Ensure that an eventual monitor ref is kept when overwriting in conflict.

src/syn_registry.erl

@@ -429,17 +429,22 @@ unregister_on_node(Name) ->
 
 -spec add_to_local_table(Name :: any(), Pid :: pid(), Meta :: any(), MonitorRef :: undefined | reference()) -> ok.
 add_to_local_table(Name, Pid, Meta, MonitorRef) ->
-    %% remove entry if previous exists
-    case find_registry_tuple_by_name(Name) of
+    %% remove entry if previous exists & get pre-existing monitor ref
+    OldMonitorRef = case find_registry_entry_by_name(Name) of
         undefined ->
-            ok;
+            undefined;
 
-        {Name, OldPid, _OldMeta} ->
-            ets:delete(syn_registry_by_pid, {OldPid, Name})
+        {Name, OldPid, _OldMeta, OldMRef, _Node} ->
+            ets:delete(syn_registry_by_pid, {OldPid, Name}),
+            OldMRef
+    end,
+    MonitorRef1 = case MonitorRef of
+        undefined -> OldMonitorRef;
+        _ -> MonitorRef
     end,
     %% overwrite & add
-    ets:insert(syn_registry_by_name, {Name, Pid, Meta, MonitorRef, node(Pid)}),
-    ets:insert(syn_registry_by_pid, {{Pid, Name}, Meta, MonitorRef, node(Pid)}),
+    ets:insert(syn_registry_by_name, {Name, Pid, Meta, MonitorRef1, node(Pid)}),
+    ets:insert(syn_registry_by_pid, {{Pid, Name}, Meta, MonitorRef1, node(Pid)}),
     ok.
 
 -spec remove_from_local_table(Name :: any(), Pid :: pid()) -> ok.

test/syn_registry_SUITE.erl

@@ -40,7 +40,8 @@
     single_node_register_gen_server/1,
     single_node_callback_on_process_exit/1,
     single_node_ensure_callback_process_exit_is_called_if_process_killed/1,
-    single_node_monitor_after_registry_crash/1
+    single_node_monitor_after_registry_crash/1,
+    single_node_keep_monitor_reference_for_pid_if_there/1
 ]).
 -export([
     two_nodes_register_monitor_and_unregister/1,
@@ -113,7 +114,8 @@ groups() ->
             single_node_register_gen_server,
             single_node_callback_on_process_exit,
             single_node_ensure_callback_process_exit_is_called_if_process_killed,
-            single_node_monitor_after_registry_crash
+            single_node_monitor_after_registry_crash,
+            single_node_keep_monitor_reference_for_pid_if_there
         ]},
         {two_nodes_process_registration, [shuffle], [
             two_nodes_register_monitor_and_unregister,
@@ -419,6 +421,20 @@ single_node_monitor_after_registry_crash(_Config) ->
     %% retrieve
     undefined = syn:whereis(<<"my proc 2">>).
 
+single_node_keep_monitor_reference_for_pid_if_there(_Config) ->
+    %% start
+    ok = syn:start(),
+    %% start processes
+    Pid = syn_test_suite_helper:start_process(),
+    %% register
+    ok = syn:register(<<"my proc">>, Pid),
+    %% get monitor
+    [{<<"my proc">>, Pid, undefined, MonitorRef, _}] = ets:lookup(syn_registry_by_name, <<"my proc">>),
+    %% insert into table and keep reference
+    syn_registry:add_to_local_table(<<"my proc">>, Pid, undefined, undefined),
+    %% check internals
+    [{<<"my proc">>, Pid, undefined, MonitorRef, _}] = ets:lookup(syn_registry_by_name, <<"my proc">>).
+
 two_nodes_register_monitor_and_unregister(Config) ->
     %% get slave
     SlaveNode = proplists:get_value(slave_node, Config),