Github user zsxwing commented on a diff in the pull request:
https://github.com/apache/spark/pull/2957#discussion_r19455903
--- Diff: core/src/main/scala/org/apache/spark/FutureAction.scala ---
@@ -210,7 +210,9 @@ class ComplexFutureAction[T] extends FutureAction[T] {
} catch {
case e: Exception => p.failure(e)
} finally {
- thread = null
+ ComplexFutureAction.this.synchronized {
--- End diff --
Here is the codes of `cancel`:
```Scala
1 override def cancel(): Unit = this.synchronized {
2 _cancelled = true
3 if (thread != null) {
4 thread.interrupt()
5 }
6 }
```
Without `ComplexFutureAction.this.synchronized`, the codes can run in the
following order:
1. In thread T1, `cancel` is called.
2. After T1 finishes L3, it's suspended.
3. Assume T2 is the thread that runs the action. Now T2 is set `thread` to
null.
4. T1 is resumed and runs L4 (thread.interrupt()), because `thread` is
null, it will throw a NPE.
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