Introduce a target cluster size hint settingmk-cluster-formation-target-cluster-size-hint

This is meant to be used by deployment tools,
core features and plugins
that expect a certain minimum
number of cluster nodes
to be present.

For example, certain setup steps
in distributed plugins might require
at least three nodes to be available.

This is just a hint, not an enforced
requirement. The default value is 1
so that for single node clusters,
there would be no behavior changes.

3 files changed, 47 insertions, 7 deletions

diff --git a/deps/rabbit/priv/schema/rabbit.schema b/deps/rabbit/priv/schema/rabbit.schema
index 29e9ce6a33..c850fa8a69 100644
--- a/deps/rabbit/priv/schema/rabbit.schema
+++ b/deps/rabbit/priv/schema/rabbit.schema
@@ -1124,6 +1124,18 @@ end}.
      {validators, ["non_zero_positive_integer"]}
     ]}.
 
+
+%% Target cluster size hint may be used by certain core features or plugins to perform
+%% actions that should only be performed when a certain number of nodes (or a quorum of a certain number)
+%% has already joined (started).
+%%
+
+{mapping, "cluster_formation.target_cluster_size_hint", "rabbit.cluster_formation.target_cluster_size_hint", [
+    {datatype, integer},
+    {validators, ["non_zero_positive_integer"]}
+]}.
+
+
 %% Classic config-driven peer discovery backend.
 %%
 %% Make clustering happen *automatically* at startup - only applied
diff --git a/deps/rabbit/src/rabbit_nodes.erl b/deps/rabbit/src/rabbit_nodes.erl
index eecf04f13e..092e8dc912 100644
--- a/deps/rabbit/src/rabbit_nodes.erl
+++ b/deps/rabbit/src/rabbit_nodes.erl
@@ -14,7 +14,7 @@
          await_running_count/2, is_single_node_cluster/0,
          boot/0]).
 -export([persistent_cluster_id/0, seed_internal_cluster_id/0, seed_user_provided_cluster_name/0]).
--export([all/0, all_running_with_hashes/0]).
+-export([all/0, all_running_with_hashes/0, target_cluster_size_hint/0, reached_target_cluster_size/0]).
 -export([lock_id/1, lock_retries/0]).
 
 -include_lib("kernel/include/inet.hrl").
@@ -30,6 +30,8 @@
 % 80 corresponds to a timeout of ca 300 seconds.
 -define(DEFAULT_LOCK_RETRIES, 80).
 
+-define(DEFAULT_TARGET_CLUSTER_SIZE, 1).
+
 %%----------------------------------------------------------------------------
 %% API
 %%----------------------------------------------------------------------------
@@ -173,15 +175,32 @@ await_running_count_with_retries(TargetCount, Retries) ->
 all_running_with_hashes() ->
     maps:from_list([{erlang:phash2(Node), Node} || Node <- all_running()]).
 
+-spec target_cluster_size_hint() -> non_neg_integer().
+target_cluster_size_hint() ->
+    cluster_formation_key_or_default(target_cluster_size_hint, ?DEFAULT_TARGET_CLUSTER_SIZE).
+
+-spec reached_target_cluster_size() -> boolean().
+reached_target_cluster_size() ->
+    running_count() >= target_cluster_size_hint().
+
+
 -spec lock_id(Node :: node()) -> {ResourceId :: string(), LockRequesterId :: node()}.
 lock_id(Node) ->
   {cookie_hash(), Node}.
 
 -spec lock_retries() -> integer().
 lock_retries() ->
-  case application:get_env(rabbit, cluster_formation) of
-      {ok, PropList} ->
-        proplists:get_value(internal_lock_retries, PropList, ?DEFAULT_LOCK_RETRIES);
-      undefined ->
-          ?DEFAULT_LOCK_RETRIES
-  end.
+    cluster_formation_key_or_default(internal_lock_retries, ?DEFAULT_LOCK_RETRIES).
+
+
+%%
+%% Implementation
+%%
+
+cluster_formation_key_or_default(Key, Default) ->
+    case application:get_env(rabbit, cluster_formation) of
+        {ok, PropList} ->
+          proplists:get_value(Key, PropList, Default);
+        undefined ->
+            Default
+    end.
\ No newline at end of file
diff --git a/deps/rabbit/test/config_schema_SUITE_data/rabbit.snippets b/deps/rabbit/test/config_schema_SUITE_data/rabbit.snippets
index 08ea31ff49..214a2d1e66 100644
--- a/deps/rabbit/test/config_schema_SUITE_data/rabbit.snippets
+++ b/deps/rabbit/test/config_schema_SUITE_data/rabbit.snippets
@@ -139,6 +139,15 @@ cluster_formation.dns.hostname = discovery.eng.example.local",
   []},
  {cluster_formation_ram_ignored,
   "cluster_formation.node_type = ram",[],[]},
+ {cluster_formation_target_cluster_size_hint,
+  "cluster_formation.target_cluster_size_hint = 3",
+  [{rabbit, [
+    {cluster_formation, [
+       {target_cluster_size_hint, 3}
+     ]}
+   ]}],
+ []},
+
  {tcp_listen_options,
   "tcp_listen_options.backlog = 128
 tcp_listen_options.nodelay = true