Make pmon:demonitor/2 respect its contract

[binarin]

Spec states that demonitor/2 should always return #state{}, but it
wasn't the case when pid wasn't found in #state.dict. This made API
unsafe to use, as it could lead to some other process storing incorrect
data as a pmon state.

This patch will prevent exceptions with stacktrace provided at the end of this message, where lists:foldl/3 was used to accumulate result of sequence of pmon:demonitor/2 applications.

Probably attempt to demonitor something that was not monitored is manifestation of another bug, but I wasn't able to pinpoint it.

** {function_clause,
       [{pmon,demonitor,
            [<23061.2976.0>,
             {dict,21,16,16,8,80,48,
                 {[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},
                 {{[[<0.9324.0>|#Ref<0.0.1.31485>]],
                   [[<0.8783.0>|#Ref<0.0.1.31479>],
                    [<0.9229.0>|#Ref<0.0.1.31483>]],
                   [],
                   [[<23061.2976.0>|{delegate_5,<23061.2976.0>}]],
                   [[<23061.9854.0>|{delegate_5,<23061.9854.0>}]],
                   [[<23061.3950.0>|{delegate_5,<23061.3950.0>}],
                    [<0.6189.0>|#Ref<0.0.0.91575>],
                    [<0.8771.0>|#Ref<0.0.1.31478>],
                    [<0.9281.0>|#Ref<0.0.1.31484>]],
                   [[<23061.11251.0>|{delegate_5,<23061.11251.0>}]],
                   [[<23062.3330.0>|{delegate_5,<23062.3330.0>}]],
                   [[<23062.3626.0>|{delegate_5,<23062.3626.0>}],
                    [<23062.3841.0>|{delegate_5,<23062.3841.0>}],
                    [<0.9220.0>|#Ref<0.0.1.31482>]],
                   [],
                   [[<0.8975.0>|#Ref<0.0.1.31480>]],
                   [[<23062.4331.0>|{delegate_5,<23062.4331.0>}]],
                   [],
                   [[<23061.4006.0>|{delegate_5,<23061.4006.0>}],
                    [<23062.2890.0>|{delegate_5,<23062.2890.0>}]],
                   [[<23061.3760.0>|{delegate_5,<23061.3760.0>}],
                    [<0.3385.0>|#Ref<0.0.0.28884>],
                    [<0.9043.0>|#Ref<0.0.1.31481>]],
                   []}}}],
            [{file,"src/pmon.erl"},{line,83}]},
        {lists,foldl,3,[{file,"lists.erl"},{line,1248}]},
        {rabbit_mirror_queue_slave,promote_me,2,
            [{file,"src/rabbit_mirror_queue_slave.erl"},{line,645}]},
        {rabbit_mirror_queue_slave,handle_call,3,
            [{file,"src/rabbit_mirror_queue_slave.erl"},{line,221}]},
        {gen_server2,handle_msg,2,[{file,"src/gen_server2.erl"},{line,1014}]},
        {proc_lib,wake_up,3,[{file,"proc_lib.erl"},{line,249}]}]}

[michaelklishin]

@binarin should this go into stable?

[binarin]

I think so, but there is no 'stable' branch in this repo yet )

[michaelklishin]

@binarin yeah, pmon is in rabbitmq-server in stable. Feel free to submit there as well.

[michaelklishin]

@binarin thank you!

[videlalvaro]

@binarin this is a great catch! It's amazing how long this bug was there actually. Thanks for debugging this out!