[SPARK-986]: Job cancelation for PySpark

* Additions to the PySpark API to cancel jobs
* Monitor Thread in PythonRDD to kill Python workers if a task is interrupted

Author: Ahir Reddy <ahirreddy@gmail.com>

Closes #541 from ahirreddy/python-cancel and squashes the following commits:

dfdf447 [Ahir Reddy] Changed success -> completed and made logging message clearer
6c860ab [Ahir Reddy] PR Comments
4b4100a [Ahir Reddy] Success flag
adba6ed [Ahir Reddy] Destroy python workers
27a2f8f [Ahir Reddy] Start the writer thread...
d422f7b [Ahir Reddy] Remove unnecesssary vals
adda337 [Ahir Reddy] Busy wait on the ocntext.interrupted flag, and then kill the python worker
d9e472f [Ahir Reddy] Revert "removed unnecessary vals"
5b9cae5 [Ahir Reddy] removed unnecessary vals
07b54d9 [Ahir Reddy] Fix canceling unit test
8ae9681 [Ahir Reddy] Don't interrupt worker
7722342 [Ahir Reddy] Monitor Thread for python workers
db04e16 [Ahir Reddy] Added canceling api to PySpark

2 files changed, 37 insertions, 1 deletions

diff --git a/core/src/main/scala/org/apache/spark/SparkEnv.scala b/core/src/main/scala/org/apache/spark/SparkEnv.scala
index 915315ed74..bea435ec34 100644
--- a/core/src/main/scala/org/apache/spark/SparkEnv.scala
+++ b/core/src/main/scala/org/apache/spark/SparkEnv.scala
@@ -97,6 +97,14 @@ class SparkEnv (
       pythonWorkers.getOrElseUpdate(key, new PythonWorkerFactory(pythonExec, envVars)).create()
     }
   }
+
+  private[spark]
+  def destroyPythonWorker(pythonExec: String, envVars: Map[String, String]) {
+    synchronized {
+      val key = (pythonExec, envVars)
+      pythonWorkers(key).stop()
+    }
+  }
 }
 
 object SparkEnv extends Logging {
diff --git a/core/src/main/scala/org/apache/spark/api/python/PythonRDD.scala b/core/src/main/scala/org/apache/spark/api/python/PythonRDD.scala
index 0d71fdbb03..1498b017a7 100644
--- a/core/src/main/scala/org/apache/spark/api/python/PythonRDD.scala
+++ b/core/src/main/scala/org/apache/spark/api/python/PythonRDD.scala
@@ -110,13 +110,41 @@ private[spark] class PythonRDD[T: ClassTag](
       }
     }.start()
 
+    // Necessary to distinguish between a task that has failed and a task that is finished
+    @volatile var complete: Boolean = false
+
+    // It is necessary to have a monitor thread for python workers if the user cancels with
+    // interrupts disabled. In that case we will need to explicitly kill the worker, otherwise the
+    // threads can block indefinitely.
+    new Thread(s"Worker Monitor for $pythonExec") {
+      override def run() {
+        // Kill the worker if it is interrupted or completed
+        // When a python task completes, the context is always set to interupted
+        while (!context.interrupted) {
+          Thread.sleep(2000)
+        }
+        if (!complete) {
+          try {
+            logWarning("Incomplete task interrupted: Attempting to kill Python Worker")
+            env.destroyPythonWorker(pythonExec, envVars.toMap)
+          } catch {
+            case e: Exception =>
+              logError("Exception when trying to kill worker", e)
+          }
+        }
+      }
+    }.start()
+
     /*
      * Partial fix for SPARK-1019: Attempts to stop reading the input stream since
      * other completion callbacks might invalidate the input. Because interruption
      * is not synchronous this still leaves a potential race where the interruption is
      * processed only after the stream becomes invalid.
      */
-    context.addOnCompleteCallback(() => context.interrupted = true)
+    context.addOnCompleteCallback{ () =>
+      complete = true // Indicate that the task has completed successfully
+      context.interrupted = true
+    }
 
     // Return an iterator that read lines from the process's stdout
     val stream = new DataInputStream(new BufferedInputStream(worker.getInputStream, bufferSize))