Deprecated features: New module to manage deprecated features (!)

This introduces a way to declare deprecated features in the code, not
only in our communication. The new module allows to disallow the use of
a deprecated feature and/or warn the user when he relies on such a
feature.

[Why]
Currently, we only tell people about deprecated features through blog
posts and the mailing-list. This might be insufficiant for our users
that a feature they use will be removed in a future version:
* They may not read our blog or mailing-list
* They may not understand that they use such a deprecated feature
* They might wait for the big removal before they plan testing
* They might not take it seriously enough

The idea behind this patch is to increase the chance that users notice
that they are using something which is about to be dropped from
RabbitMQ. Anopther benefit is that they should be able to test how
RabbitMQ will behave in the future before the actual removal. This
should allow them to test and plan changes.

[How]
When a feature is deprecated in other large projects (such as FreeBSD
where I took the idea from), it goes through a lifecycle:
1. The feature is still available, but users get a warning somehow when
   they use it. They can disable it to test.
2. The feature is still available, but disabled out-of-the-box. Users
   can re-enable it (and get a warning).
3. The feature is disconnected from the build. Therefore, the code
   behind it is still there, but users have to recompile the thing to be
   able to use it.
4. The feature is removed from the source code. Users have to adapt or
   they can't upgrade anymore.

The solution in this patch offers the same lifecycle. A deprecated
feature will be in one of these deprecation phases:
1. `optout`: The feature is available. Users get a warning if they use
   it. They can disable it from the configuration.
2. `optin`: The feature is available but disabled by default. Users get
   an error if they use it and RabbitMQ behaves like the feature is
   removed. They can re-enable is from the configuration and get a
   warning.
3. `disconnected`: The feature is present in the source code, but is
   disabled and can't be re-enabled without recompiling RabbitMQ. Users
   get the same behavior as if the code was removed.
4. `removed`: The feature's code is gone.

The whole thing is based on the feature flags subsystem, but it has the
following differences with other feature flags:
* The semantic is reversed: the feature flag behind a deprecated feature
  is disabled when the deprecated feature is permitted, or enabled when
  the deprecated feature is denied.
* The feature flag behind a deprecated feature is enabled out-of-the-box
  (meaning the deprecated feature is denied):
    * if the deprecation phase is `optout` and the configuration denies
      the deprecated feature
    * if the deprecation phase is `optin` and the configuration doesn't
      permit the deprecated feature
    * if the deprecation phase is `disconnected` or `removed`
* Feature flags behind deprecated feature don't appear in feature flags
  listings.

Otherwise, deprecated features' feature flags are managed like other
feature flags, in particular inside clusters.

To declare a deprecated feature:

    -rabbit_deprecated_feature(
       {my_deprecated_feature,
        #{deprecation_phase => optout,
          warning => "This feature will be removed in RabbitMQ X.0"
         }}).

Then, to check the state of a deprecated feature in the code:

    case rabbit_deprecated_features:is_permitted(my_deprecated_feature) of
        true ->
            %% The deprecated feature is still permitted.
            ok;
        false ->
            %% The deprecated feature is gone or should be considered
            %% unavailable.
            error
    end.

Warnings and errors are logged automatically. A message is generated
automatically, but it is possible to define a message in the deprecated
feature flag declaration like in the example above.

Here is an example of a logged warning that was generated automatically:

    2023-03-01 12:23:29.627795+01:00 [warning] <12086.105.0> Deprecated features: `my_deprecated_feature`: Feature `my_deprecated_feature` is deprecated; it is enabled by default, except if configured otherwise

To override the default state of `optout` and `optin` deprecation
phases, users can set the following configuration:

    # In rabbitmq.conf:
    permit_deprecated_features.my_deprecated_feature = true # or false

The actual behavior protected by a deprecated feature check is out of
scope for this subsystem. It is the repsonsibility of each deprecated
feature code to determine what to do when the deprecated feature is
denied.

Author: dumbbell

deps/rabbit/BUILD.bazel

@@ -379,6 +379,14 @@ rabbitmq_integration_suite(
     size = "medium",
 )
 
+rabbitmq_integration_suite(
+    name = "deprecated_features_SUITE",
+    size = "medium",
+    additional_beam = [
+        ":feature_flags_v2_SUITE_beam_files",
+    ],
+)
+
 rabbitmq_integration_suite(
     name = "disconnect_detected_during_alarm_SUITE",
     size = "medium",

deps/rabbit/Makefile

@@ -241,7 +241,7 @@ ct-slow: CT_SUITES = $(SLOW_CT_SUITES)
 # --------------------------------------------------------------------
 
 RMQ_ERLC_OPTS += -I $(DEPS_DIR)/rabbit_common/include
-EDOC_OPTS += {preprocess,true}
+EDOC_OPTS += {preprocess,true},{includes,["."]}
 
 ifdef INSTRUMENT_FOR_QC
 RMQ_ERLC_OPTS += -DINSTR_MOD=gm_qc

deps/rabbit/docs/rabbitmq.conf.example

@@ -548,6 +548,25 @@
 ## NB: Change these only if you understand what you are doing!
 ##
 
+## To permit or deny a deprecated feature when it is in its `optout` or
+## `optin` deprecation phase, the default state can be overriden from the
+## configuration.
+##
+## When a deprecated feature is "opt-out" (first phase of the deprecation
+## period), it means the feature is available by default and can be turned off by
+## setting it to false in the configuration.
+##
+## When a deprecated feature is "opt-in" (second phase of the deprecation
+## period), it menas the feature is unavailable by default but can be turned back
+## on by setting it to true in the configuration.
+##
+## When a deprecated feature is "disconnected" or "removed" (last two phases of
+## the deprecation period), it is no longer possible to turn it back on from the
+## configuration.
+##
+# permit_deprecated_features.a_deprecated_feature = true
+# permit_deprecated_features.another_deprecated_feature = false
+
 ## Timeout used when waiting for Mnesia tables in a cluster to
 ## become available.
 ##

deps/rabbit/priv/schema/rabbit.schema

@@ -2088,6 +2088,31 @@ end}.
     {datatype, integer}
 ]}.
 
+%%
+%% Feature flags and deprecated features
+%% =====================================
+%%
+
+{mapping,
+ "permit_deprecated_features.$name", "rabbit.permit_deprecated_features",
+ [{datatype, {enum, [true, false]}}]
+}.
+
+%% This converts:
+%%     permit_deprecated_features.my_feature = true
+%% to:
+%%     {rabbit, [{permit_deprecated_features, #{my_feature => true}}]}.
+
+{translation, "rabbit.permit_deprecated_features",
+ fun(Conf) ->
+         Settings = cuttlefish_variable:filter_by_prefix(
+                      "permit_deprecated_features", Conf),
+         maps:from_list(
+           [{list_to_atom(FeatureName), State}
+            || {["permit_deprecated_features", FeatureName], State}
+               <- Settings])
+ end}.
+
 % ==========================
 % Kernel section
 % ==========================