SPARK-11286 Make Outbox stopped exception singleton

[tedyu]

In two places in Outbox.scala , new SparkException is created for Outbox stopped condition.
Create a singleton for Outbox stopped exception and use it instead of creating exception every time.

[rxin]

Ted - why is this change necessary? These only happen in error cases so I'm not sure how useful it is to optimize then. On top of that, this seems to change the stacktrace coming from the exceptions.

[vanzin]

Yeah, please don't do this. Broken stack traces are terrible.

[tedyu]

Exception creation can be done here instead of inside the loop, keeping usable stack trace

[vanzin]

But why? What's being optimized here? It's such a rare occurrence that it's better to pay the allocation cost when it happens than waste the memory to keep the singleton around.

[tedyu]

In that case the exception is no longer a singleton - it is created inside handleNetworkFailure().
Network failure should be rare but when it happens, there may be non-trivial number of messages to be notified.

[vanzin]

I doubt there will be many messages in the outbox at a time. This code mostly fixes a race during initialization of the RPC channel, later the outbox would be mostly a bypass. #9210 even bypasses the outbox altogether in a lot of cases.

[tedyu]

Thanks for providing richer background information.

[SparkQA]

Test build #44230 has finished for PR 9254 at commit 79c6e00.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * case class Data(feature: Int)\n