vendor RWLock

Author: tomato42

src/ecdsa/_rwlock.py

@@ -0,0 +1,85 @@
+# Copyright Mateusz Kobos, (c) 2011
+# https://code.activestate.com/recipes/577803-reader-writer-lock-with-priority-for-writers/
+# released under the MIT licence
+
+import threading
+
+
+__author__ = "Mateusz Kobos"
+
+
+class RWLock:
+    """
+    Read-Write locking primitive
+
+    Synchronization object used in a solution of so-called second
+    readers-writers problem. In this problem, many readers can simultaneously
+    access a share, and a writer has an exclusive access to this share.
+    Additionally, the following constraints should be met:
+    1) no reader should be kept waiting if the share is currently opened for
+       reading unless a writer is also waiting for the share,
+    2) no writer should be kept waiting for the share longer than absolutely
+       necessary.
+
+    The implementation is based on [1, secs. 4.2.2, 4.2.6, 4.2.7]
+    with a modification -- adding an additional lock (C{self.__readers_queue})
+    -- in accordance with [2].
+
+    Sources:
+    [1] A.B. Downey: "The little book of semaphores", Version 2.1.5, 2008
+    [2] P.J. Courtois, F. Heymans, D.L. Parnas:
+        "Concurrent Control with 'Readers' and 'Writers'",
+        Communications of the ACM, 1971 (via [3])
+    [3] http://en.wikipedia.org/wiki/Readers-writers_problem
+    """
+
+    def __init__(self):
+        """
+        A lock giving an even higher priority to the writer in certain
+        cases (see [2] for a discussion).
+        """
+        self.__read_switch = _LightSwitch()
+        self.__write_switch = _LightSwitch()
+        self.__no_readers = threading.Lock()
+        self.__no_writers = threading.Lock()
+        self.__readers_queue = threading.Lock()
+
+    def reader_acquire(self):
+        self.__readers_queue.acquire()
+        self.__no_readers.acquire()
+        self.__read_switch.acquire(self.__no_writers)
+        self.__no_readers.release()
+        self.__readers_queue.release()
+
+    def reader_release(self):
+        self.__read_switch.release(self.__no_writers)
+
+    def writer_acquire(self):
+        self.__write_switch.acquire(self.__no_readers)
+        self.__no_writers.acquire()
+
+    def writer_release(self):
+        self.__no_writers.release()
+        self.__write_switch.release(self.__no_readers)
+
+
+class _LightSwitch:
+    """An auxiliary "light switch"-like object. The first thread turns on the
+    "switch", the last one turns it off (see [1, sec. 4.2.2] for details)."""
+    def __init__(self):
+        self.__counter = 0
+        self.__mutex = threading.Lock()
+
+    def acquire(self, lock):
+        self.__mutex.acquire()
+        self.__counter += 1
+        if self.__counter == 1:
+            lock.acquire()
+        self.__mutex.release()
+
+    def release(self, lock):
+        self.__mutex.acquire()
+        self.__counter -= 1
+        if self.__counter == 0:
+            lock.release()
+        self.__mutex.release()

