[SPARK-25560][SQL] Allow FunctionInjection in SparkExtensions

This allows an implementer of Spark Session Extensions to utilize a
method "injectFunction" which will add a new function to the default
Spark Session Catalogue.

## What changes were proposed in this pull request?

Adds a new function to SparkSessionExtensions

    def injectFunction(functionDescription: FunctionDescription)

Where function description is a new type

  type FunctionDescription = (FunctionIdentifier, FunctionBuilder)

The functions are loaded in BaseSessionBuilder when the function registry does not have a parent
function registry to get loaded from.

## How was this patch tested?

New unit tests are added for the extension in SparkSessionExtensionSuite

Closes apache#22576 from RussellSpitzer/SPARK-25560.

Authored-by: Russell Spitzer <Russell.Spitzer@gmail.com>
Signed-off-by: Herman van Hovell <hvanhovell@databricks.com>

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

@@ -20,6 +20,10 @@ package org.apache.spark.sql
 import scala.collection.mutable
 
 import org.apache.spark.annotation.{DeveloperApi, Experimental, InterfaceStability}
+import org.apache.spark.sql.catalyst.FunctionIdentifier
+import org.apache.spark.sql.catalyst.analysis.FunctionRegistry
+import org.apache.spark.sql.catalyst.analysis.FunctionRegistry.FunctionBuilder
+import org.apache.spark.sql.catalyst.expressions.ExpressionInfo
 import org.apache.spark.sql.catalyst.parser.ParserInterface
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.catalyst.rules.Rule
@@ -68,6 +72,7 @@ class SparkSessionExtensions {
   type CheckRuleBuilder = SparkSession => LogicalPlan => Unit
   type StrategyBuilder = SparkSession => Strategy
   type ParserBuilder = (SparkSession, ParserInterface) => ParserInterface
+  type FunctionDescription = (FunctionIdentifier, ExpressionInfo, FunctionBuilder)
 
   private[this] val resolutionRuleBuilders = mutable.Buffer.empty[RuleBuilder]
 
@@ -171,4 +176,21 @@ class SparkSessionExtensions {
   def injectParser(builder: ParserBuilder): Unit = {
     parserBuilders += builder
   }
+
+  private[this] val injectedFunctions = mutable.Buffer.empty[FunctionDescription]
+
+  private[sql] def registerFunctions(functionRegistry: FunctionRegistry) = {
+    for ((name, expressionInfo, function) <- injectedFunctions) {
+      functionRegistry.registerFunction(name, expressionInfo, function)
+    }
+    functionRegistry
+  }
+
+  /**
+  * Injects a custom function into the [[org.apache.spark.sql.catalyst.analysis.FunctionRegistry]]
+  * at runtime for all sessions.
+  */
+  def injectFunction(functionDescription: FunctionDescription): Unit = {
+    injectedFunctions += functionDescription
+  }
 }

sql/core/src/main/scala/org/apache/spark/sql/internal/BaseSessionStateBuilder.scala

@@ -95,7 +95,8 @@ abstract class BaseSessionStateBuilder(
    * This either gets cloned from a pre-existing version or cloned from the built-in registry.
    */
   protected lazy val functionRegistry: FunctionRegistry = {
-    parentState.map(_.functionRegistry).getOrElse(FunctionRegistry.builtin).clone()
+    parentState.map(_.functionRegistry.clone())
+      .getOrElse(extensions.registerFunctions(FunctionRegistry.builtin.clone()))
   }
 
   /**

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

@@ -18,12 +18,12 @@ package org.apache.spark.sql
 
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.sql.catalyst.{FunctionIdentifier, TableIdentifier}
-import org.apache.spark.sql.catalyst.expressions.Expression
+import org.apache.spark.sql.catalyst.expressions.{Expression, ExpressionInfo, Literal}
 import org.apache.spark.sql.catalyst.parser.{CatalystSqlParser, ParserInterface}
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.catalyst.rules.Rule
 import org.apache.spark.sql.execution.{SparkPlan, SparkStrategy}
-import org.apache.spark.sql.types.{DataType, StructType}
+import org.apache.spark.sql.types.{DataType, IntegerType, StructType}
 
 /**
  * Test cases for the [[SparkSessionExtensions]].
@@ -90,6 +90,16 @@ class SparkSessionExtensionSuite extends SparkFunSuite {
     }
   }
 
+  test("inject function") {
+    val extensions = create { extensions =>
+      extensions.injectFunction(MyExtensions.myFunction)
+    }
+    withSession(extensions) { session =>
+      assert(session.sessionState.functionRegistry
+        .lookupFunction(MyExtensions.myFunction._1).isDefined)
+    }
+  }
+
   test("use custom class for extensions") {
     val session = SparkSession.builder()
       .master("local[1]")
@@ -98,6 +108,8 @@ class SparkSessionExtensionSuite extends SparkFunSuite {
     try {
       assert(session.sessionState.planner.strategies.contains(MySparkStrategy(session)))
       assert(session.sessionState.analyzer.extendedResolutionRules.contains(MyRule(session)))
+      assert(session.sessionState.functionRegistry
+        .lookupFunction(MyExtensions.myFunction._1).isDefined)
     } finally {
       stop(session)
     }
@@ -136,9 +148,17 @@ case class MyParser(spark: SparkSession, delegate: ParserInterface) extends Pars
     delegate.parseDataType(sqlText)
 }
 
+object MyExtensions {
+
+  val myFunction = (FunctionIdentifier("myFunction"),
+    new ExpressionInfo("noClass", "myDb", "myFunction", "usage", "extended usage" ),
+    (myArgs: Seq[Expression]) => Literal(5, IntegerType))
+}
+
 class MyExtensions extends (SparkSessionExtensions => Unit) {
   def apply(e: SparkSessionExtensions): Unit = {
     e.injectPlannerStrategy(MySparkStrategy)
     e.injectResolutionRule(MyRule)
+    e.injectFunction(MyExtensions.myFunction)
   }
 }