[SPARK-20674][SQL] Support registering UserDefinedFunction as named UDF

## What changes were proposed in this pull request?
For some reason we don't have an API to register UserDefinedFunction as named UDF. It is a no brainer to add one, in addition to the existing register functions we have.

## How was this patch tested?
Added a test case in UDFSuite for the new API.

Author: Reynold Xin <[email protected]>

Closes #17915 from rxin/SPARK-20674.

(cherry picked from commit d099f41)
Signed-off-by: Xiao Li <[email protected]>

Author: rxin

sql/core/src/main/scala/org/apache/spark/sql/UDFRegistration.scala

@@ -70,15 +70,31 @@ class UDFRegistration private[sql] (functionRegistry: FunctionRegistry) extends
    * @param name the name of the UDAF.
    * @param udaf the UDAF needs to be registered.
    * @return the registered UDAF.
+   *
+   * @since 1.5.0
    */
-  def register(
-      name: String,
-      udaf: UserDefinedAggregateFunction): UserDefinedAggregateFunction = {
+  def register(name: String, udaf: UserDefinedAggregateFunction): UserDefinedAggregateFunction = {
     def builder(children: Seq[Expression]) = ScalaUDAF(children, udaf)
     functionRegistry.registerFunction(name, builder)
     udaf
   }
 
+  /**
+   * Register a user-defined function (UDF), for a UDF that's already defined using the DataFrame
+   * API (i.e. of type UserDefinedFunction).
+   *
+   * @param name the name of the UDF.
+   * @param udf the UDF needs to be registered.
+   * @return the registered UDF.
+   *
+   * @since 2.2.0
+   */
+  def register(name: String, udf: UserDefinedFunction): UserDefinedFunction = {
+    def builder(children: Seq[Expression]) = udf.apply(children.map(Column.apply) : _*).expr
+    functionRegistry.registerFunction(name, builder)
+    udf
+  }
+
   // scalastyle:off line.size.limit
 
   /* register 0-22 were generated by this script

sql/core/src/test/scala/org/apache/spark/sql/UDFSuite.scala

@@ -93,6 +93,13 @@ class UDFSuite extends QueryTest with SharedSQLContext {
     assert(sql("SELECT strLenScala('test')").head().getInt(0) === 4)
   }
 
+  test("UDF defined using UserDefinedFunction") {
+    import functions.udf
+    val foo = udf((x: Int) => x + 1)
+    spark.udf.register("foo", foo)
+    assert(sql("select foo(5)").head().getInt(0) == 6)
+  }
+
   test("ZeroArgument UDF") {
     spark.udf.register("random0", () => { Math.random()})
     assert(sql("SELECT random0()").head().getDouble(0) >= 0.0)