New submission from Max Polk:
In the docs (both Python 2 and 3) for the multiprocessing module, the Proxy
section needs to be explicit about the fact that is does NOT create locks
around access to shared objects.
Why? Because on the same page, we read about
multiprocessing.managers.SyncManager's Queue method to create a shared
queue.Queue object and return a proxy for it, next to other methods like
SyncManager.list to create a process safe list.
But later, a willful violation of these semantics exists in the Using a remote
manager section where a Proxy is implicitly created (via the register method
of multiprocessing.managers.BaseManager) surrounding a Queue.Queue object.
Note that it did not create a proxy around a SyncManager.Queue, it created it
around a Queue.Queue. A user who copies this code and replaces Queue.Queue
with a plain Python list gets the sense that all the needed locks will be
created to protect the shared list.
However, due to lack of documentation in the Proxy section, the user will not
know it's an unsafe list, and Proxy isn't helping them. I'm guessing that
Queue.Queue has its own locks to protect it in a process-shared setting, and we
lucked out in this example, but an unwary reader's luck runs out if they
replace it with a plain Python list.
Therefore, may I suggest two changes: (1) replace Queue.Queue with
SyncManager.Queue in the Using a remote manager section to avoid misleading
readers; and (2) be explicit in Proxy class docs that no locks are created to
protect against concurrent access, and maybe add that the user must go to the
multiprocessing.managers.SyncManager methods (Queue, list, etc) to get process
safe objects to place behind the Proxy.
--
assignee: docs@python
components: Documentation
messages: 205039
nosy: docs@python, maxpolk
priority: normal
severity: normal
status: open
title: multiprocessing Proxy docs need locking semantics explained
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http://bugs.python.org/issue19864
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