Add comments to explain the stack trace difference

Author: zsxwing

core/src/main/scala/org/apache/spark/TaskEndReason.scala

@@ -83,6 +83,13 @@ case class FetchFailed(
  * :: DeveloperApi ::
  * Task failed due to a runtime exception. This is the most common failure case and also captures
  * user program exceptions.
+ *
+ * `stackTrace` contains the stack trace of the exception itself. It still exists for backward
+ * compatibility. It's better that using `this(e: Throwable, metrics: Option[TaskMetrics])` to
+ * create `ExceptionFailure` as it will handle the backward compatibility properly.
+ *
+ * `fullStackTrace` is a better representation of the stack trace because it contains the whole
+ * stack trace including the exception and its causes
  */
 @DeveloperApi
 case class ExceptionFailure(
@@ -101,10 +108,23 @@ case class ExceptionFailure(
     if (fullStackTrace == null) {
       // fullStackTrace is added in 1.2.0
       // If fullStackTrace is null, use the old error string for backward compatibility
-      Utils.exceptionString(className, description, stackTrace)
+      exceptionString(className, description, stackTrace)
     } else {
       fullStackTrace
     }
+
+  /**
+   * Return a nice string representation of the exception, including the stack trace.
+   * Note: It does not include the exception's causes, and is only used for backward compatibility.
+   */
+  private def exceptionString(
+      className: String,
+      description: String,
+      stackTrace: Array[StackTraceElement]): String = {
+    val desc = if (description == null) "" else description
+    val st = if (stackTrace == null) "" else stackTrace.map("        " + _).mkString("\n")
+    s"$className: $desc\n$st"
+  }
 }
 
 /**

core/src/main/scala/org/apache/spark/shuffle/FetchFailedException.scala

@@ -33,18 +33,9 @@ private[spark] class FetchFailedException(
     mapId: Int,
     reduceId: Int,
     message: String,
-    cause: Throwable)
+    cause: Throwable = null)
   extends Exception(message, cause) {
 
-  def this(
-      bmAddress: BlockManagerId,
-      shuffleId: Int,
-      mapId: Int,
-      reduceId: Int,
-      message: String) {
-    this(bmAddress, shuffleId, mapId, reduceId, message, null)
-  }
-
   def this(
       bmAddress: BlockManagerId,
       shuffleId: Int,

core/src/main/scala/org/apache/spark/ui/jobs/StageTable.scala

@@ -207,15 +207,15 @@ private[ui] class FailedStageTable(
         failureReason
       })
     val details = if (isMultiline) {
-    // scalastyle:off
-    <span onclick="this.parentNode.querySelector('.stacktrace-details').classList.toggle('collapsed')"
-          class="expand-details">
-      +details
-    </span> ++
-      <div class="stacktrace-details collapsed">
-        <pre>{failureReason}</pre>
-      </div>
-    // scalastyle:on
+      // scalastyle:off
+      <span onclick="this.parentNode.querySelector('.stacktrace-details').classList.toggle('collapsed')"
+            class="expand-details">
+        +details
+      </span> ++
+        <div class="stacktrace-details collapsed">
+          <pre>{failureReason}</pre>
+        </div>
+      // scalastyle:on
     } else {
       ""
     }

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

@@ -1596,32 +1596,21 @@ private[spark] object Utils extends Logging {
       .orNull
   }
 
-  /** Return a nice string representation of the exception, including the stack trace. */
+  /**
+   * Return a nice string representation of the exception. It will call "printStackTrace" to
+   * recursively generate the stack trace including the exception and its causes.
+   */
   def exceptionString(e: Throwable): String = {
     if (e == null) {
       ""
     } else {
+      // Use e.printStackTrace here because e.getStackTrace doesn't include the cause
       val stringWriter = new StringWriter()
       e.printStackTrace(new PrintWriter(stringWriter))
       stringWriter.toString
     }
   }
 
-  /**
-   * Return a nice string representation of the exception, including the stack trace.
-   * It's only used for backward compatibility.
-   * Note: deprecated because it does not include the exception's cause.
-   */
-  @deprecated("Use exceptionString(Throwable) instead", "1.2.0")
-  def exceptionString(
-      className: String,
-      description: String,
-      stackTrace: Array[StackTraceElement]): String = {
-    val desc = if (description == null) "" else description
-    val st = if (stackTrace == null) "" else stackTrace.map("        " + _).mkString("\n")
-    s"$className: $desc\n$st"
-  }
-
   /**
    * Convert all spark properties set in the given SparkConf to a sequence of java options.
    */