Add an optional init step for peer discovery mechanisms

Sometimes they need to perform some initialisation very early on,
e.g. to configure HTTP client proxy settings or similar.

Part of rabbitmq/rabbitmq-peer-discovery-common#5.

[#153615554]

Author: michaelklishin

src/rabbit_mnesia.erl

@@ -107,11 +107,13 @@ init() ->
                             "Assuming we need to join an existing cluster or initialise from scratch...~n",
                             [dir()]),
             rabbit_peer_discovery:log_configured_backend(),
+            rabbit_peer_discovery:maybe_init(),
             init_with_lock();
         false ->
             NodeType = node_type(),
             init_db_and_upgrade(cluster_nodes(all), NodeType,
                                 NodeType =:= ram, _Retry = true),
+            rabbit_peer_discovery:maybe_init(),
             rabbit_peer_discovery:maybe_register()
     end,
     %% We intuitively expect the global name server to be synced when

src/rabbit_peer_discovery.erl

@@ -20,7 +20,7 @@
 %% API
 %%
 
--export([discover_cluster_nodes/0, backend/0, node_type/0,
+-export([maybe_init/0, discover_cluster_nodes/0, backend/0, node_type/0,
          normalize/1, format_discovered_nodes/1, log_configured_backend/0,
          register/0, unregister/0, maybe_register/0, maybe_unregister/0,
          maybe_inject_randomized_delay/0, lock/0, unlock/1]).
@@ -88,6 +88,22 @@ lock_acquisition_failure_mode() ->
 log_configured_backend() ->
   rabbit_log:info("Configured peer discovery backend: ~s~n", [backend()]).
 
+maybe_init() ->
+    Backend = backend(),
+    case erlang:function_exported(Backend, init, 0) of
+        true  ->
+            rabbit_log:debug("Peer discovery backend supports initialisation."),
+            case Backend:init() of
+                ok ->
+                    rabbit_log:debug("Peer discovery backend initialisation succeeded."),
+                    ok;
+                {error, Error} ->
+                    rabbit_log:warn("Peer discovery backend initialisation failed: ~p.", [Error]),
+                    ok
+            end;
+        false -> ok
+    end.
+
 
 -spec discover_cluster_nodes() -> {ok, Nodes :: list()} |
                                   {ok, {Nodes :: list(), NodeType :: rabbit_types:node_type()}} |