src/ecdsa/test_rw_lock.py

@@ -0,0 +1,175 @@
+# Copyright Mateusz Kobos, (c) 2011
+# https://code.activestate.com/recipes/577803-reader-writer-lock-with-priority-for-writers/
+# released under the MIT licence
+
+import unittest
+import threading
+import time
+import copy
+from ._rwlock import RWLock
+
+
+class Writer(threading.Thread):
+    def __init__(self, buffer_, rw_lock, init_sleep_time, sleep_time, to_write):
+        """
+        @param buffer_: common buffer_ shared by the readers and writers
+        @type buffer_: list
+        @type rw_lock: L{RWLock}
+        @param init_sleep_time: sleep time before doing any action
+        @type init_sleep_time: C{float}
+        @param sleep_time: sleep time while in critical section
+        @type sleep_time: C{float}
+        @param to_write: data that will be appended to the buffer
+        """
+        threading.Thread.__init__(self)
+        self.__buffer = buffer_
+        self.__rw_lock = rw_lock
+        self.__init_sleep_time = init_sleep_time
+        self.__sleep_time = sleep_time
+        self.__to_write = to_write
+        self.entry_time = None
+        """Time of entry to the critical section"""
+        self.exit_time = None
+        """Time of exit from the critical section"""
+
+    def run(self):
+        time.sleep(self.__init_sleep_time)
+        self.__rw_lock.writer_acquire()
+        self.entry_time = time.time()
+        time.sleep(self.__sleep_time)
+        self.__buffer.append(self.__to_write)
+        self.exit_time = time.time()
+        self.__rw_lock.writer_release()
+
+
+class Reader(threading.Thread):
+    def __init__(self, buffer_, rw_lock, init_sleep_time, sleep_time):
+        """
+        @param buffer_: common buffer shared by the readers and writers
+        @type buffer_: list
+        @type rw_lock: L{RWLock}
+        @param init_sleep_time: sleep time before doing any action
+        @type init_sleep_time: C{float}
+        @param sleep_time: sleep time while in critical section
+        @type sleep_time: C{float}
+        """
+        threading.Thread.__init__(self)
+        self.__buffer = buffer_
+        self.__rw_lock = rw_lock
+        self.__init_sleep_time = init_sleep_time
+        self.__sleep_time = sleep_time
+        self.buffer_read = None
+        """a copy of a the buffer read while in critical section"""
+        self.entry_time = None
+        """Time of entry to the critical section"""
+        self.exit_time = None
+        """Time of exit from the critical section"""
+
+    def run(self):
+        time.sleep(self.__init_sleep_time)
+        self.__rw_lock.reader_acquire()
+        self.entry_time = time.time()
+        time.sleep(self.__sleep_time)
+        self.buffer_read = copy.deepcopy(self.__buffer)
+        self.exit_time = time.time()
+        self.__rw_lock.reader_release()
+
+
+class RWLockTestCase(unittest.TestCase):
+    def test_readers_nonexclusive_access(self):
+        (buffer_, rw_lock, threads) = self.__init_variables()
+
+        threads.append(Reader(buffer_, rw_lock, 0, 0))
+        threads.append(Writer(buffer_, rw_lock, 0.2, 0.4, 1))
+        threads.append(Reader(buffer_, rw_lock, 0.3, 0.3))
+        threads.append(Reader(buffer_, rw_lock, 0.5, 0))
+
+        self.__start_and_join_threads(threads)
+
+        ## The third reader should enter after the second one but it should
+        ## exit before the second one exits 
+        ## (i.e. the readers should be in the critical section 
+        ## at the same time)
+
+        self.assertEqual([], threads[0].buffer_read)
+        self.assertEqual([1], threads[2].buffer_read)
+        self.assertEqual([1], threads[3].buffer_read)
+        self.assert_(threads[1].exit_time <= threads[2].entry_time)
+        self.assert_(threads[2].entry_time <= threads[3].entry_time)
+        self.assert_(threads[3].exit_time < threads[2].exit_time)
+
+    def test_writers_exclusive_access(self):
+        (buffer_, rw_lock, threads) = self.__init_variables()
+
+        threads.append(Writer(buffer_, rw_lock, 0, 0.4, 1))
+        threads.append(Writer(buffer_, rw_lock, 0.1, 0, 2))
+        threads.append(Reader(buffer_, rw_lock, 0.2, 0))
+
+        self.__start_and_join_threads(threads)
+
+        ## The second writer should wait for the first one to exit
+
+        self.assertEqual([1, 2], threads[2].buffer_read)
+        self.assert_(threads[0].exit_time <= threads[1].entry_time)
+        self.assert_(threads[1].exit_time <= threads[2].exit_time)
+
+    def test_writer_priority(self):
+        (buffer_, rw_lock, threads) = self.__init_variables()
+
+        threads.append(Writer(buffer_, rw_lock, 0, 0, 1))
+        threads.append(Reader(buffer_, rw_lock, 0.1, 0.4))
+        threads.append(Writer(buffer_, rw_lock, 0.2, 0, 2))
+        threads.append(Reader(buffer_, rw_lock, 0.3, 0))
+        threads.append(Reader(buffer_, rw_lock, 0.3, 0))
+
+        self.__start_and_join_threads(threads)
+
+        ## The second writer should go before the second and the third reader
+
+        self.assertEqual([1], threads[1].buffer_read)
+        self.assertEqual([1, 2], threads[3].buffer_read)
+        self.assertEqual([1, 2], threads[4].buffer_read)
+        self.assert_(threads[0].exit_time < threads[1].entry_time)
+        self.assert_(threads[1].exit_time <= threads[2].entry_time)
+        self.assert_(threads[2].exit_time <= threads[3].entry_time)
+        self.assert_(threads[2].exit_time <= threads[4].entry_time)
+
+    def test_many_writers_priority(self):
+        (buffer_, rw_lock, threads) = self.__init_variables()
+
+        threads.append(Writer(buffer_, rw_lock, 0, 0, 1))
+        threads.append(Reader(buffer_, rw_lock, 0.1, 0.6))
+        threads.append(Writer(buffer_, rw_lock, 0.2, 0.1, 2))
+        threads.append(Reader(buffer_, rw_lock, 0.3, 0))
+        threads.append(Reader(buffer_, rw_lock, 0.4, 0))
+        threads.append(Writer(buffer_, rw_lock, 0.5, 0.1, 3))
+
+        self.__start_and_join_threads(threads)
+
+        ## The two last writers should go first -- after the first reader and
+        ## before the second and the third reader
+
+        self.assertEqual([1], threads[1].buffer_read)
+        self.assertEqual([1, 2, 3], threads[3].buffer_read)
+        self.assertEqual([1, 2, 3], threads[4].buffer_read)
+        self.assert_(threads[0].exit_time < threads[1].entry_time)
+        self.assert_(threads[1].exit_time <= threads[2].entry_time)
+        self.assert_(threads[1].exit_time <= threads[5].entry_time)
+        self.assert_(threads[2].exit_time <= threads[3].entry_time)
+        self.assert_(threads[2].exit_time <= threads[4].entry_time)
+        self.assert_(threads[5].exit_time <= threads[3].entry_time)
+        self.assert_(threads[5].exit_time <= threads[4].entry_time)
+
+    @staticmethod
+    def __init_variables():
+        buffer_ = []
+        rw_lock = RWLock()
+        threads = []
+        return (buffer_, rw_lock, threads)
+
+    @staticmethod
+    def __start_and_join_threads(threads):
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()