[SPARK-15601][CORE] CircularBuffer's toString() to print only the contents written if buffer isn't full

## What changes were proposed in this pull request?

1. The class allocated 4x space than needed as it was using `Int` to store the `Byte` values

2. If CircularBuffer isn't full, currently toString() will print some garbage chars along with the content written as is tries to print the entire array allocated for the buffer. The fix is to keep track of buffer getting full and don't print the tail of the buffer if it isn't full (suggestion by sameeragarwal over apache#12194 (comment))

3. Simplified `toString()`

## How was this patch tested?

Added new test case

Author: Tejas Patil <tejasp@fb.com>

Closes apache#13351 from tejasapatil/circular_buffer.

Author: tejasapatil

core/src/main/scala/org/apache/spark/util/Utils.scala

@@ -2344,29 +2344,24 @@ private[spark] class RedirectThread(
  * the toString method.
  */
 private[spark] class CircularBuffer(sizeInBytes: Int = 10240) extends java.io.OutputStream {
-  var pos: Int = 0
-  var buffer = new Array[Int](sizeInBytes)
+  private var pos: Int = 0
+  private var isBufferFull = false
+  private val buffer = new Array[Byte](sizeInBytes)
 
-  def write(i: Int): Unit = {
-    buffer(pos) = i
+  def write(input: Int): Unit = {
+    buffer(pos) = input.toByte
     pos = (pos + 1) % buffer.length
+    isBufferFull = isBufferFull || (pos == 0)
   }
 
   override def toString: String = {
-    val (end, start) = buffer.splitAt(pos)
-    val input = new java.io.InputStream {
-      val iterator = (start ++ end).iterator
-
-      def read(): Int = if (iterator.hasNext) iterator.next() else -1
-    }
-    val reader = new BufferedReader(new InputStreamReader(input, StandardCharsets.UTF_8))
-    val stringBuilder = new StringBuilder
-    var line = reader.readLine()
-    while (line != null) {
-      stringBuilder.append(line)
-      stringBuilder.append("\n")
-      line = reader.readLine()
+    if (!isBufferFull) {
+      return new String(buffer, 0, pos, StandardCharsets.UTF_8)
     }
-    stringBuilder.toString()
+
+    val nonCircularBuffer = new Array[Byte](sizeInBytes)
+    System.arraycopy(buffer, pos, nonCircularBuffer, 0, buffer.length - pos)
+    System.arraycopy(buffer, 0, nonCircularBuffer, buffer.length - pos, pos)
+    new String(nonCircularBuffer, StandardCharsets.UTF_8)
   }
 }

core/src/test/scala/org/apache/spark/util/UtilsSuite.scala

@@ -17,7 +17,7 @@
 
 package org.apache.spark.util
 
-import java.io.{ByteArrayInputStream, ByteArrayOutputStream, File, FileOutputStream}
+import java.io.{ByteArrayInputStream, ByteArrayOutputStream, File, FileOutputStream, PrintStream}
 import java.lang.{Double => JDouble, Float => JFloat}
 import java.net.{BindException, ServerSocket, URI}
 import java.nio.{ByteBuffer, ByteOrder}
@@ -681,14 +681,37 @@ class UtilsSuite extends SparkFunSuite with ResetSystemProperties with Logging {
     assert(!Utils.isInDirectory(nullFile, childFile3))
   }
 
-  test("circular buffer") {
+  test("circular buffer: if nothing was written to the buffer, display nothing") {
+    val buffer = new CircularBuffer(4)
+    assert(buffer.toString === "")
+  }
+
+  test("circular buffer: if the buffer isn't full, print only the contents written") {
+    val buffer = new CircularBuffer(10)
+    val stream = new PrintStream(buffer, true, "UTF-8")
+    stream.print("test")
+    assert(buffer.toString === "test")
+  }
+
+  test("circular buffer: data written == size of the buffer") {
+    val buffer = new CircularBuffer(4)
+    val stream = new PrintStream(buffer, true, "UTF-8")
+
+    // fill the buffer to its exact size so that it just hits overflow
+    stream.print("test")
+    assert(buffer.toString === "test")
+
+    // add more data to the buffer
+    stream.print("12")
+    assert(buffer.toString === "st12")
+  }
+
+  test("circular buffer: multiple overflow") {
     val buffer = new CircularBuffer(25)
-    val stream = new java.io.PrintStream(buffer, true, "UTF-8")
+    val stream = new PrintStream(buffer, true, "UTF-8")
 
-    // scalastyle:off println
-    stream.println("test circular test circular test circular test circular test circular")
-    // scalastyle:on println
-    assert(buffer.toString === "t circular test circular\n")
+    stream.print("test circular test circular test circular test circular test circular")
+    assert(buffer.toString === "st circular test circular")
   }
 
   test("nanSafeCompareDoubles") {