Prepare for merge

[larshesel]

A few of the pre-merge items to cross off of the list...

[larshesel]

Hey @dergraf , this is interesting. Notice a42f86c and that I reverted it in 8f725d3. Dialyzer correctly complains:

apps/vmq_server/src/vmq_metadata.erl
  32: Call to missing or unexported function vmq_plugin_mgr:disable_plugin/2

As there is no arity two disable_plugin function in vmq_plugin_mgr. But with the fix a test like

rebar3 ct --suite ./apps/vmq_server/test/vmq_clean_session_SUITE.erl --group mqttv4 --case clean_session_qos1_test

Just hangs in the suite teardown which calls vmq_server:stop() - which ends up calling vmq_plugin_mgr:disable_plugin. The test passes when the undefined function is used, but hangs when the correct one is used... got an idea what the problem could be? I reverted the commit with the fix to be able to run the test suites for now

[larshesel]

Hey @dergraf I'm done with this - please review. I investigated the dialyzer error about vmq_plugin_mgr:disable_plugin/2 when it's fixed, vmq_plugin_mgr:disable_plugin(vmq_plumtree) simply doesn't return. Maybe you could take a look at that?

[dergraf]

Analyzed the case. So the root of the problem is a deadlock in the OTP application controller when calling vmq_server:stop().

vmq_server:stop(
    application:stop(vmq_server,
        vmq_server_app:stop(
             vmq_metadata:stop(
                  vmq_plugin_mgr:stop(vmq_plumtree
                      application:stop(vmq_plumtree ===>>>BOOM!

As application:stop is a blocking call, we can't stop another application in the terminate path of an application.

Simple fix would be to run vmq_metadata:stop inside a own process.

[dergraf]

I've a PR for the issue above in #829
So I'd say revert the related changes here... and merge this PR.