Github user greghogan commented on the pull request:
https://github.com/apache/flink/pull/1464#issuecomment-165499126
I haven't found a prohibition on returning the second input to a
user-defined reduce function. I think fixing this in code is preferable to
documenting the problem or throwing an error, especially since the problem only
exists with object reuse enabled.
Object reuse is configured in JavaProgramTestBase but must be set in the
ExecutionEnvironmentFactory by TestEnvironment. I don't see that any of the
many tests subclassing JavaProgramTestBase were testing with object reuse
enabled.
It is rather difficult to test for object reuse. ObjectReuseITCase simply
captured a few isolated instances of strange behavior (with the original tests
3 and 4 being perfect copies). I would look to duplicate the first two tests in
the appropriate JavaProgramTestBase subclass and delete ObjectReuseITCase.
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