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https://issues.apache.org/jira/browse/ZOOKEEPER-2471?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16155862#comment-16155862
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Dan Benediktson commented on ZOOKEEPER-2471:
--------------------------------------------
Hey, sorry, I've been meaning to get back to this, but I wasn't expecting to
sign up for porting the exponential backoff retry until this was in. Polishing
that patch up so that it can be accepted in mainline will take me a fair bit
more time, since our version of the code straight-up replaced the existing
logic with jittered exponential backoff (we haven't run a JVM ZK client without
jittered exponential backoff in > 1.5 years), and I doubt Apache ZK would be
willing to accept that. I simply don't have time right now to do that work, and
won't for at least a month. It also makes me a bit nervous to offer a pull
request for the exponential backoff feature without this fix already checked
in, since this was an extremely expensive bug for us, but I'm sympathetic to
the desire to unit test it; we simply didn't have time to concoct a unit test
back when we needed the fix urgently, since the nature of Zookeeper code makes
it generally pretty difficult to add unit tests for most areas, including this
one.
> Java Zookeeper Client incorrectly considers time spent sleeping as time spent
> connecting, potentially resulting in infinite reconnect loop
> ------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: ZOOKEEPER-2471
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2471
> Project: ZooKeeper
> Issue Type: Bug
> Components: java client
> Affects Versions: 3.5.3
> Environment: all
> Reporter: Dan Benediktson
> Assignee: Dan Benediktson
> Attachments: ZOOKEEPER-2471.patch
>
>
> ClientCnxnSocket uses a member variable "now" to track the current time, and
> lastSend / lastHeard variables to track socket liveness. Implementations, and
> even ClientCnxn itself, are expected to call both updateNow() to reset "now"
> to System.currentTimeMillis, and then call updateLastSend()/updateLastHeard()
> on IO completions.
> This is a fragile contract, so it's not surprising that there's a bug
> resulting from it: ClientCnxn.SendThread.run() calls updateLastSendAndHeard()
> as soon as startConnect() returns, but it does not call updateNow() first. I
> expect when this was written, either the expectation was that startConnect()
> was an asynchronous operation and that updateNow() would have been called
> very recently, or simply the requirement to call updateNow() was forgotten at
> this point. As far as I can see, this bug has been present since the
> "updateNow" method was first introduced in the distant past. As it turns out,
> since startConnect() calls HostProvider.next(), which can sleep, quite a lot
> of time can pass, leaving a big gap between "now" and now.
> If you are using very short session timeouts (one of our ZK ensembles has
> many clients using a 1-second timeout), this is potentially disastrous,
> because the sleep time may exceed the connection timeout itself, which can
> potentially result in the Java client being stuck in a perpetual reconnect
> loop. The exact code path it goes through in this case is complicated,
> because there has to be a previously-closed socket still waiting in the
> selector (otherwise, the first timeout evaluation will not fail because "now"
> still hasn't been updated, and then the actual connect timeout will be
> applied in ClientCnxnSocket.doTransport()) so that select() will harvest the
> IO from the previous socket and updateNow(), resulting in the next loop
> through ClientCnxnSocket.SendThread.run() observing the spurious timeout and
> failing. In practice it does happen to us fairly frequently; we only got to
> the bottom of the bug yesterday. Worse, when it does happen, the Zookeeper
> client object is rendered unusable: it's stuck in a perpetual reconnect loop
> where it keeps sleeping, opening a socket, and immediately closing it.
> I have a patch. Rather than calling updateNow() right after startConnect(),
> my fix is to remove the "now" member variable and the updateNow() method
> entirely, and to instead just call System.currentTimeMillis() whenever time
> needs to be evaluated. I realize there is a benefit (aside from a trivial
> micro-optimization not worth worrying about) to having the time be "fixed",
> particularly for truth in the logging: if time is fixed by an updateNow()
> call, then the log for a timeout will still show exactly the same value the
> code reasoned about. However, this benefit is in my opinion not enough to
> merit the fragility of the contract which led to this (for us) highly
> impactful and difficult-to-find bug in the first place.
> I'm currently running ant tests locally against my patch on trunk, and then
> I'll upload it here.
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