### What changes were proposed in this pull request?
This patch fixes the bug of UnsafeRow which misses to handle the UDT specifically, in `isFixedLength` and `isMutable`. These methods don't check its SQL type for UDT, always treating UDT as variable-length, and non-mutable.
It doesn't bring any issue if UDT is used to represent complicated type, but when UDT is used to represent some type which is matched with fixed length of SQL type, it exposes the chance of correctness issues, as these informations sometimes decide how the value should be handled.
We got report from user mailing list which suspected as mapGroupsWithState looks like handling UDT incorrectly, but after some investigation it was from GenerateUnsafeRowJoiner in shuffle phase.
0e2ca11d80/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/GenerateUnsafeRowJoiner.scala (L32-L43)
Here updating position should not happen on fixed-length column, but due to this bug, the value of UDT having fixed-length as sql type would be modified, which actually corrupts the value.
### Why are the changes needed?
Misclassifying of the type of length for UDT can corrupt the value when the row is presented to the input of GenerateUnsafeRowJoiner, which brings correctness issue.
### Does this PR introduce any user-facing change?
No.
### How was this patch tested?
New UT added.
Closes#27747 from HeartSaVioR/SPARK-30993.
Authored-by: Jungtaek Lim (HeartSaVioR) <kabhwan.opensource@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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