Cleanup unused handlers when a trace is removed, documentation

README.org

@@ -198,3 +198,12 @@
 
   To view the active log backends and traces, you can use the lager:status()
   function. To clear all active traces, you can use lager:clear_all_traces().
+
+  To delete a specific trace, store a handle for the trace when you create it,
+  that you later pass to lager:stop_trace/1:
+
+#+BEGIN_EXAMPLE
+  {ok, Trace} = lager:trace_file("log/error.log", [{module, mymodule}]),
+  ...
+  lager:stop_trace(Trace)
+#+END_EXAMPLE

src/lager.erl

@@ -137,16 +137,36 @@ trace_console(Filter, Level) ->
             Error
     end.
 
-stop_trace(Trace) ->
+stop_trace({_Filter, _Level, Target} = Trace) ->
     {MinLevel, Traces} = lager_mochiglobal:get(loglevel),
-    lager_mochiglobal:put(loglevel, {MinLevel, lists:delete(Trace, Traces)}),
-    %% TODO - somehow determine if the handler is safe to stop, its not
-    %$ straightforward to tell.
+    NewTraces =  lists:delete(Trace, Traces),
+    lager_mochiglobal:put(loglevel, {MinLevel, NewTraces}),
+    case get_loglevel(Target) of
+        none ->
+            %% check no other traces point here
+            case lists:keyfind(Target, 3, NewTraces) of
+                false ->
+                    gen_event:delete_handler(lager_event, Target, []);
+                _ ->
+                    ok
+            end;
+        _ ->
+            ok
+    end,
     ok.
 
 clear_all_traces() ->
     {MinLevel, _Traces} = lager_mochiglobal:get(loglevel),
-    lager_mochiglobal:put(loglevel, {MinLevel, []}).
+    lager_mochiglobal:put(loglevel, {MinLevel, []}),
+    [begin
+                case get_loglevel(Handler) of
+                    none ->
+                        gen_event:delete_handler(lager_event, Handler, []);
+                    _ ->
+                        ok
+                end
+        end || Handler <- gen_event:which_handlers(lager_event)],
+    ok.
 
 status() ->
     Handlers = gen_event:which_handlers(lager_event),

src/lager_console_backend.erl

@@ -183,7 +183,7 @@ console_log_test_() ->
                             500 ->
                                 ?assert(true)
                         end,
-                        ok = lager:trace_console([{module, ?MODULE}]),
+                        {ok, _} = lager:trace_console([{module, ?MODULE}]),
                         lager:debug("Test message"),
                         receive
                             {io_request, From1, ReplyAs1, {put_chars, unicode, Msg1}} ->
@@ -210,7 +210,7 @@ console_log_test_() ->
                             500 ->
                                 ?assert(true)
                         end,
-                        ok = lager:trace_console([{module, ?MODULE}]),
+                        {ok, _} = lager:trace_console([{module, ?MODULE}]),
                         lager:error("Test message"),
                         receive
                             {io_request, From1, ReplyAs1, {put_chars, unicode, Msg1}} ->

src/lager_file_backend.erl

@@ -376,7 +376,7 @@ filesystem_test_() ->
             {"tracing to a dedicated file should work",
                 fun() ->
                         file:delete("foo.log"),
-                        lager:trace_file("foo.log", [{module, ?MODULE}]),
+                        {ok, _} = lager:trace_file("foo.log", [{module, ?MODULE}]),
                         lager:error("Test message"),
                         {ok, Bin3} = file:read_file("foo.log"),
                         ?assertMatch([_, _, "[error]", _, "Test message\n"], re:split(Bin3, " ", [{return, list}, {parts, 5}]))