[SPARK-20146][SQL] fix comment missing issue for thrift server

## What changes were proposed in this pull request?

The column comment was missing while constructing the Hive TableSchema. This fix will preserve the original comment.

## How was this patch tested?

I have added a new test case to test the column with/without comment.

Author: bomeng <bmeng@us.ibm.com>

Closes #17470 from bomeng/SPARK-20146.

sql/hive-thriftserver/src/main/scala/org/apache/spark/sql/hive/thriftserver/SparkExecuteStatementOperation.scala

@@ -292,7 +292,7 @@ object SparkExecuteStatementOperation {
   def getTableSchema(structType: StructType): TableSchema = {
     val schema = structType.map { field =>
       val attrTypeString = if (field.dataType == NullType) "void" else field.dataType.catalogString
-      new FieldSchema(field.name, attrTypeString, "")
+      new FieldSchema(field.name, attrTypeString, field.getComment.getOrElse(""))
     }
     new TableSchema(schema.asJava)
   }

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

@@ -18,7 +18,7 @@
 package org.apache.spark.sql.hive.thriftserver
 
 import org.apache.spark.SparkFunSuite
-import org.apache.spark.sql.types.{NullType, StructField, StructType}
+import org.apache.spark.sql.types.{IntegerType, NullType, StringType, StructField, StructType}
 
 class SparkExecuteStatementOperationSuite extends SparkFunSuite {
   test("SPARK-17112 `select null` via JDBC triggers IllegalArgumentException in ThriftServer") {
@@ -30,4 +30,16 @@ class SparkExecuteStatementOperationSuite extends SparkFunSuite {
     assert(columns.get(0).getType() == org.apache.hive.service.cli.Type.NULL_TYPE)
     assert(columns.get(1).getType() == org.apache.hive.service.cli.Type.NULL_TYPE)
   }
+
+  test("SPARK-20146 Comment should be preserved") {
+    val field1 = StructField("column1", StringType).withComment("comment 1")
+    val field2 = StructField("column2", IntegerType)
+    val tableSchema = StructType(Seq(field1, field2))
+    val columns = SparkExecuteStatementOperation.getTableSchema(tableSchema).getColumnDescriptors()
+    assert(columns.size() == 2)
+    assert(columns.get(0).getType() == org.apache.hive.service.cli.Type.STRING_TYPE)
+    assert(columns.get(0).getComment() == "comment 1")
+    assert(columns.get(1).getType() == org.apache.hive.service.cli.Type.INT_TYPE)
+    assert(columns.get(1).getComment() == "")
+  }
 }