[SPARK-15822] [SQL] Prevent byte array backed classes from referencing freed memory

## What changes were proposed in this pull request?
`UTF8String` and all `Unsafe*` classes are backed by either on-heap or off-heap byte arrays. The code generated version `SortMergeJoin` buffers the left hand side join keys during iteration. This was actually problematic in off-heap mode when one of the keys is a `UTF8String` (or any other 'Unsafe*` object) and the left hand side iterator was exhausted (and released its memory); the buffered keys would reference freed memory. This causes Seg-faults and all kinds of other undefined behavior when we would use one these buffered keys.

This PR fixes this problem by creating copies of the buffered variables. I have added a general method to the `CodeGenerator` for this. I have checked all places in which this could happen, and only `SortMergeJoin` had this problem.

This PR is largely based on the work of robbinspg and he should be credited for this.

closes https://github.com/apache/spark/pull/13707

## How was this patch tested?
Manually tested on problematic workloads.

Author: Pete Robbins <robbinspg@gmail.com>
Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #13723 from hvanhovell/SPARK-15822-2.

1 files changed, 16 insertions, 0 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala
index ff97cd3211..6392ff42d7 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala
@@ -130,6 +130,22 @@ class CodegenContext {
     mutableStates += ((javaType, variableName, initCode))
   }
 
+  /**
+   * Add buffer variable which stores data coming from an [[InternalRow]]. This methods guarantees
+   * that the variable is safely stored, which is important for (potentially) byte array backed
+   * data types like: UTF8String, ArrayData, MapData & InternalRow.
+   */
+  def addBufferedState(dataType: DataType, variableName: String, initCode: String): ExprCode = {
+    val value = freshName(variableName)
+    addMutableState(javaType(dataType), value, "")
+    val code = dataType match {
+      case StringType => s"$value = $initCode.clone();"
+      case _: StructType | _: ArrayType | _: MapType => s"$value = $initCode.copy();"
+      case _ => s"$value = $initCode;"
+    }
+    ExprCode(code, "false", value)
+  }
+
   def declareMutableStates(): String = {
     // It's possible that we add same mutable state twice, e.g. the `mergeExpressions` in
     // `TypedAggregateExpression`, we should call `distinct` here to remove the duplicated ones.