Future blocks on a shutdown ExectionContext

[da-liii]

Found this bug working on apache/spark#22304

This is the simplified code to reproduce the bug:

https://github.com/sadhen/blocking-future

This bug exists in 2.12.6and 2.13.0-M4. Fixed in 2.13.0-M5.

[SethTisue]

thanks for the report. I see you asked over at scala/scala#6610 whether there's any possibility of a fix happening in the 2.12.x series. (I don't know the answer.)