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From Kapil Thangavelu <kapil.f...@gmail.com>
Subject avoiding deadlocks on client handle close w/ python/c api
Date Tue, 04 May 2010 01:48:46 GMT
Hi Folks,

I'm constructing an async api on top of the zookeeper python bindings for
twisted. The intent was to make a thin wrapper that would wrap the existing
async api with one that allows for integration with the twisted python event
loop (http://www.twistedmatrix.com) primarily using the async apis.

One issue i'm running into while developing a unit tests, deadlocks occur if
we attempt to close a handle while there are any outstanding async requests
(aget, acreate, etc). Normally on close both the io thread terminates and
the completion thread are terminated and joined, however w\ith outstanding
async requests, the completion thread won't be in a joinable state, and we
effectively hang when the main thread does the join.

I'm curious if this would be considered bug, afaics ideal behavior would be
on close of a handle, to effectively clear out any remaining callbacks and
let the completion thread terminate.

i've tried adding some bookkeeping to the api to guard against closing while
there is an outstanding completion request, but its an imperfect solution do
to the nature of the event loop integration. The problem is that the python
callback invoked by the completion thread in turn schedules a function for
the main thread. In twisted the api for this is implemented by appending the
function to a list attribute on the reactor and then writing a byte to a
pipe to wakeup the main thread. If a thread switch to the main thread occurs
before the completion thread callback returns, the scheduled function runs
and the rest of the application keeps processing, of which the last step for
the unit tests is to close the connection, which results in a deadlock.

i've included some of the client log and gdb stack traces from a deadlock'd
client process.

thanks,

Kapil

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