[SPARK-29758][SQL][2.4] Fix truncation of requested string fields in json_tuple

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/jsonExpressions.scala

@@ -472,11 +472,6 @@ case class JsonTuple(children: Seq[Expression])
     parser.getCurrentToken match {
       // if the user requests a string field it needs to be returned without enclosing
       // quotes which is accomplished via JsonGenerator.writeRaw instead of JsonGenerator.write
-      case JsonToken.VALUE_STRING if parser.hasTextCharacters =>
-        // slight optimization to avoid allocating a String instance, though the characters
-        // still have to be decoded... Jackson doesn't have a way to access the raw bytes
-        generator.writeRaw(parser.getTextCharacters, parser.getTextOffset, parser.getTextLength)
-
       case JsonToken.VALUE_STRING =>
         // the normal String case, pass it through to the output without enclosing quotes
         generator.writeRaw(parser.getText)

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

@@ -535,4 +535,14 @@ class JsonFunctionsSuite extends QueryTest with SharedSQLContext {
       to_json(struct($"t"), Map("timestampFormat" -> "yyyy-MM-dd HH:mm:ss.SSSSSS")))
     checkAnswer(df, Row(s"""{"t":"$s"}"""))
   }
+
+  test("json_tuple - do not truncate results") {
+    val len = 2800
+    val str = Array.tabulate(len)(_ => "a").mkString
+    val json_tuple_result = Seq(s"""{"test":"$str"}""").toDF("json")
+      .withColumn("result", json_tuple('json, "test"))
+      .select('result)
+      .as[String].head.length
+    assert(json_tuple_result === len)
+  }
 }