[SPARK-14769][SQL] Create built-in functionality for variable substitution

## What changes were proposed in this pull request?
In order to fully merge the Hive parser and the SQL parser, we'd need to support variable substitution in Spark. The implementation of the substitute algorithm is mostly copied from Hive, but I simplified the overall structure quite a bit and added more comprehensive test coverage.

Note that this pull request does not yet use this functionality anywhere.

## How was this patch tested?
Added VariableSubstitutionSuite for unit tests.

Author: Reynold Xin <rxin@databricks.com>

Closes #12538 from rxin/SPARK-14769.

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

@@ -457,6 +457,18 @@ object SQLConf {
       .booleanConf
       .createWithDefault(true)
 
+  val VARIABLE_SUBSTITUTE_ENABLED =
+    SQLConfigBuilder("spark.sql.variable.substitute")
+      .doc("This enables substitution using syntax like ${var} ${system:var} and ${env:var}.")
+      .booleanConf
+      .createWithDefault(true)
+
+  val VARIABLE_SUBSTITUTE_DEPTH =
+    SQLConfigBuilder("spark.sql.variable.substitute.depth")
+      .doc("The maximum replacements the substitution engine will do.")
+      .intConf
+      .createWithDefault(40)
+
   // TODO: This is still WIP and shouldn't be turned on without extensive test coverage
   val COLUMNAR_AGGREGATE_MAP_ENABLED = SQLConfigBuilder("spark.sql.codegen.aggregate.map.enabled")
     .internal()
@@ -615,6 +627,10 @@ private[sql] class SQLConf extends Serializable with CatalystConf with Logging {
 
   def columnarAggregateMapEnabled: Boolean = getConf(COLUMNAR_AGGREGATE_MAP_ENABLED)
 
+  def variableSubstituteEnabled: Boolean = getConf(VARIABLE_SUBSTITUTE_ENABLED)
+
+  def variableSubstituteDepth: Int = getConf(VARIABLE_SUBSTITUTE_DEPTH)
+
   override def orderByOrdinal: Boolean = getConf(ORDER_BY_ORDINAL)
 
   override def groupByOrdinal: Boolean = getConf(GROUP_BY_ORDINAL)

sql/core/src/main/scala/org/apache/spark/sql/internal/VariableSubstitution.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.internal
+
+import java.util.regex.Pattern
+
+import org.apache.spark.sql.AnalysisException
+
+/**
+ * A helper class that enables substitution using syntax like
+ * `${var}`, `${system:var}` and `${env:var}`.
+ *
+ * Variable substitution is controlled by [[SQLConf.variableSubstituteEnabled]].
+ */
+class VariableSubstitution(conf: SQLConf) {
+
+  private val pattern = Pattern.compile("\\$\\{[^\\}\\$ ]+\\}")
+
+  /**
+   * Given a query, does variable substitution and return the result.
+   */
+  def substitute(input: String): String = {
+    // Note that this function is mostly copied from Hive's SystemVariables, so the style is
+    // very Java/Hive like.
+    if (input eq null) {
+      return null
+    }
+
+    if (!conf.variableSubstituteEnabled) {
+      return input
+    }
+
+    var eval = input
+    val depth = conf.variableSubstituteDepth
+    val builder = new StringBuilder
+    val m = pattern.matcher("")
+
+    var s = 0
+    while (s <= depth) {
+      m.reset(eval)
+      builder.setLength(0)
+
+      var prev = 0
+      var found = false
+      while (m.find(prev)) {
+        val group = m.group()
+        var substitute = substituteVariable(group.substring(2, group.length - 1))
+        if (substitute.isEmpty) {
+          substitute = group
+        } else {
+          found = true
+        }
+        builder.append(eval.substring(prev, m.start())).append(substitute)
+        prev = m.end()
+      }
+
+      if (!found) {
+        return eval
+      }
+
+      builder.append(eval.substring(prev))
+      eval = builder.toString
+      s += 1
+    }
+
+    if (s > depth) {
+      throw new AnalysisException(
+        "Variable substitution depth is deeper than " + depth + " for input " + input)
+    } else {
+      return eval
+    }
+  }
+
+  /**
+   * Given a variable, replaces with the substitute value (default to "").
+   */
+  private def substituteVariable(variable: String): String = {
+    var value: String = null
+
+    if (variable.startsWith("system:")) {
+      value = System.getProperty(variable.substring("system:".length()))
+    }
+
+    if (value == null && variable.startsWith("env:")) {
+      value = System.getenv(variable.substring("env:".length()))
+    }
+
+    if (value == null && conf != null && variable.startsWith("hiveconf:")) {
+      value = conf.getConfString(variable.substring("hiveconf:".length()), "")
+    }
+
+    if (value == null && conf != null && variable.startsWith("sparkconf:")) {
+      value = conf.getConfString(variable.substring("sparkconf:".length()), "")
+    }
+
+    if (value == null && conf != null && variable.startsWith("spark:")) {
+      value = conf.getConfString(variable.substring("spark:".length()), "")
+    }
+
+    if (value == null && conf != null) {
+      value = conf.getConfString(variable, "")
+    }
+
+    value
+  }
+}

sql/core/src/test/scala/org/apache/spark/sql/internal/VariableSubstitutionSuite.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.internal
+
+import org.apache.spark.SparkFunSuite
+import org.apache.spark.sql.AnalysisException
+
+class VariableSubstitutionSuite extends SparkFunSuite {
+
+  private lazy val conf = new SQLConf
+  private lazy val sub = new VariableSubstitution(conf)
+
+  test("system property") {
+    System.setProperty("varSubSuite.var", "abcd")
+    assert(sub.substitute("${system:varSubSuite.var}") == "abcd")
+  }
+
+  test("environmental variables") {
+    assert(sub.substitute("${env:SPARK_TESTING}") == "1")
+  }
+
+  test("Spark configuration variable") {
+    conf.setConfString("some-random-string-abcd", "1234abcd")
+    assert(sub.substitute("${hiveconf:some-random-string-abcd}") == "1234abcd")
+    assert(sub.substitute("${sparkconf:some-random-string-abcd}") == "1234abcd")
+    assert(sub.substitute("${spark:some-random-string-abcd}") == "1234abcd")
+    assert(sub.substitute("${some-random-string-abcd}") == "1234abcd")
+  }
+
+  test("multiple substitutes") {
+    val q = "select ${bar} ${foo} ${doo} this is great"
+    conf.setConfString("bar", "1")
+    conf.setConfString("foo", "2")
+    conf.setConfString("doo", "3")
+    assert(sub.substitute(q) == "select 1 2 3 this is great")
+  }
+
+  test("test nested substitutes") {
+    val q = "select ${bar} ${foo} this is great"
+    conf.setConfString("bar", "1")
+    conf.setConfString("foo", "${bar}")
+    assert(sub.substitute(q) == "select 1 1 this is great")
+  }
+
+  test("depth limit") {
+    val q = "select ${bar} ${foo} ${doo}"
+    conf.setConfString(SQLConf.VARIABLE_SUBSTITUTE_DEPTH.key, "2")
+
+    // This should be OK since it is not nested.
+    conf.setConfString("bar", "1")
+    conf.setConfString("foo", "2")
+    conf.setConfString("doo", "3")
+    assert(sub.substitute(q) == "select 1 2 3")
+
+    // This should not be OK since it is nested in 3 levels.
+    conf.setConfString("bar", "1")
+    conf.setConfString("foo", "${bar}")
+    conf.setConfString("doo", "${foo}")
+    intercept[AnalysisException] {
+      sub.substitute(q)
+    }
+  }
+}