rabbit: Fix plugins' run_boot_steps() vs. start order

Before `rabbit` startup code was rewritten as part of
rabbitmq/rabbitmq-server#2180 to make it closer to a regular Erlang
application, plugins' boot steps were executed before plugins were
started.

This commit restores this behavior. Indeed the initial patch inverted
them by starting the plugins first, then executed the boot steps.

It also brings another improvement in the process: a dependency has its
boot steps executed and is started before a plugin which depends on it
is considered. This should improve consistency.

Note that the `start_apps/2` function, which is run when a user enables
a plugin at runtime must be improved as well. There is a work in
progress in rabbitmq/rabbitmq-server#2219.

1 files changed, 14 insertions, 4 deletions

diff --git a/src/rabbit.erl b/src/rabbit.erl
index 7f16d0b1d1..292272738d 100644
--- a/src/rabbit.erl
+++ b/src/rabbit.erl
@@ -983,18 +983,28 @@ do_run_postlaunch_phase() ->
         rabbit_log_prelaunch:debug("== Plugins =="),
 
         rabbit_log_prelaunch:debug("Setting plugins up"),
+        %% `Plugins` contains all the enabled plugins, plus their
+        %% dependencies. The order is important: dependencies appear
+        %% before plugin which depend on them.
         Plugins = rabbit_plugins:setup(),
         rabbit_log_prelaunch:debug(
           "Starting the following plugins: ~p", [Plugins]),
+        %% We can load all plugins and refresh their feature flags at
+        %% once, because it does not involve running code from the
+        %% plugins.
         app_utils:load_applications(Plugins),
         ok = rabbit_feature_flags:refresh_feature_flags_after_app_load(
                Plugins),
+        %% However, we want to run their boot steps and actually start
+        %% them one by one, to ensure a dependency is fully started
+        %% before a plugin which depends on it gets a chance to start.
         lists:foreach(
           fun(Plugin) ->
-              case application:ensure_all_started(Plugin) of
-                  {ok, _} -> rabbit_boot_steps:run_boot_steps([Plugin]);
-                  Error   -> throw(Error)
-              end
+                  ok = rabbit_boot_steps:run_boot_steps([Plugin]),
+                  case application:ensure_all_started(Plugin) of
+                      {ok, _} -> ok;
+                      Error   -> throw(Error)
+                  end
           end, Plugins),
 
         rabbit_log_prelaunch:debug("Marking RabbitMQ as running"),