add set_loghwm method in order to change high water mark on the fly

src/lager.erl

@@ -29,7 +29,7 @@
         trace/2, trace/3, trace_file/2, trace_file/3, trace_file/4, trace_console/1, trace_console/2,
         clear_all_traces/0, stop_trace/1, status/0, 
         get_loglevel/1, set_loglevel/2, set_loglevel/3, get_loglevels/0,
-        update_loglevel_config/0, posix_error/1,
+        update_loglevel_config/0, posix_error/1, set_loghwm/2, set_loghwm/3,		
         safe_format/3, safe_format_chop/3, dispatch_log/5, dispatch_log/9, 
         do_log/9, pr/2]).
 
@@ -320,6 +320,14 @@ get_loglevels() ->
     [gen_event:call(lager_event, Handler, get_loglevel, infinity) ||
         Handler <- gen_event:which_handlers(lager_event)].
 
+%% @doc Set the loghwm for a particular backend.
+set_loghwm(Handler, Hwm) when is_integer(Hwm) ->
+    gen_event:call(lager_event, Handler, {set_loghwm, Hwm}, infinity).
+
+%% @doc Set the loghwm (log high water mark) for file backends with multiple identifiers
+set_loghwm(Handler, Ident, Hwm) when is_integer(Hwm) ->
+    gen_event:call(lager_event, {Handler, Ident}, {set_loghwm, Hwm}, infinity).
+
 %% @private
 add_trace_to_loglevel_config(Trace) ->
     {MinLevel, Traces} = lager_config:get(loglevel),

src/lager_file_backend.erl

@@ -132,6 +132,15 @@ handle_call({set_loglevel, Level}, #state{name=Ident} = State) ->
     end;
 handle_call(get_loglevel, #state{level=Level} = State) ->
     {ok, Level, State};
+handle_call({set_loghwm, Hwm}, #state{shaper=Shaper, name=Name} = State) ->
+	case validate_logfile_proplist([{file, Name}, {high_water_mark, Hwm}]) of
+        false ->
+            {ok, {error, bad_log_hwm}, State};
+        _ ->
+			NewShaper = Shaper#lager_shaper{hwm=Hwm},
+			?INT_LOG(notice, "Changed loghwm of ~s to ~p", [Name, Hwm]),
+			{ok, {last_loghwm, Shaper#lager_shaper.hwm}, State#state{shaper=NewShaper}}
+	end;
 handle_call(_Request, State) ->
     {ok, ok, State}.