Only start syslog application if it is required

[lukebakken]

Fixes #1718

Test using the following rabbitmq.config file:

%% {rfc5424, udp, [{ip, {127,0,0,1}}]}
%% {rfc5424, udp}
[
    {syslog, [
        {dest_port, 5140},
        {dest_host, "127.0.0.1"},
        {protocol,
            {rfc5424, udp, [{ip, {192,168,1,5}}]}
        }
    ]},
    {rabbit, [
        {log, [
            {syslog, [
                {level, info},
                {enabled, true}
            ]},                                                                                                                                                                                                                                                                 
            {console, [                                                                                                                                                                                                                                                         
                {level, info},                                                                                                                                                                                                                                                  
                {enabled, true}                                                                                                                                                                                                                                                 
            ]}                                                                                                                                                                                                                                                                  
        ]},                                                                                                                                                                                                                                                                     
        {loopback_users,[]}                                                                                                                                                                                                                                                     
    ]}                                                                                                                                                                                                                                                                          
].

You can experiment by using different values in protocol, or by leaving that term out completely. If you leave out ip, 127.0.0.1 will be used. Use netstat to confirm:

netstat -pan | fgrep beam

You will see a line beginning with udp and it should be bound to the local address you expect.

If you leave out all syslog configuration, you will not see beam.smp listening on any udp ports.

[michaelklishin]

This seems to work as advertised w.r.t. UDP listeners.

However, I get extra standard output (error?) log messages from error logger with this PR. On master:

(rabbit@warp10)1>
  ##  ##
  ##  ##      RabbitMQ x.y.z.g1be4734. Copyright (C) 2007-2018 Pivotal Software, Inc.
  ##########  Licensed under the MPL.  See http://www.rabbitmq.com/
  ######  ##
  ##########  Logs: /var/folders/gp/53t98z011678vk9rkcb_s6ph0000gn/T/rabbitmq-test-instances/rabbit/log/rabbit.log
                    /var/folders/gp/53t98z011678vk9rkcb_s6ph0000gn/T/rabbitmq-test-instances/rabbit/log/rabbit_upgrade.log

              Starting broker...
 completed with 0 plugins.

On rabbitmq-server-1718-syslog-localhost:

(rabbit@warp10)1>
  ##  ##
  ##  ##      RabbitMQ x.y.z.gc44722a. Copyright (C) 2007-2018 Pivotal Software, Inc.
  ##########  Licensed under the MPL.  See http://www.rabbitmq.com/
  ######  ##
  ##########  Logs: /var/folders/gp/53t98z011678vk9rkcb_s6ph0000gn/T/rabbitmq-test-instances/rabbit/log/rabbit.log
                    /var/folders/gp/53t98z011678vk9rkcb_s6ph0000gn/T/rabbitmq-test-instances/rabbit/log/rabbit_upgrade.log

              Starting broker...
=INFO REPORT==== 12-Oct-2018::06:29:49.425017 ===
Limiting to approx 7068 file handles (6359 sockets)

=INFO REPORT==== 12-Oct-2018::06:29:49.545610 ===
started TCP listener on [::]:5672

Note that the above happens without any configuration file (e.g. gmake run-broker RABBITMQ_CONFIG_FILE=/tmp/non-existent), so nothing instructs RabbitMQ to log to console.

From rabbitmq environment:

{log,
    [{file,
         [{file,
              "/var/folders/gp/53t98z011678vk9rkcb_s6ph0000gn/T/rabbitmq-test-instances/rabbit/log/rabbit.log"}]},
     {categories,
         [{upgrade,
              [{file,
                   "/var/folders/gp/53t98z011678vk9rkcb_s6ph0000gn/T/rabbitmq-test-instances/rabbit/log/rabbit_upgrade.log"}]}]}]}

[michaelklishin]

@lukebakken I merged master into this branch so cherry-picking the merge commit to v3.7.x might introduce a 3.8.0-specific change 😓. Sorry. Cherry picking individual commits should be safe.

[lukebakken]

@michaelklishin that output is probably due to lager starting at a different time than prior to this change. Let me see if I can figure it out.

[lukebakken]

@michaelklishin FYI I undid the merge commit you mention in this comment

[lukebakken]

@michaelklishin ready for round #2. This issue pointed me in the right direction: erlang-lager/lager#481