RABBITMQ_ERLANG_COOKIE environment variable takes precedence over .erlang.cookie file

If RABBITMQ_ERLANG_COOKIE environment variable is set, the CLI commands will use this value instead of the .erlang.cookie file value.

If these 2 values do not match, the CLI commands will fail to communicate with the rabbit node. This has been mentioned in https://github.com/rabbitmq/rabbitmq-cli/issues/252 before, and has a relevant rabbitmq-users thread.

To reproduce:

Start RabbitMQ in development mode

$ pwd
/Users/gerhard/github.com/rabbitmq/3.9.x/deps/rabbit
$ make run-broker
...

rabbitmqctl prefers the env var and fails to cluster with rabbit node

$ RABBITMQ_ERLANG_COOKIE=foo sbin/rabbitmqctl eval 'rabbit:status().'
Error: unable to perform an operation on node 'rabbit@focker'. Please see diagnostics information and suggestions below.

Most common reasons for this are:

 * Target node is unreachable (e.g. due to hostname resolution, TCP connection or firewall issues)
 * CLI tool fails to authenticate with the server (e.g. due to CLI tool's Erlang cookie not matching that of the server)
 * Target node is not running

In addition to the diagnostics info below:

 * See the CLI, clustering and networking guides on https://rabbitmq.com/documentation.html to learn more
 * Consult server logs on node rabbit@focker
 * If target node is configured to use long node names, don't forget to use --longnames with CLI tools

DIAGNOSTICS
===========

attempted to contact: [rabbit@focker]

rabbit@focker:
  * connected to epmd (port 4369) on focker
  * epmd reports node 'rabbit' uses port 25672 for inter-node and CLI tool traffic
  * TCP connection succeeded but Erlang distribution failed
  * suggestion: check if the Erlang cookie identical for all server nodes and CLI tools
  * suggestion: check if all server nodes and CLI tools use consistent hostnames when addressing each other
  * suggestion: check if inter-node connections may be configured to use TLS. If so, all nodes and CLI tools must do that
   * suggestion: see the CLI, clustering and networking guides on https://rabbitmq.com/documentation.html to learn more

Current node details:
 * node name: 'rabbitmqcli-49409-rabbit@focker'
 * effective user's home directory: /Users/gerhard
 * Erlang cookie hash: rL0Y20zC+Fzt72VPzMSk2A==

The erl equivalent ignores the RABBITMQ_ERLANG_COOKIE env var and works as expected

$ RABBITMQ_ERLANG_COOKIE=foo erl -sname rabbitmqcli-49409-rabbit -eval 'io:format("~p~n", [rpc:call(rabbit@focker, rabbit, status, [])]), init:stop().'
Erlang/OTP 23 [erts-11.0.2] [source] [64-bit] [smp:20:20] [ds:20:20:10] [async-threads:1] [hipe] [dtrace]

Eshell V11.0.2  (abort with ^G)
(rabbitmqcli-49409-rabbit@focker)1> [{pid,48497},
 {running_applications,
     [{rabbit,"RabbitMQ","3.8.0+rc.1.349.g167c376"},
      {mnesia,"MNESIA  CXC 138 12","4.17"},
      {lager,"Erlang logging framework","3.8.0"},
...

The proposal is to deprecate RABBITMQ_ERLANG_COOKIE in RabbitMQ release series 3.8, and remove in 3.9

As a better alternative, --set-cookie flag should be added to address the use-cases where RABBITMQ_ERLANG_COOKIE was needed (see #180 & the related #182). This flag is similar to erl's -setcookie

As discussed with @michaelklishin & @dumbbell

re https://github.com/docker-library/rabbitmq/pull/422#issuecomment-650074731

rabbitmq / rabbitmq-cli

RABBITMQ_ERLANG_COOKIE environment variable takes precedence over .erlang.cookie file #443