[SPARK-4301] StreamingContext should not allow start() to be called after calling stop()

In Spark 1.0.0+, calling `stop()` on a StreamingContext that has not been started is a no-op which has no side-effects. This allows users to call `stop()` on a fresh StreamingContext followed by `start()`. I believe that this almost always indicates an error and is not behavior that we should support. Since we don't allow `start() stop() start()` then I don't think it makes sense to allow `stop() start()`.

The current behavior can lead to resource leaks when StreamingContext constructs its own SparkContext: if I call `stop(stopSparkContext=True)`, then I expect StreamingContext's underlying SparkContext to be stopped irrespective of whether the StreamingContext has been started. This is useful when writing unit test fixtures.

Prior discussions:
- #3053 (diff)
- #3121 (comment)

Author: Josh Rosen <[email protected]>

Closes #3160 from JoshRosen/SPARK-4301 and squashes the following commits:

dbcc929 [Josh Rosen] Address more review comments
bdbe5da [Josh Rosen] Stop SparkContext after stopping scheduler, not before.
03e9c40 [Josh Rosen] Always stop SparkContext, even if stop(false) has already been called.
832a7f4 [Josh Rosen] Address review comment
5142517 [Josh Rosen] Add tests; improve Scaladoc.
813e471 [Josh Rosen] Revert workaround added in https://github.com/apache/spark/pull/3053/files#diff-e144dbee130ed84f9465853ddce65f8eR49
5558e70 [Josh Rosen] StreamingContext.stop() should stop SparkContext even if StreamingContext has not been started yet.

(cherry picked from commit 7b41b17)
Signed-off-by: Tathagata Das <[email protected]>

Author: JoshRosen

streaming/src/main/scala/org/apache/spark/streaming/StreamingContext.scala

@@ -436,10 +436,10 @@ class StreamingContext private[streaming] (
 
   /**
    * Start the execution of the streams.
+   *
+   * @throws SparkException if the context has already been started or stopped.
    */
   def start(): Unit = synchronized {
-    // Throw exception if the context has already been started once
-    // or if a stopped context is being started again
     if (state == Started) {
       throw new SparkException("StreamingContext has already been started")
     }
@@ -472,8 +472,10 @@ class StreamingContext private[streaming] (
   /**
    * Stop the execution of the streams immediately (does not wait for all received data
    * to be processed).
-   * @param stopSparkContext Stop the associated SparkContext or not
    *
+   * @param stopSparkContext if true, stops the associated SparkContext. The underlying SparkContext
+   *                         will be stopped regardless of whether this StreamingContext has been
+   *                         started.
    */
   def stop(stopSparkContext: Boolean = true): Unit = synchronized {
     stop(stopSparkContext, false)
@@ -482,25 +484,27 @@ class StreamingContext private[streaming] (
   /**
    * Stop the execution of the streams, with option of ensuring all received data
    * has been processed.
-   * @param stopSparkContext Stop the associated SparkContext or not
-   * @param stopGracefully Stop gracefully by waiting for the processing of all
+   *
+   * @param stopSparkContext if true, stops the associated SparkContext. The underlying SparkContext
+   *                         will be stopped regardless of whether this StreamingContext has been
+   *                         started.
+   * @param stopGracefully if true, stops gracefully by waiting for the processing of all
    *                       received data to be completed
    */
   def stop(stopSparkContext: Boolean, stopGracefully: Boolean): Unit = synchronized {
-    // Warn (but not fail) if context is stopped twice,
-    // or context is stopped before starting
-    if (state == Initialized) {
-      logWarning("StreamingContext has not been started yet")
-      return
+    state match {
+      case Initialized => logWarning("StreamingContext has not been started yet")
+      case Stopped => logWarning("StreamingContext has already been stopped")
+      case Started =>
+        scheduler.stop(stopGracefully)
+        logInfo("StreamingContext stopped successfully")
+        waiter.notifyStop()
     }
-    if (state == Stopped) {
-      logWarning("StreamingContext has already been stopped")
-      return
-    } // no need to throw an exception as its okay to stop twice
-    scheduler.stop(stopGracefully)
-    logInfo("StreamingContext stopped successfully")
-    waiter.notifyStop()
+    // Even if the streaming context has not been started, we still need to stop the SparkContext.
+    // Even if we have already stopped, we still need to attempt to stop the SparkContext because
+    // a user might stop(stopSparkContext = false) and then call stop(stopSparkContext = true).
     if (stopSparkContext) sc.stop()
+    // The state should always be Stopped after calling `stop()`, even if we haven't started yet:
     state = Stopped
   }
 }

streaming/src/test/scala/org/apache/spark/streaming/StreamingContextSuite.scala

@@ -46,10 +46,6 @@ class StreamingContextSuite extends FunSuite with BeforeAndAfter with Timeouts w
   after {
     if (ssc != null) {
       ssc.stop()
-      if (ssc.sc != null) {
-        // Calling ssc.stop() does not always stop the associated SparkContext.
-        ssc.sc.stop()
-      }
       ssc = null
     }
     if (sc != null) {
@@ -137,11 +133,16 @@ class StreamingContextSuite extends FunSuite with BeforeAndAfter with Timeouts w
     ssc.stop()
   }
 
-  test("stop before start and start after stop") {
+  test("stop before start") {
     ssc = new StreamingContext(master, appName, batchDuration)
     addInputStream(ssc).register()
     ssc.stop()  // stop before start should not throw exception
-    ssc.start()
+  }
+
+  test("start after stop") {
+    // Regression test for SPARK-4301
+    ssc = new StreamingContext(master, appName, batchDuration)
+    addInputStream(ssc).register()
     ssc.stop()
     intercept[SparkException] {
       ssc.start() // start after stop should throw exception
@@ -161,6 +162,18 @@ class StreamingContextSuite extends FunSuite with BeforeAndAfter with Timeouts w
     ssc.stop()
   }
 
+  test("stop(stopSparkContext=true) after stop(stopSparkContext=false)") {
+    ssc = new StreamingContext(master, appName, batchDuration)
+    addInputStream(ssc).register()
+    ssc.stop(stopSparkContext = false)
+    assert(ssc.sc.makeRDD(1 to 100).collect().size === 100)
+    ssc.stop(stopSparkContext = true)
+    // Check that the SparkContext is actually stopped:
+    intercept[Exception] {
+      ssc.sc.makeRDD(1 to 100).collect()
+    }
+  }
+
   test("stop gracefully") {
     val conf = new SparkConf().setMaster(master).setAppName(appName)
     conf.set("spark.cleaner.ttl", "3600")