We have query plans that through multiple transformations can grow extremely long in length. These would eventually throw OutOfMemory exceptions (https://issues.apache.org/jira/browse/SPARK-26103 & related https://issues.apache.org/jira/browse/SPARK-25380).
We backported the changes from https://github.com/apache/spark/pull/23169 into our distribution of Spark, based on 2.4.4, and attempted to use the added `spark.sql.maxPlanStringLength`. While this works in some cases, large query plans can still lead to issues stemming from `StringConcat` in sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala.
The following unit test exhibits the issue, which continues to fail in the master branch of spark:
Looking at the append method here: https://github.com/apache/spark/blob/fc6af9d900ec6f6a1cbe8f987857a69e6ef600d1/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala#L118-L128
It seems like regardless of whether the string to be append is added fully to the internal buffer, added as a substring to reach `maxLength`, or not added at all the internal `length` field is incremented by the length of `s`. Eventually this will overflow an int and cause L123 to substring with a negative index.