This looks to be causing a regression for me when running locally:
I now see this error:
16:50:19 INFO - mozproxy Starting mitmproxy playback using command: /Users/acreskey/dev/src/build/obj-release/testing/mozproxy/mitmdump -k -s ""/Users/acreskey/dev/src/mozilla-central/testing/raptor/raptor/playback/alternate-server-replay.py /Users/acreskey/dev/src/build/obj-release/testing/mozproxy/amazon.mp""
16:50:20 INFO - mozproxy Aborting: mitmproxy playback process failed to work
16:50:20 INFO - mozproxy Stopping mitmproxy playback, killing process 1852
16:50:20 INFO - mozproxy Mitmproxy exited with error code -15
But if I run the given playback command from the terminal, mitmproxy starts up:
~/dev/src/mozilla-central$ /Users/acreskey/dev/src/build/obj-release/testing/mozproxy/mitmdump -k -s ""/Users/acreskey/dev/src/mozilla-central/testing/raptor/raptor/playback/alternate-server-replay.py /Users/acreskey/dev/src/build/obj-release/testg/mozproxy/amazon.mp""
Loading script: /Users/acreskey/dev/src/mozilla-central/testing/raptor/raptor/playback/alternate-server-replay.py
Replaying from files: 
Proxy server listening at http://0.0.0.0:8080
I'm on a MacbookPro. I did make sure that there was no other instance of mitmdump running.
I see this too when trying to run Raptor tests. I'm not able to get any test running locally. I will have a look into that.