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concurrent.futures.ThreadPoolExecutor offers a higher level interfaceto push tasks to a background thread without blocking execution of thecalling thread, while still being able to retrieve their results when needed.

The maximum value allowed for the timeout parameter of blocking functions(Lock.acquire(), RLock.acquire(), Condition.wait(), etc.).Specifying a timeout greater than this value will raise anOverflowError.

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join() raises a RuntimeError if an attempt is madeto join the current thread as that would cause a deadlock. It is alsoan error to join() a thread before it has been startedand attempts to do so raise the same exception.

A primitive lock is a synchronization primitive that is not owned by aparticular thread when locked. In Python, it is currently the lowest levelsynchronization primitive available, implemented directly by the _threadextension module.

When more than one thread is blocked in acquire() waiting for thestate to turn to unlocked, only one thread proceeds when a release()call resets the state to unlocked; which one of the waiting threads proceedsis not defined, and may vary across implementations.

When invoked with the floating-point timeout argument set to a positivevalue, block for at most the number of seconds specified by timeoutand as long as the lock cannot be acquired. A timeout argument of -1specifies an unbounded wait. It is forbidden to specify a timeoutwhen blocking is False.

To lock the lock, a thread calls its acquire() method; thisreturns once the thread owns the lock. To unlock the lock, a thread callsits release() method. acquire()/release()call pairs may be nested; only the final release() (therelease() of the outermost pair) resets the lock to unlocked andallows another thread blocked in acquire() to proceed.

This class implements reentrant lock objects. A reentrant lock must bereleased by the thread that acquired it. Once a thread has acquired areentrant lock, the same thread may acquire it again without blocking; thethread must release it once for each time it has acquired it.

When invoked without arguments: if this thread already owns the lock, incrementthe recursion level by one, and return immediately. Otherwise, if anotherthread owns the lock, block until the lock is unlocked. Once the lock isunlocked (not owned by any thread), then grab ownership, set the recursion levelto one, and return. If more than one thread is blocked waiting until the lockis unlocked, only one at a time will be able to grab ownership of the lock.There is no return value in this case.

When invoked with the blocking argument set to False, do not block. If a callwithout an argument would block, return False immediately; otherwise, do thesame thing as when called without arguments, and return True.

When invoked with the floating-point timeout argument set to a positivevalue, block for at most the number of seconds specified by timeoutand as long as the lock cannot be acquired. Return True if the lock hasbeen acquired, False if the timeout has elapsed.

Release a lock, decrementing the recursion level. If after the decrement it iszero, reset the lock to unlocked (not owned by any thread), and if any otherthreads are blocked waiting for the lock to become unlocked, allow exactly oneof them to proceed. If after the decrement the recursion level is stillnonzero, the lock remains locked and owned by the calling thread.

A condition variable obeys the context management protocol:using the with statement acquires the associated lock for the duration ofthe enclosed block. The acquire() andrelease() methods also call the corresponding methods ofthe associated lock.

Other methods must be called with the associated lock held. Thewait() method releases the lock, and then blocks untilanother thread awakens it by calling notify() ornotify_all(). Once awakened, wait()re-acquires the lock and returns. It is also possible to specify a timeout.

The typical programming style using condition variables uses the lock tosynchronize access to some shared state; threads that are interested in aparticular change of state call wait() repeatedly until theysee the desired state, while threads that modify the state callnotify() or notify_all() when they changethe state in such a way that it could possibly be a desired state for oneof the waiters. For example, the following code is a genericproducer-consumer situation with unlimited buffer capacity:

This method releases the underlying lock, and then blocks until it isawakened by a notify() or notify_all() call for the samecondition variable in another thread, or until the optional timeoutoccurs. Once awakened or timed out, it re-acquires the lock and returns.

When the underlying lock is an RLock, it is not released usingits release() method, since this may not actually unlock the lockwhen it was acquired multiple times recursively. Instead, an internalinterface of the RLock class is used, which really unlocks iteven when it has been recursively acquired several times. Another internalinterface is then used to restore the recursion level when the lock isreacquired.

