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From David Nickerson <davidnickerson4mailingli...@gmail.com>
Subject Memory leak caused by a watch that will never be triggered
Date Wed, 30 May 2012 18:35:34 GMT
I writing a lock implementation that agrees with the lock recipe in the
documentation<http://zookeeper.apache.org/doc/current/recipes.html#sc_recipes_Locks>.
In step 5, the thread needs to wait for a notification that it may have the
lock. Since I'm working with many threads, each thread needs to wait on its
own object. I decided that it would be easiest if the thread waits on the
watcher that is passed as a parameter to zk.exists in step 4. This way,
when a thread may have the lock, its watcher gets called directly.

(The other option would be to have only one watcher. Then, when
watcher.process() gets called, the watcher would need to figure out which
thread it should wake up.)

The only problem is that I might have a memory leak. In step 5, if exists()
returns false, then the node does not exist and it will never exist because
it is sequential. This means that the client will forever hold a watch for
a node that will never be created, and that watch will forever reference my
watcher object. Therefore, even if I try to dump that watcher object (since
I don't need it anymore), the client will still hold a reference to it and
it won't be garbage collected.

The only way that I've come up with to plug the memory leak is to recreate
the znode as non-sequential, thereby triggering the watch I set, and then
delete the znode to prevent a deadlock.

Can anyone confirm that this is indeed a memory leak, and does anyone know
a better way to prevent it?

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