[SPARK-21957][SQL][FOLLOWUP] Support CURRENT_USER without tailing parentheses

sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4

@@ -817,6 +817,7 @@ valueExpression
 
 primaryExpression
     : name=(CURRENT_DATE | CURRENT_TIMESTAMP)                                                  #currentDatetime
+    | CURRENT_USER                                                                             #currentUser
     | CASE whenClause+ (ELSE elseExpression=expression)? END                                   #searchedCase
     | CASE value=expression whenClause+ (ELSE elseExpression=expression)? END                  #simpleCase
     | name=(CAST | TRY_CAST) '(' expression AS dataType ')'                                    #cast

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

@@ -1652,6 +1652,7 @@ class Analyzer(override val catalogManager: CatalogManager)
   private val literalFunctions: Seq[(String, () => Expression, Expression => String)] = Seq(
     (CurrentDate().prettyName, () => CurrentDate(), toPrettySQL(_)),
     (CurrentTimestamp().prettyName, () => CurrentTimestamp(), toPrettySQL(_)),
+    (CurrentUser().prettyName, () => CurrentUser(), toPrettySQL),
     (VirtualColumn.hiveGroupingIdName, () => GroupingID(Nil), _ => VirtualColumn.hiveGroupingIdName)
   )
 

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala

@@ -1698,6 +1698,9 @@ class AstBuilder extends SqlBaseBaseVisitor[AnyRef] with SQLConfHelper with Logg
     }
   }
 
+  override def visitCurrentUser(ctx: CurrentUserContext): Expression = withOrigin(ctx) {
+    if (conf.ansiEnabled) CurrentUser() else UnresolvedAttribute.quoted(ctx.CURRENT_USER().getText)
+  }
   /**
    * Create a [[Cast]] expression.
    */

sql/hive-thriftserver/src/test/scala/org/apache/spark/sql/hive/thriftserver/ThriftServerWithSparkContextSuite.scala

@@ -20,7 +20,7 @@ package org.apache.spark.sql.hive.thriftserver
 import java.sql.SQLException
 import java.util.concurrent.atomic.AtomicBoolean
 
-import org.apache.hive.service.cli.HiveSQLException
+import org.apache.hive.service.cli.{HiveSQLException, OperationHandle}
 
 import org.apache.spark.TaskKilled
 import org.apache.spark.scheduler.{SparkListener, SparkListenerTaskEnd}
@@ -125,9 +125,23 @@ trait ThriftServerWithSparkContextSuite extends SharedThriftServer {
     withCLIServiceClient(clientUser) { client =>
       val sessionHandle = client.openSession(clientUser, "")
       val confOverlay = new java.util.HashMap[java.lang.String, java.lang.String]
-      val opHandle = client.executeStatement(sessionHandle, sql, confOverlay)
-      val rowSet = client.fetchResults(opHandle)
-      assert(rowSet.toTRowSet.getColumns.get(0).getStringVal.getValues.get(0) === clientUser)
+      val exec: String => OperationHandle = client.executeStatement(sessionHandle, _, confOverlay)
+
+      exec(s"set ${SQLConf.ANSI_ENABLED.key}=false")
+
+      val opHandle1 = exec("select current_user(), current_user")
+      val rowSet1 = client.fetchResults(opHandle1)
+      rowSet1.toTRowSet.getColumns.forEach { col =>
+        assert(col.getStringVal.getValues.get(0) === clientUser)
+      }
+
+      exec(s"set ${SQLConf.ANSI_ENABLED.key}=true")
+      val opHandle2 = exec("select current_user")
+      assert(client.fetchResults(opHandle2).toTRowSet.getColumns.get(0)
+        .getStringVal.getValues.get(0) === clientUser)
+
+      val e = intercept[HiveSQLException](exec("select current_user()"))
+      assert(e.getMessage.contains("current_user"))
     }
   }
 }