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venv/Lib/site-packages/ecdsa/_rwlock.py

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+# 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()