[SPARK-10344] [SQL] Add tests for extraStrategies

Actually using this API requires access to a lot of classes that we might make private by accident.  I've added some tests to prevent this.

Author: Michael Armbrust <michael@databricks.com>

Closes #8516 from marmbrus/extraStrategiesTests.

(cherry picked from commit 5c3d16a)
Signed-off-by: Yin Huai <yhuai@databricks.com>

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

@@ -0,0 +1,67 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package test.org.apache.spark.sql
+
+import org.apache.spark.rdd.RDD
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.expressions.{Literal, GenericInternalRow, Attribute}
+import org.apache.spark.sql.catalyst.plans.logical.{Project, LogicalPlan}
+import org.apache.spark.sql.execution.SparkPlan
+import org.apache.spark.sql.{Row, Strategy, QueryTest}
+import org.apache.spark.sql.test.SharedSQLContext
+import org.apache.spark.unsafe.types.UTF8String
+
+case class FastOperator(output: Seq[Attribute]) extends SparkPlan {
+
+  override protected def doExecute(): RDD[InternalRow] = {
+    val str = Literal("so fast").value
+    val row = new GenericInternalRow(Array[Any](str))
+    sparkContext.parallelize(Seq(row))
+  }
+
+  override def children: Seq[SparkPlan] = Nil
+}
+
+object TestStrategy extends Strategy {
+  def apply(plan: LogicalPlan): Seq[SparkPlan] = plan match {
+    case Project(Seq(attr), _) if attr.name == "a" =>
+      FastOperator(attr.toAttribute :: Nil) :: Nil
+    case _ => Nil
+  }
+}
+
+class ExtraStrategiesSuite extends QueryTest with SharedSQLContext {
+  import testImplicits._
+
+  test("insert an extraStrategy") {
+    try {
+      sqlContext.experimental.extraStrategies = TestStrategy :: Nil
+
+      val df = sqlContext.sparkContext.parallelize(Seq(("so slow", 1))).toDF("a", "b")
+      checkAnswer(
+        df.select("a"),
+        Row("so fast"))
+
+      checkAnswer(
+        df.select("a", "b"),
+        Row("so slow", 1))
+    } finally {
+      sqlContext.experimental.extraStrategies = Nil
+    }
+  }
+}

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

@@ -23,7 +23,7 @@ import org.apache.spark.sql.{ColumnName, SQLContext}
 /**
  * Helper trait for SQL test suites where all tests share a single [[TestSQLContext]].
  */
-private[sql] trait SharedSQLContext extends SQLTestUtils {
+trait SharedSQLContext extends SQLTestUtils {
 
   /**
    * The [[TestSQLContext]] to use for all tests in this suite.