[SPARK-16274][SQL] Implement xpath_boolean

## What changes were proposed in this pull request?
This patch implements xpath_boolean expression for Spark SQL, a xpath function that returns true or false. The implementation is modelled after Hive's xpath_boolean, except that how the expression handles null inputs. Hive throws a NullPointerException at runtime if either of the input is null. This implementation returns null if either of the input is null.

## How was this patch tested?
Created two new test suites. One for unit tests covering the expression, and the other for end-to-end test in SQL.

Author: petermaxlee <petermaxlee@gmail.com>

Closes #13964 from petermaxlee/SPARK-16274.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala

@@ -25,6 +25,7 @@ import org.apache.spark.sql.AnalysisException
 import org.apache.spark.sql.catalyst.analysis.FunctionRegistry.FunctionBuilder
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.expressions.aggregate._
+import org.apache.spark.sql.catalyst.expressions.xml._
 import org.apache.spark.sql.catalyst.util.StringKeyHashMap
 
 
@@ -301,6 +302,7 @@ object FunctionRegistry {
     expression[UnBase64]("unbase64"),
     expression[Unhex]("unhex"),
     expression[Upper]("upper"),
+    expression[XPathBoolean]("xpath_boolean"),
 
     // datetime functions
     expression[AddMonths]("add_months"),

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/xml/XPathBoolean.scala

+/*
+ * 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 org.apache.spark.sql.catalyst.expressions.xml
+
+import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.catalyst.expressions.codegen.CodegenFallback
+import org.apache.spark.sql.types.{AbstractDataType, BooleanType, DataType, StringType}
+import org.apache.spark.unsafe.types.UTF8String
+
+
+@ExpressionDescription(
+  usage = "_FUNC_(xml, xpath) - Evaluates a boolean xpath expression.",
+  extended = "> SELECT _FUNC_('<a><b>1</b></a>','a/b');\ntrue")
+case class XPathBoolean(xml: Expression, path: Expression)
+  extends BinaryExpression with ExpectsInputTypes with CodegenFallback {
+
+  @transient private lazy val xpathUtil = new UDFXPathUtil
+
+  // If the path is a constant, cache the path string so that we don't need to convert path
+  // from UTF8String to String for every row.
+  @transient lazy val pathLiteral: String = path match {
+    case Literal(str: UTF8String, _) => str.toString
+    case _ => null
+  }
+
+  override def prettyName: String = "xpath_boolean"
+
+  override def dataType: DataType = BooleanType
+
+  override def inputTypes: Seq[AbstractDataType] = Seq(StringType, StringType)
+
+  override def left: Expression = xml
+  override def right: Expression = path
+
+  override protected def nullSafeEval(xml: Any, path: Any): Any = {
+    val xmlString = xml.asInstanceOf[UTF8String].toString
+    if (pathLiteral ne null) {
+      xpathUtil.evalBoolean(xmlString, pathLiteral)
+    } else {
+      xpathUtil.evalBoolean(xmlString, path.asInstanceOf[UTF8String].toString)
+    }
+  }
+}

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/xml/XPathExpressionSuite.scala

+/*
+ * 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 org.apache.spark.sql.catalyst.expressions.xml
+
+import org.apache.spark.SparkFunSuite
+import org.apache.spark.sql.catalyst.dsl.expressions._
+import org.apache.spark.sql.catalyst.expressions.{ExpressionEvalHelper, Literal}
+import org.apache.spark.sql.types.StringType
+
+/**
+ * Test suite for various xpath functions.
+ */
+class XPathExpressionSuite extends SparkFunSuite with ExpressionEvalHelper {
+
+  private def testBoolean[T](xml: String, path: String, expected: T): Unit = {
+    checkEvaluation(
+      XPathBoolean(Literal.create(xml, StringType), Literal.create(path, StringType)),
+      expected)
+  }
+
+  test("xpath_boolean") {
+    testBoolean("<a><b>b</b></a>", "a/b", true)
+    testBoolean("<a><b>b</b></a>", "a/c", false)
+    testBoolean("<a><b>b</b></a>", "a/b = \"b\"", true)
+    testBoolean("<a><b>b</b></a>", "a/b = \"c\"", false)
+    testBoolean("<a><b>10</b></a>", "a/b < 10", false)
+    testBoolean("<a><b>10</b></a>", "a/b = 10", true)
+
+    // null input
+    testBoolean(null, null, null)
+    testBoolean(null, "a", null)
+    testBoolean("<a><b>10</b></a>", null, null)
+
+    // exception handling for invalid input
+    intercept[Exception] {
+      testBoolean("<a>/a>", "a", null)
+    }
+  }
+
+  test("xpath_boolean path cache invalidation") {
+    // This is a test to ensure the expression is not reusing the path for different strings
+    val expr = XPathBoolean(Literal("<a><b>b</b></a>"), 'path.string.at(0))
+    checkEvaluation(expr, true, create_row("a/b"))
+    checkEvaluation(expr, false, create_row("a/c"))
+  }
+}

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

+/*
+ * 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 org.apache.spark.sql
+
+import org.apache.spark.sql.test.SharedSQLContext
+
+/**
+ * End-to-end tests for XML expressions.
+ */
+class XmlFunctionsSuite extends QueryTest with SharedSQLContext {
+  import testImplicits._
+
+  test("xpath_boolean") {
+    val df = Seq("<a><b>b</b></a>" -> "a/b").toDF("xml", "path")
+    checkAnswer(df.selectExpr("xpath_boolean(xml, path)"), Row(true))
+  }
+}

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveSessionCatalog.scala

@@ -241,7 +241,7 @@ private[sql] class HiveSessionCatalog(
     "elt", "hash", "java_method", "histogram_numeric",
     "map_keys", "map_values",
     "parse_url", "percentile", "percentile_approx", "reflect", "sentences", "stack", "str_to_map",
-    "xpath", "xpath_boolean", "xpath_double", "xpath_float", "xpath_int", "xpath_long",
+    "xpath", "xpath_double", "xpath_float", "xpath_int", "xpath_long",
     "xpath_number", "xpath_short", "xpath_string",
 
     // table generating function