From 1838418b9193140f6ba5f5716884b62aad7b662d Mon Sep 17 00:00:00 2001
From: Siri Hansen <siri@erlang.org>
Date: Wed, 3 Oct 2018 13:00:41 +0200
Subject: [logger] Start using handler callback changing_config/3 in built-in
 handlers

The new parameter to this function, SetOrUpdate, indicates how
unspecified configuration data fields shall be set. The rule is that
if SetOrUpdate equals set, then default values shall be used, and if
SetOrUpdate equals update, then existing configuration values shall be
used. Consequently, these examples now apply to logger_std_h and
logger_disk_log_h:

set_handler_config(default, config, #{sync_mode_qlen => 20}) sets the
value of sync_mode_qlen to 20, and resets all other (writable) fields
in the config map to their default values.

update_handler_config(default, config, #{sync_mode_qlen => 20}) sets
the value of sync_mode_qlen to 20, and leaves all other fields in the
config map unchanged.
---
 lib/kernel/src/logger_server.erl | 75 +++++++++++++++++++++++++++++-----------
 1 file changed, 55 insertions(+), 20 deletions(-)

(limited to 'lib/kernel/src/logger_server.erl')

diff --git a/lib/kernel/src/logger_server.erl b/lib/kernel/src/logger_server.erl
index 9e043f9362..b7735dbcf7 100644
--- a/lib/kernel/src/logger_server.erl
+++ b/lib/kernel/src/logger_server.erl
@@ -31,6 +31,9 @@
          update_config/2, update_config/3,
          update_formatter_config/2]).
 
+%% Helper
+-export([diff_maps/2]).
+
 %% gen_server callbacks
 -export([init/1, handle_call/3, handle_cast/2, handle_info/2,
          terminate/2]).
@@ -244,16 +247,22 @@ handle_call({change_config,SetOrUpd,HandlerId,Config0}, From,
                     update -> OldConfig
                 end,
             Config = maps:merge(Default,Config0),
-            call_h_async(
-              fun() ->
-                      call_h(Module,changing_config,[SetOrUpd,OldConfig,Config],
-                             {ok,Config})
-              end,
-              fun({ok,Config1}) ->
-                      logger_config:set(Tid,HandlerId,Config1);
-                 (Error) ->
-                      Error
-              end,From,State);
+            case check_config_change(OldConfig,Config) of
+                ok ->
+                    call_h_async(
+                      fun() ->
+                              call_h(Module,changing_config,
+                                     [SetOrUpd,OldConfig,Config],
+                                     {ok,Config})
+                      end,
+                      fun({ok,Config1}) ->
+                              logger_config:set(Tid,HandlerId,Config1);
+                         (Error) ->
+                              Error
+                      end,From,State);
+                Error ->
+                    {reply,Error,State}
+            end;
         _ ->
             {reply,{error,{not_found,HandlerId}},State}
     end;
@@ -262,16 +271,22 @@ handle_call({change_config,SetOrUpd,HandlerId,Key,Value}, From,
     case logger_config:get(Tid,HandlerId) of
         {ok,#{module:=Module}=OldConfig} ->
             Config = OldConfig#{Key=>Value},
-            call_h_async(
-              fun() ->
-                      call_h(Module,changing_config,[SetOrUpd,OldConfig,Config],
-                             {ok,Config})
-              end,
-              fun({ok,Config1}) ->
-                      logger_config:set(Tid,HandlerId,Config1);
-                 (Error) ->
-                      Error
-              end,From,State);
+            case check_config_change(OldConfig,Config) of
+                ok ->
+                    call_h_async(
+                      fun() ->
+                              call_h(Module,changing_config,
+                                     [SetOrUpd,OldConfig,Config],
+                                     {ok,Config})
+                      end,
+                      fun({ok,Config1}) ->
+                              logger_config:set(Tid,HandlerId,Config1);
+                         (Error) ->
+                              Error
+                      end,From,State);
+                Error ->
+                    {reply,Error,State}
+            end;
         _ ->
             {reply,{error,{not_found,HandlerId}},State}
     end;
@@ -486,6 +501,15 @@ check_formatter({Mod,Config}) ->
 check_formatter(Formatter) ->
     throw({invalid_formatter,Formatter}).
 
+%% When changing configuration for a handler, the id and module fields
+%% can not be changed.
+check_config_change(#{id:=Id,module:=Module},#{id:=Id,module:=Module}) ->
+    ok;
+check_config_change(OldConfig,NewConfig) ->
+    {Old,New} = logger_server:diff_maps(maps:with([id,module],OldConfig),
+                                        maps:with([id,module],NewConfig)),
+    {error,{illegal_config_change,Old,New}}.
+
 call_h(Module, Function, Args, DefRet) ->
     %% Not calling code:ensure_loaded + erlang:function_exported here,
     %% since in some rare terminal cases, the code_server might not
@@ -558,3 +582,14 @@ call_h_reply(Unexpected,State) ->
                    {process,?SERVER},
                    {message,Unexpected}]),
     {noreply,State}.
+
+%% Return two maps containing only the fields that differ.
+diff_maps(M1,M2) ->
+    diffs(lists:sort(maps:to_list(M1)),lists:sort(maps:to_list(M2)),#{},#{}).
+
+diffs([H|T1],[H|T2],D1,D2) ->
+    diffs(T1,T2,D1,D2);
+diffs([{K,V1}|T1],[{K,V2}|T2],D1,D2) ->
+    diffs(T1,T2,D1#{K=>V1},D2#{K=>V2});
+diffs([],[],D1,D2) ->
+    {D1,D2}.
-- 
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