GitHub user JoshRosen opened a pull request:
https://github.com/apache/spark/pull/15185
[SPARK-17618] Fix invalid comparisons between UnsafeRow and other row
formats
## What changes were proposed in this pull request?
This patch addresses a correctness bug in Spark 1.6.x in where `coalesce()`
declares that it can process `UnsafeRows` but mis-declares that it always
outputs safe rows. If UnsafeRow and other Row types are compared for equality
then we will get spurious `false` comparisons, leading to wrong answers in
operators which perform whole-row comparison (such as `distinct()` or
`except()`). An example of a query impacted by this bug is given in the [JIRA
ticket](https://issues.apache.org/jira/browse/SPARK-17618).
The problem is that the validity of our row format conversion rules depends
on operators which handle `unsafeRows` (signalled by overriding
`canProcessUnsafeRows`) correctly reporting their output row format (which is
done by overriding `outputsUnsafeRows`). In #9024, we overrode
`canProcessUnsafeRows` but forgot to override `outputsUnsafeRows`, leading to
the incorrect `equals()` comparison.
Our interface design is flawed because correctness depends on operators
correctly overriding multiple methods this problem could have been prevented by
a design which coupled row format methods / metadata into a single method /
class so that all three methods had to be overridden at the same time.
This patch addresses this issue by adding missing `outputsUnsafeRows`
overrides. In order to ensure that bugs in this logic are uncovered sooner, I
have modified `UnsafeRow.equals()` to throw an `IllegalArgumentException` if it
is called with an object that is not an `UnsafeRow`.
## How was this patch tested?
I believe that the stronger misuse-checking in `UnsafeRow.equals()` is
sufficient to detect and prevent this class of bug.
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/JoshRosen/spark SPARK-17618
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/spark/pull/15185.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #15185
----
commit d09a81e79cf4febc35b42fb0ff57d772f70b8e49
Author: Josh Rosen <[email protected]>
Date: 2016-09-21T19:56:25Z
Override outputsUnsafeRows when overriding canProcessUnsafeRows
commit 9d4cf441a8d7d0af8eba3d473e996e0b39fd5560
Author: Josh Rosen <[email protected]>
Date: 2016-09-21T19:57:40Z
Throw IllegalArgumentException when performing illegal UnsafeRow equals
comparison.
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