2 files changed, 43 insertions, 25 deletions
diff --git a/core/src/main/scala/org/apache/spark/util/Utils.scala b/core/src/main/scala/org/apache/spark/util/Utils.scala
index 7e204fa218..1a9dbcae8c 100644
--- a/core/src/main/scala/org/apache/spark/util/Utils.scala
+++ b/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)
   }
 }
diff --git a/core/src/test/scala/org/apache/spark/util/UtilsSuite.scala b/core/src/test/scala/org/apache/spark/util/UtilsSuite.scala
index 4aa4854c36..6698749866 100644
--- a/core/src/test/scala/org/apache/spark/util/UtilsSuite.scala
+++ b/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") {