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Diffstat (limited to 'pyload/lib/ReadWriteLock.py')
| -rw-r--r-- | pyload/lib/ReadWriteLock.py | 232 |
1 files changed, 232 insertions, 0 deletions
diff --git a/pyload/lib/ReadWriteLock.py b/pyload/lib/ReadWriteLock.py new file mode 100644 index 000000000..cc82f3d48 --- /dev/null +++ b/pyload/lib/ReadWriteLock.py @@ -0,0 +1,232 @@ +# -*- coding: iso-8859-15 -*- +"""locks.py - Read-Write lock thread lock implementation + +See the class documentation for more info. + +Copyright (C) 2007, Heiko Wundram. +Released under the BSD-license. + +http://code.activestate.com/recipes/502283-read-write-lock-class-rlock-like/ +""" + +# Imports +# ------- + +from threading import Condition, Lock, currentThread +from time import time + + +# Read write lock +# --------------- + +class ReadWriteLock(object): + """Read-Write lock class. A read-write lock differs from a standard + threading.RLock() by allowing multiple threads to simultaneously hold a + read lock, while allowing only a single thread to hold a write lock at the + same point of time. + + When a read lock is requested while a write lock is held, the reader + is blocked; when a write lock is requested while another write lock is + held or there are read locks, the writer is blocked. + + Writers are always preferred by this implementation: if there are blocked + threads waiting for a write lock, current readers may request more read + locks (which they eventually should free, as they starve the waiting + writers otherwise), but a new thread requesting a read lock will not + be granted one, and block. This might mean starvation for readers if + two writer threads interweave their calls to acquireWrite() without + leaving a window only for readers. + + In case a current reader requests a write lock, this can and will be + satisfied without giving up the read locks first, but, only one thread + may perform this kind of lock upgrade, as a deadlock would otherwise + occur. After the write lock has been granted, the thread will hold a + full write lock, and not be downgraded after the upgrading call to + acquireWrite() has been match by a corresponding release(). + """ + + def __init__(self): + """Initialize this read-write lock.""" + + # Condition variable, used to signal waiters of a change in object + # state. + self.__condition = Condition(Lock()) + + # Initialize with no writers. + self.__writer = None + self.__upgradewritercount = 0 + self.__pendingwriters = [] + + # Initialize with no readers. + self.__readers = {} + + def acquire(self, blocking=True, timeout=None, shared=False): + if shared: + self.acquireRead(timeout) + else: + self.acquireWrite(timeout) + + def acquireRead(self, timeout=None): + """Acquire a read lock for the current thread, waiting at most + timeout seconds or doing a non-blocking check in case timeout is <= 0. + + In case timeout is None, the call to acquireRead blocks until the + lock request can be serviced. + + In case the timeout expires before the lock could be serviced, a + RuntimeError is thrown.""" + + if timeout is not None: + endtime = time() + timeout + me = currentThread() + self.__condition.acquire() + try: + if self.__writer is me: + # If we are the writer, grant a new read lock, always. + self.__writercount += 1 + return + while True: + if self.__writer is None: + # Only test anything if there is no current writer. + if self.__upgradewritercount or self.__pendingwriters: + if me in self.__readers: + # Only grant a read lock if we already have one + # in case writers are waiting for their turn. + # This means that writers can't easily get starved + # (but see below, readers can). + self.__readers[me] += 1 + return + # No, we aren't a reader (yet), wait for our turn. + else: + # Grant a new read lock, always, in case there are + # no pending writers (and no writer). + self.__readers[me] = self.__readers.get(me, 0) + 1 + return + if timeout is not None: + remaining = endtime - time() + if remaining <= 0: + # Timeout has expired, signal caller of this. + raise RuntimeError("Acquiring read lock timed out") + self.__condition.wait(remaining) + else: + self.__condition.wait() + finally: + self.__condition.release() + + def acquireWrite(self, timeout=None): + """Acquire a write lock for the current thread, waiting at most + timeout seconds or doing a non-blocking check in case timeout is <= 0. + + In case the write lock cannot be serviced due to the deadlock + condition mentioned above, a ValueError is raised. + + In case timeout is None, the call to acquireWrite blocks until the + lock request can be serviced. + + In case the timeout expires before the lock could be serviced, a + RuntimeError is thrown.""" + + if timeout is not None: + endtime = time() + timeout + me, upgradewriter = currentThread(), False + self.__condition.acquire() + try: + if self.__writer is me: + # If we are the writer, grant a new write lock, always. + self.__writercount += 1 + return + elif me in self.__readers: + # If we are a reader, no need to add us to pendingwriters, + # we get the upgradewriter slot. + if self.__upgradewritercount: + # If we are a reader and want to upgrade, and someone + # else also wants to upgrade, there is no way we can do + # this except if one of us releases all his read locks. + # Signal this to user. + raise ValueError( + "Inevitable dead lock, denying write lock" + ) + upgradewriter = True + self.__upgradewritercount = self.__readers.pop(me) + else: + # We aren't a reader, so add us to the pending writers queue + # for synchronization with the readers. + self.__pendingwriters.append(me) + while True: + if not self.__readers and self.__writer is None: + # Only test anything if there are no readers and writers. + if self.__upgradewritercount: + if upgradewriter: + # There is a writer to upgrade, and it's us. Take + # the write lock. + self.__writer = me + self.__writercount = self.__upgradewritercount + 1 + self.__upgradewritercount = 0 + return + # There is a writer to upgrade, but it's not us. + # Always leave the upgrade writer the advance slot, + # because he presumes he'll get a write lock directly + # from a previously held read lock. + elif self.__pendingwriters[0] is me: + # If there are no readers and writers, it's always + # fine for us to take the writer slot, removing us + # from the pending writers queue. + # This might mean starvation for readers, though. + self.__writer = me + self.__writercount = 1 + self.__pendingwriters = self.__pendingwriters[1:] + return + if timeout is not None: + remaining = endtime - time() + if remaining <= 0: + # Timeout has expired, signal caller of this. + if upgradewriter: + # Put us back on the reader queue. No need to + # signal anyone of this change, because no other + # writer could've taken our spot before we got + # here (because of remaining readers), as the test + # for proper conditions is at the start of the + # loop, not at the end. + self.__readers[me] = self.__upgradewritercount + self.__upgradewritercount = 0 + else: + # We were a simple pending writer, just remove us + # from the FIFO list. + self.__pendingwriters.remove(me) + raise RuntimeError("Acquiring write lock timed out") + self.__condition.wait(remaining) + else: + self.__condition.wait() + finally: + self.__condition.release() + + def release(self): + """Release the currently held lock. + + In case the current thread holds no lock, a ValueError is thrown.""" + + me = currentThread() + self.__condition.acquire() + try: + if self.__writer is me: + # We are the writer, take one nesting depth away. + self.__writercount -= 1 + if not self.__writercount: + # No more write locks; take our writer position away and + # notify waiters of the new circumstances. + self.__writer = None + self.__condition.notifyAll() + elif me in self.__readers: + # We are a reader currently, take one nesting depth away. + self.__readers[me] -= 1 + if not self.__readers[me]: + # No more read locks, take our reader position away. + del self.__readers[me] + if not self.__readers: + # No more readers, notify waiters of the new + # circumstances. + self.__condition.notifyAll() + else: + raise ValueError("Trying to release unheld lock") + finally: + self.__condition.release() |