Wake up all threads waiting on this condition. This method acts likenotify(), but wakes up all waiting threads instead of one. If thecalling thread has not acquired the lock when this method is called, aRuntimeError is raised.

A semaphore manages an internal counter which is decremented by eachacquire() call and incremented by each release()call. The counter can never go below zero; when acquire()finds that it is zero, it blocks, waiting until some other thread callsrelease().

This class implements semaphore objects. A semaphore manages an atomiccounter representing the number of release() calls minus the number ofacquire() calls, plus an initial value. The acquire() methodblocks if necessary until it can return without making the counter negative.If not given, value defaults to 1.

If the internal counter is zero on entry, block until awoken by a call torelease(). Once awoken (and the counter is greaterthan 0), decrement the counter by 1 and return True. Exactly onethread will be awoken by each call to release(). Theorder in which threads are awoken should not be relied on.

When invoked with blocking set to False, do not block. If a callwithout an argument would block, return False immediately; otherwise, dothe same thing as when called without arguments, and return True.

Class implementing event objects. An event manages a flag that can be set totrue with the set() method and reset to false with theclear() method. The wait() method blocks until the flag is true.The flag is initially false.

Block until the internal flag is true. If the internal flag is true onentry, return immediately. Otherwise, block until another thread callsset() to set the flag to true, or until the optional timeout occurs.

This class provides a simple synchronization primitive for use by a fixed numberof threads that need to wait for each other. Each of the threads tries to passthe barrier by calling the wait() method and will block untilall of the threads have made their wait() calls. At this point,the threads are released simultaneously.

Put the barrier into a broken state. This causes any active or futurecalls to wait() to fail with the BrokenBarrierError. Usethis for example if one of the threads needs to abort, to avoid deadlocking theapplication.

All of the objects provided by this module that have acquire andrelease methods can be used as context managers for a withstatement. The acquire method will be called when the block isentered, and release will be called when the block is exited. Hence,the following snippet:

This page discusses the Bucket Lock feature, which lets you configure aCloud Storage bucket's retention policy. This policy governs how longobjects in the bucket must be retained. The feature also lets you lock thebucket's retention policy, permanently preventing the policy from beingreduced or removed.

You cannot destroy Cloud Key Management Service key versions that encrypt objects inlocked buckets if the objects haven't met their retention expiration times.For more information, see Key versions used to encrypt locked objects.

You can use Object Lifecycle Management to automatically delete objectsin a bucket, including in a bucket with a locked policy.A lifecycle rule won't delete an object until after the objectfulfills the retention policy.

When you lock a bucket's retention policy, you prevent the policy fromever being removed or the retention period from ever being reduced (althoughyou can still increase the retention period). If you try to remove or reducethe policy duration of a locked bucket, you get a 400 BadRequestExceptionerror. Once a retention policy is locked, you cannot delete the bucket untilevery object in the bucket has met the retention period.

Locking a bucket's retention policy is irreversible, and you should befamiliar with the implications of doing so prior to using this feature. When youuse an unlocked policy, you have the ability to remove the policy, allowingyou to still delete objects when desired. When you lock a policy, you mustdelete the entire bucket in order to "remove" the policy. However, you can'tdelete the bucket if there are objects in it that haven't fulfilled theirretention period. Thus, to "remove" a locked policy, you have to wait untilevery object in the bucket has fulfilled its retention period, at which pointyou can delete the bucket.

Additionally, when you lock a retention policy, Cloud Storage automaticallyapplies a lien to the projects.delete permission for the project thatcontains the bucket. While in place, the lien prevents the project from beingdeleted. To delete the project, you must first remove all such liens.Note that removing a lien requires the resourcemanager.projects.updateLienspermission, which is part of the roles/owner androles/resourcemanager.lienModifier roles.

PostgreSQL provides various lock modes to control concurrent access to data in tables. These modes can be used for application-controlled locking in situations where MVCC does not give the desired behavior. Also, most PostgreSQL commands automatically acquire locks of appropriate modes to ensure that referenced tables are not dropped or modified in incompatible ways while the command executes. (For example, TRUNCATE cannot safely be executed concurrently with other operations on the same table, so it obtains an ACCESS EXCLUSIVE lock on the table to enforce that.) ff782bc1db