[HOTFIX][CORE] fix a concurrence issue in NewAccumulator

## What changes were proposed in this pull request?

`AccumulatorContext` is not thread-safe, that's why all of its methods are synchronized. However, there is one exception: the `AccumulatorContext.originals`. `NewAccumulator` use it to check if it's registered, which is wrong as it's not synchronized.

This PR mark `AccumulatorContext.originals` as `private` and now all access to `AccumulatorContext` is synchronized.

## How was this patch tested?

I verified it locally. To be safe, we can let jenkins test it many times to make sure this problem is gone.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #12773 from cloud-fan/debug.

1 files changed, 2 insertions, 2 deletions

diff --git a/sql/core/src/test/scala/org/apache/spark/sql/CachedTableSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/CachedTableSuite.scala
index 0e6356b578..1095a73c58 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/CachedTableSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/CachedTableSuite.scala
@@ -334,10 +334,10 @@ class CachedTableSuite extends QueryTest with SQLTestUtils with SharedSQLContext
     sql("SELECT * FROM t2").count()
 
     AccumulatorContext.synchronized {
-      val accsSize = AccumulatorContext.originals.size
+      val accsSize = AccumulatorContext.numAccums
       sqlContext.uncacheTable("t1")
       sqlContext.uncacheTable("t2")
-      assert((accsSize - 2) == AccumulatorContext.originals.size)
+      assert((accsSize - 2) == AccumulatorContext.numAccums)
     }
   }