized
across processes with a common filesystem. A future Dogpile
release may include this additional class at some point.
"""
def __init__(self):
# counts how many asynchronous methods are executing
self.async = 0
# pointer to thread that is the current sync operation
self.current_sync_operation = None
# condition object to lock on
self.condition = threading.Condition(threading.Lock())
def acquire_read_lock(self, wait = True):
"""Acquire the 'read' lock."""
self.condition.acquire()
try:
# see if a synchronous operation is waiting to start
# or is already running, in which case we wait (or just
# give up and return)
if wait:
while self.current_sync_operation is not None:
self.condition.wait()
else:
if self.current_sync_operation is not None:
return False
self.async += 1
log.debug("%s acquired read lock", self)
finally:
self.condition.release()
if not wait:
return True
def release_read_lock(self):
"""Release the 'read' lock."""
self.condition.acquire()
try:
self.async -= 1
# check if we are the last asynchronous reader thread
# out the door.
if self.async == 0:
# yes. so if a sync operation is waiting, notifyAll to wake
# it up
if self.current_sync_operation is not None:
self.condition.notifyAll()
elif self.async < 0:
raise LockError("Synchronizer error - too many "
"release_read_locks called")
log.debug("%s released read lock", self)
finally:
self.condition.release()
def acquire_write_lock(self, wait = True):
"""Acquire the 'write' lock."""
self.condition.acquire()
try:
# here, we are not a synchronous reader, and after returning,
# assuming waiting or immediate availability, we will be.
if wait:
# if another sync is working, wait
while self.current_sync_operation is not None:
self.condition.wait()
else:
# if another sync is working,
# we dont want to wait, so forget it
if self.current_sync_operation is not None:
return False
# establish ourselves as the current sync
# this indicates to other read/write operations
# that they should wait until this is None again
self.current_sync_operation = threading.currentThread()
# now wait again for asyncs to finish
if self.async > 0:
if wait:
# wait
self.condition.wait()
else:
# we dont want to wait, so forget it
self.current_sync_operation = None
return False
log.debug("%s acquired write lock", self)
finally:
self.condition.release()
if not wait:
return True
def release_write_lock(self):
"""Release the 'write' lock."""
self.condition.acquire()
try:
if self.current_sync_operation is not threading.currentThread():
raise LockError("Synchronizer error - current thread doesn't "
"have the write lock")
# reset the current sync operation so
# another can get it
self.current_sync_operation = None
# tell everyone to get ready
self.condition.notifyAll()
log.debug("%s released write lock", self)
finally:
# everyone go !!
self.condition.release()
|
