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From "Gabor Gevay (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-3291) Object reuse bug in MergeIterator.HeadStream.nextHead
Date Fri, 12 Feb 2016 16:10:19 GMT

    [ https://issues.apache.org/jira/browse/FLINK-3291?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15144755#comment-15144755

Gabor Gevay commented on FLINK-3291:

The initial idea of a contract for MutableObjectiterator.next(reuse) was the following:
1. The caller may not hold onto reuse any more
2. The iterator implementor may not hold onto the returned object any more.
Given that this was long ago (5 years probably, since I created that interface), I am pretty
sure that contract is not obeyed everywhere.

OK, this clears things up; thanks for chiming in, [~StephanEwen]!

Then the things we should do are
1. Add this contract to the javadoc of `MutableObjectiterator.next(reuse)`.
2. Go with [~greghogan]'s solution \[1\] to fix the problem brought up by this Jira.
3. Check all calls to `MutableObjectiterator.next(reuse)`. (A few suspicious ones are in `CrossDriver`,
`UnionWithTempOperator`, `MutableHashTable.ProbeIterator.next`, and `ReusingBuildFirstHashJoinIterator.callWithNextKey`.)

Gabor Gevay, at this point are the changes to MergeIterator fixing a bug? Do you want to fix
up and clarify the documentation for MutableObjectIterator and verify the implementing classes?
My changes to `MergeIterator` become unnecessary if the contracts of `MutableObjectIterator.next(reuse)`
are what Stephan said.
I'll open a Jira for fixing the javadoc of `MutableObjectIterator.next(reuse)` to include
the contract (and possibly also add this info to a new wiki page that will be linked from
the new version of the object reuse documentation brewing at \[2\]), and check all call sites
whether they obey the contract.

\[1\] https://github.com/apache/flink/pull/1626
\[2\] https://docs.google.com/document/d/1cgkuttvmj4jUonG7E2RdFVjKlfQDm_hE6gvFcgAfzXg/edit

> Object reuse bug in MergeIterator.HeadStream.nextHead
> -----------------------------------------------------
>                 Key: FLINK-3291
>                 URL: https://issues.apache.org/jira/browse/FLINK-3291
>             Project: Flink
>          Issue Type: Bug
>          Components: Distributed Runtime
>    Affects Versions: 1.0.0
>            Reporter: Gabor Gevay
>            Assignee: Gabor Gevay
>            Priority: Critical
> MergeIterator.HeadStream.nextHead saves a reference into `this.head` of the `reuse` object
that it got as an argument. This object might be modified later by the caller.
> This actually happens when ReduceDriver.run calls input.next (which will actually be
MergeIterator.next(E reuse)) in the inner while loop of the objectReuseEnabled branch, and
that calls top.nextHead with the reference that it got from ReduceDriver, which erroneously
saves the reference, and then ReduceDriver later uses that same object for doing the reduce.
> Another way in which this fails is when MergeIterator.next(E reuse) gives `reuse` to
different `top`s in different calls, and then the heads end up being the same object.
> You can observe the latter situation in action by running ReducePerformance here:
> https://github.com/ggevay/flink/tree/merge-iterator-object-reuse-bug
> Set memory to -Xmx200m (so that the MergeIterator actually has merging to do), put a
breakpoint at the beginning of MergeIterator.next(reuse), and then watch `reuse`, and the
heads of the first two elements of `this.heap` in the debugger. They will get to be the same
object after hitting continue about 6 times.
> You can also look at the count that is printed at the end, which shouldn't be larger
than the key range. Also, if you look into the output file /tmp/xxxobjectreusebug, for example
the key 999977 appears twice.
> The good news is that I think I can see an easy fix that doesn't affect performance:
MergeIterator.HeadStream could have a reuse object of its own as a member, and give that to
iterator.next in nextHead(E reuse). And then we wouldn't need the overload of nextHead that
has the reuse parameter, and MergeIterator.next(E reuse) could just call its other overload.

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