init

1 files changed, 71 insertions, 0 deletions

diff --git a/rwlock.py b/rwlock.py
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index 0000000..8823777
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+++ b/rwlock.py
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+"""Simple reader-writer locks in Python
+Many readers can hold the lock XOR one and only one writer"""
+import threading
+
+version = """$Id: rwlock.py,v 1.1 2004/12/22 22:32:00 majid Exp $"""
+
+class RWLock:
+  """
+A simple reader-writer lock Several readers can hold the lock
+simultaneously, XOR one writer. Write locks have priority over reads to
+prevent write starvation.
+"""
+  def __init__(self):
+    self.rwlock = 0
+    self.writers_waiting = 0
+    self.monitor = threading.Lock()
+    self.readers_ok = threading.Condition(self.monitor)
+    self.writers_ok = threading.Condition(self.monitor)
+  def acquire_read(self):
+    """Acquire a read lock. Several threads can hold this typeof lock.
+It is exclusive with write locks."""
+    self.monitor.acquire()
+    while self.rwlock < 0 or self.writers_waiting:
+      self.readers_ok.wait()
+    self.rwlock += 1
+    self.monitor.release()
+  def acquire_write(self):
+    """Acquire a write lock. Only one thread can hold this lock, and
+only when no read locks are also held."""
+    self.monitor.acquire()
+    while self.rwlock != 0:
+      self.writers_waiting += 1
+      self.writers_ok.wait()
+      self.writers_waiting -= 1
+    self.rwlock = -1
+    self.monitor.release()
+  def promote(self):
+    """Promote an already-acquired read lock to a write lock
+    WARNING: it is very easy to deadlock with this method"""
+    self.monitor.acquire()
+    self.rwlock -= 1
+    while self.rwlock != 0:
+      self.writers_waiting += 1
+      self.writers_ok.wait()
+      self.writers_waiting -= 1
+    self.rwlock = -1
+    self.monitor.release()
+  def demote(self):
+    """Demote an already-acquired write lock to a read lock"""
+    self.monitor.acquire()
+    self.rwlock = 1
+    self.readers_ok.notifyAll()
+    self.monitor.release()
+  def release(self):
+    """Release a lock, whether read or write."""
+    self.monitor.acquire()
+    if self.rwlock < 0:
+      self.rwlock = 0
+    else:
+      self.rwlock -= 1
+    wake_writers = self.writers_waiting and self.rwlock == 0
+    wake_readers = self.writers_waiting == 0
+    self.monitor.release()
+    if wake_writers:
+      self.writers_ok.acquire()
+      self.writers_ok.notify()
+      self.writers_ok.release()
+    elif wake_readers:
+      self.readers_ok.acquire()
+      self.readers_ok.notifyAll()
+      self.readers_ok.release()