never invalidate a queue with None
never invalidate a queue with None

I saw this *extremely* intermittently (and wasn't able to reproduce it at all),
but I think there was a race when invalidating an output queue in the abort()
method of PipelineThread. It was possible for a thread's input queue to be
invalidated (with val=None) while abort_flag was False (and abort_lock was not
held). This would cause coroutines to see a None value when the pipeline was
shutting down. Always poisoning a queue when it's invalidated should solve this
by making the thread break before sending the value to the coroutine.

jcoady9/beets,krig/beets,Freso/beets,jbaiter/beets,Dishwishy/beets,marcuskrahl/beets,madmouser1/beets,tima/beets,mathstuf/beets,mathstuf/beets,lengtche/beets,jbaiter/beets,xsteadfastx/beets,andremiller/beets,shanemikel/beets,lightwang1/beets,sampsyo/beets,pkess/beets,ibmibmibm/beets,ruippeixotog/beets,arabenjamin/beets,untitaker/beets,imsparsh/beets,Kraymer/beets,tima/beets,Kraymer/beets,shanemikel/beets,andremiller/beets,jackwilsdon/beets,randybias/beets,shamangeorge/beets,jmwatte/beets,kelvinhammond/beets,PierreRust/beets,pkess/beets,xsteadfastx/beets,Andypsamp/CODfinalJUNIT,Dishwishy/beets,Wen777/beets,sampsyo/beets,parapente/beets,beetbox/beets,drm00/beets,swt30/beets,shamangeorge/beets,lightwang1/beets,gabrielaraujof/beets,ibmibmibm/beets,jayme-github/beets,untitaker/beets,Freso/beets,kareemallen/beets,asteven/beets,shanemikel/beets,PierreRust/beets,Dishwishy/beets,Andypsamp/CODfinalJUNIT,gabrielaraujof/beets,andremiller/beets,bj-yinyan/beets,Andypsamp/CODfinalJUNIT,YetAnotherNerd/beets,mosesfistos1/beetbox,aspidites/beets,moodboom/beets,moodboom/beets,beetbox/beets,Kraymer/beets,SusannaMaria/beets,aspidites/beets,drm00/beets,ibmibmibm/beets,lengtche/beets,shanemikel/beets,YetAnotherNerd/beets,randybias/beets,MyTunesFreeMusic/privacy-policy,parapente/beets,m-urban/beets,dfc/beets,diego-plan9/beets,swt30/beets,mclearc/beets,randybias/beets,shamangeorge/beets,Dishwishy/beets,jcoady9/beets,LordSputnik/beets,Andypsamp/CODjunit,madmouser1/beets,imsparsh/beets,ruippeixotog/beets,moodboom/beets,pkess/beets,arabenjamin/beets,mclearc/beets,gabrielaraujof/beets,asteven/beets,MyTunesFreeMusic/privacy-policy,gabrielaraujof/beets,xsteadfastx/beets,multikatt/beets,beetbox/beets,ngokevin/beets,pdf/beets,dfc/beets,LordSputnik/beets,tima/beets,mclearc/beets,xsteadfastx/beets,lengtche/beets,Andypsamp/CODjunit,m-urban/beets,sampsyo/beets,kelvinhammond/beets,lightwang1/beets,m-urban/beets,marcuskrahl/beets,marcuskrahl/beets,jayme-github/beets,Andypsamp/CODjunit,Freso/beets,diego-plan9/beets,diego-plan9/beets,jcoady9/beets,artemutin/beets,jackwilsdon/beets,dfc/beets,LordSputnik/beets,kareemallen/beets,bj-yinyan/beets,bj-yinyan/beets,Andypsamp/CODjunit,marcuskrahl/beets,ibmibmibm/beets,imsparsh/beets,ttsda/beets,madmouser1/beets,Andypsamp/CODfinalJUNIT,sadatay/beets,Andypsamp/CODfinalJUNIT,YetAnotherNerd/beets,YetAnotherNerd/beets,mathstuf/beets,madmouser1/beets,Wen777/beets,ruippeixotog/beets,jmwatte/beets,MyTunesFreeMusic/privacy-policy,asteven/beets,mried/beets,artemutin/beets,multikatt/beets,jcoady9/beets,mosesfistos1/beetbox,krig/beets,multikatt/beets,artemutin/beets,mried/beets,jackwilsdon/beets,jackwilsdon/beets,asteven/beets,mathstuf/beets,mried/beets,pdf/beets,arabenjamin/beets,drm00/beets,SusannaMaria/beets,parapente/beets,Andypsamp/CODjunit,untitaker/beets,SusannaMaria/beets,drm00/beets,imsparsh/beets,sampsyo/beets,pkess/beets,SusannaMaria/beets,untitaker/beets,tima/beets,jmwatte/beets,ngokevin/beets,ruippeixotog/beets,mried/beets,beetbox/beets,sadatay/beets,swt30/beets,shamangeorge/beets,kelvinhammond/beets,Kraymer/beets,moodboom/beets,sadatay/beets,sadatay/beets,ttsda/beets,mosesfistos1/beetbox,kelvinhammond/beets,swt30/beets,m-urban/beets,Freso/beets,Wen777/beets,arabenjamin/beets,multikatt/beets,krig/beets,bj-yinyan/beets,randybias/beets,parapente/beets,ttsda/beets,dfc/beets,PierreRust/beets,kareemallen/beets,lengtche/beets,kareemallen/beets,jmwatte/beets,diego-plan9/beets,PierreRust/beets,artemutin/beets,mosesfistos1/beetbox,ttsda/beets,lightwang1/beets,MyTunesFreeMusic/privacy-policy,LordSputnik/beets