rabbit_peer_discovery: Compute start time once (backport #13490)

deps/rabbit/src/rabbit_peer_discovery.erl

@@ -627,8 +627,7 @@ query_node_props2([{Node, Members} | Rest], NodesAndProps, FromNode) ->
                            ["Peer discovery: temporary hidden node '~ts' "
                             "queries properties from node '~ts'",
                             [node(), Node]], FromNode),
-                         StartTime = get_node_start_time(
-                                       Node, microsecond, FromNode),
+                         StartTime = get_node_start_time(Node, FromNode),
                          IsReady = is_node_db_ready(Node, FromNode),
                          NodeAndProps = {Node, Members, StartTime, IsReady},
                          NodesAndProps1 = [NodeAndProps | NodesAndProps],
@@ -656,9 +655,8 @@ query_node_props2([], NodesAndProps, _FromNode) ->
     ?assert(length(NodesAndProps1) =< length(nodes(hidden))),
     NodesAndProps1.
 
--spec get_node_start_time(Node, Unit, FromNode) -> StartTime when
+-spec get_node_start_time(Node, FromNode) -> StartTime when
       Node :: node(),
-      Unit :: erlang:time_unit(),
       FromNode :: node(),
       StartTime :: non_neg_integer().
 %% @doc Returns the start time of the given `Node' in `Unit'.
@@ -679,15 +677,21 @@ query_node_props2([], NodesAndProps, _FromNode) ->
 %%
 %% @private
 
-get_node_start_time(Node, Unit, FromNode) ->
-    NativeStartTime = erpc_call(
-                        Node, erlang, system_info, [start_time], FromNode),
-    TimeOffset = erpc_call(Node, erlang, time_offset, [], FromNode),
-    SystemStartTime = NativeStartTime + TimeOffset,
-    StartTime = erpc_call(
-                  Node, erlang, convert_time_unit,
-                  [SystemStartTime, native, Unit], FromNode),
-    StartTime.
+get_node_start_time(Node, FromNode) ->
+    try
+        erpc_call(Node,rabbit_boot_state, get_start_time, [], FromNode)
+    catch
+        error:{exception, _, _} ->
+            NativeStartTime = erpc_call(
+                                Node, erlang, system_info, [start_time],
+                                FromNode),
+            TimeOffset = erpc_call(Node, erlang, time_offset, [], FromNode),
+            SystemStartTime = NativeStartTime + TimeOffset,
+            StartTime = erpc_call(
+                          Node, erlang, convert_time_unit,
+                          [SystemStartTime, native, microsecond], FromNode),
+            StartTime
+    end.
 
 -spec is_node_db_ready(Node, FromNode) -> IsReady when
       Node :: node(),

deps/rabbitmq_prelaunch/src/rabbit_boot_state.erl

@@ -17,9 +17,12 @@
          set/1,
          wait_for/2,
          has_reached/1,
-         has_reached_and_is_active/1]).
+         has_reached_and_is_active/1,
+         get_start_time/0,
+         record_start_time/0]).
 
 -define(PT_KEY_BOOT_STATE, {?MODULE, boot_state}).
+-define(PT_KEY_START_TIME, {?MODULE, start_time}).
 
 -type boot_state() :: stopped |
                       booting |
@@ -95,3 +98,56 @@ has_reached_and_is_active(TargetBootState) ->
             andalso
             not has_reached(CurrentBootState, stopping)
     end.
+
+-spec get_start_time() -> StartTime when
+      StartTime :: integer().
+%% @doc Returns the start time of the Erlang VM.
+%%
+%% This time was recorded by {@link record_start_time/0} as early as possible
+%% and is immutable.
+
+get_start_time() ->
+    persistent_term:get(?PT_KEY_START_TIME).
+
+-spec record_start_time() -> ok.
+%% @doc Records the start time of the Erlang VM.
+%%
+%% The time is expressed in microseconds since Epoch. It can be compared to
+%% other non-native times. This is used by the Peer Discovery subsystem to
+%% sort nodes and select a seed node if the peer discovery backend did not
+%% select one.
+%%
+%% This time is recorded once. Calling this function multiple times won't
+%% overwrite the value.
+
+record_start_time() ->
+    Key = ?PT_KEY_START_TIME,
+    try
+        %% Check if the start time was recorded.
+        _ = persistent_term:get(Key),
+        ok
+    catch
+        error:badarg ->
+            %% The start time was not recorded yet. Acquire a lock and check
+            %% again in case another process got the lock first and recorded
+            %% the start time.
+            Node = node(),
+            LockId = {?PT_KEY_START_TIME, self()},
+            true = global:set_lock(LockId, [Node]),
+            try
+                _ = persistent_term:get(Key),
+                ok
+            catch
+                error:badarg ->
+                    %% We are really the first to get the lock and we can
+                    %% record the start time.
+                    NativeStartTime = erlang:system_info(start_time),
+                    TimeOffset = erlang:time_offset(),
+                    SystemStartTime = NativeStartTime + TimeOffset,
+                    StartTime = erlang:convert_time_unit(
+                                  SystemStartTime, native, microsecond),
+                    persistent_term:put(Key, StartTime)
+            after
+                global:del_lock(LockId, [Node])
+            end
+    end.

deps/rabbitmq_prelaunch/src/rabbit_prelaunch.erl

@@ -48,6 +48,7 @@ run_prelaunch_first_phase() ->
 do_run() ->
     %% Indicate RabbitMQ is booting.
     clear_stop_reason(),
+    rabbit_boot_state:record_start_time(),
     rabbit_boot_state:set(booting),
 
     %% Configure dbg if requested.