[SPARK-19496][SQL] to_date udf to return null when input date is invalid

## What changes were proposed in this pull request?

Currently the udf  `to_date` has different return value with an invalid date input.

```
SELECT to_date('2015-07-22', 'yyyy-dd-MM') ->  return `2016-10-07`
SELECT to_date('2014-31-12')    -> return null
```

As discussed in JIRA [SPARK-19496](https://issues.apache.org/jira/browse/SPARK-19496), we should return null in both situations when the input date is invalid

## How was this patch tested?
unit test added

Author: windpiger <songjun@outlook.com>

Closes #16870 from windpiger/to_date.

1 files changed, 4 insertions, 0 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateTimeUtils.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateTimeUtils.scala
index af70efbb0a..9e1de0fd2f 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateTimeUtils.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateTimeUtils.scala
@@ -98,6 +98,10 @@ object DateTimeUtils {
   def newDateFormat(formatString: String, timeZone: TimeZone): DateFormat = {
     val sdf = new SimpleDateFormat(formatString, Locale.US)
     sdf.setTimeZone(timeZone)
+    // Enable strict parsing, if the input date/format is invalid, it will throw an exception.
+    // e.g. to parse invalid date '2016-13-12', or '2016-01-12' with  invalid format 'yyyy-aa-dd',
+    // an exception will be throwed.
+    sdf.setLenient(false)
     sdf
   }