Add OS process listeners

We don't use this in dbcore but if we ever merge this back to CouchDB
then we'll want to have this feature.

src/couch_event_os_listener.erl

@@ -0,0 +1,75 @@
+% Licensed under the Apache License, Version 2.0 (the "License"); you may not
+% use this file except in compliance with the License. You may obtain a copy of
+% the License at
+%
+%   http://www.apache.org/licenses/LICENSE-2.0
+%
+% Unless required by applicable law or agreed to in writing, software
+% distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+% WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+% License for the specific language governing permissions and limitations under
+% the License.
+
+-module(couch_event_os_listener).
+-behavior(gen_server).
+
+
+-export([
+    start_link/0
+]).
+
+-export([
+    init/1,
+    terminate/2,
+    handle_call/3,
+    handle_cast/2,
+    handle_info/2,
+    code_change/3
+]).
+
+
+start_link(Exe) when is_list(Exe) ->
+    gen_server:start_link(?MODULE, Exe, []).
+
+
+init(Exe) ->
+    process_flag(trap_exit, true),
+    ok = couch_event:register_all(self()),
+    couch_os_process:start_link(Exe, []).
+
+
+terminate(_Reason, Pid) when is_pid(Pid) ->
+    couch_os_process:stop(Pid);
+terminate(_Reason, _Pid) ->
+    ok.
+
+
+handle_call(Msg, From, Pid) ->
+    couch_log:notice("~s ignoring call ~w from ~w", [?MODULE, Msg, From]),
+    {reply, ignored, Pid, 0}.
+
+
+handle_cast(Msg, Pid) ->
+    couch_log:notice("~s ignoring cast ~w", [?MODULE, Msg]),
+    {noreply, Pid, 0}.
+
+
+handle_info({'$couch_event', DbName, Event}, Pid) ->
+    Obj = {[
+        {db, DbName},
+        {type, list_to_binary(atom_to_list(Event))}
+    ]},
+    ok = couch_os_process:send(Pid, Obj),
+    {noreply, Pid};
+
+handle_info({'EXIT', Pid, Reason}, Pid) ->
+    couch_log:error("Update notificatio process ~w died: ~w", [Pid, Reason]),
+    {stop, normal, nil};
+
+handle_info(Msg, Pid) ->
+    couch_log:notice("~s ignoring info ~w", [?MODULE, Msg]),
+    {noreply, Pid, 0}.
+
+
+code_change(_OldVsn, St, _Extra) ->
+    {ok, St}.

src/couch_event_os_sup.erl

@@ -0,0 +1,73 @@
+% Licensed under the Apache License, Version 2.0 (the "License"); you may not
+% use this file except in compliance with the License. You may obtain a copy of
+% the License at
+%
+%   http://www.apache.org/licenses/LICENSE-2.0
+%
+% Unless required by applicable law or agreed to in writing, software
+% distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+% WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+% License for the specific language governing permissions and limitations under
+% the License.
+
+
+% This causes an OS process to spawned and it is notified every time a database
+% is updated.
+%
+% The notifications are in the form of a the database name sent as a line of
+% text to the OS processes stdout.
+
+
+-module(couch_event_os_sup).
+-behaviour(supervisor).
+-behaviour(config_listener).
+
+
+-export([
+    start_link/0,
+    init/1
+]).
+
+-export([
+    handle_config_change/5
+]).
+
+
+start_link() ->
+    supervisor:start_link({local, ?MODULE}, ?MODULE, []).
+
+
+init([]) ->
+    ok = config:listen_for_changes(?MODULE, nil),
+
+    UpdateNotifierExes = config:get("update_notification"),
+    Children = [child(Id, Exe) || {Id, Exe} <- UpdateNotifierExes],
+
+    {ok, {
+        {one_for_one, 10, 3600},
+        Children
+    }.
+
+
+handle_config_change("update_notification", Id, deleted, _, _) ->
+    supervisor:terminate_child(?MODULE, Id),
+    supervisor:delete_child(?MODULE, Id),
+    {ok, nil};
+handle_config_change("update_notification", Id, Exe, _, _) when is_list(Exe) ->
+    supervisor:terminate_child(?MODULE, Id),
+    supervisor:delete_child(?MODULE, Id),
+    supervisor:start_child(?MODULE, child(Id, Exe)),
+    {ok, nil};
+handle_config_change(_, _, _, _, _) ->
+    {ok, nil}.
+
+
+child(Id, Arg) ->
+    {
+        Id,
+        {couch_event_os_listener, start_link, [Arg]},
+        permanent,
+        1000,
+        supervisor,
+        [couch_event_os_listener]
+    }.

src/couch_event_sup2.erl

@@ -45,6 +45,13 @@ init(_) ->
             5000,
             worker,
             [couch_event_dist]
+        },
+        {couch_event_os_sup,
+            {couch_event_os_sup, start_link, []},
+            permanent,
+            5000,
+            supervisor,
+            [couch_event_os_sup]
         }
     ],
     {ok, {{one_for_one, 5, 10}, Children}}.