Refactor OS process management

Squashed commit of the following:
commit a9cd9681f6c88f0f3c019e98e2edfef55cad0129
commit eb38bca08ffbf778b69fbb2d612e23733af82ff5
commit 98a03a079ab24f2c7bd9e0d6d7fac5fa62bfd4eb
commit 9b8ec059165d981e4cd743008ecdf393a4f37f61
commit 3a891c1dd9a17fdd267c423b340dd09c31c89d7a
commit 68351dd181c8a92b5baa9ac23f25c7c191484394
commit e4384a517e2efeac9231701898a6c67213642319
commit cd954661422d0ef146b5bd7792f835dcc4220c84
commit 3bcca92c7c0102d5722dfc6b2c332766cfe0370c
commit 82d15f40f503b2609cf785ce2837e1280edaaa43
commit 70051abbd699e076452d772587c32ee5e09bdcbc
commit 7f01d37781e7774015f6cb34f795b28db9ecc9f5

BugzID: 11572

See also COUCHDB-901

A new config setting is introduced.  The following block controls the
maximum number of OS processes that will be reused.  Additional OS
processes will still be spawned on-demand, but they'll be terminated
when the clients are through with them.

[query_server_config]
os_process_soft_limit = 100

1 files changed, 186 insertions, 0 deletions

diff --git a/apps/couch/src/couch_proc_manager.erl b/apps/couch/src/couch_proc_manager.erl
new file mode 100644
index 00000000..ca2e8ac5
--- /dev/null
+++ b/apps/couch/src/couch_proc_manager.erl
@@ -0,0 +1,186 @@
+-module(couch_proc_manager).
+-behaviour(gen_server).
+-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, 
+    code_change/3]).
+
+-export([start_link/0, get_proc_count/0]).
+
+-include("couch_db.hrl").
+
+-record(state, {tab}).
+
+start_link() ->
+    gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
+
+get_proc_count() ->
+    gen_server:call(?MODULE, get_proc_count).
+
+init([]) ->
+    process_flag(trap_exit, true),
+    {ok, #state{tab = ets:new(procs, [{keypos, #proc.pid}])}}.
+
+handle_call(get_table, _From, State) ->
+    {reply, State#state.tab, State};
+
+handle_call(get_proc_count, _From, State) ->
+    {reply, ets:info(State#state.tab, size), State};
+
+handle_call({get_proc, #doc{body={Props}}=DDoc, DDocKey}, {Client, _}, State) ->
+    Lang = couch_util:get_value(<<"language">>, Props, <<"javascript">>),
+    try get_procs(State#state.tab, Lang) of
+    Procs ->
+        case proc_with_ddoc(DDoc, DDocKey, Procs) of
+        {ok, Proc0} ->
+            Proc = Proc0#proc{client = erlang:monitor(process, Client)},
+            ets:insert(State#state.tab, Proc),
+            {reply, {ok, Proc, get_query_server_config()}, State};
+        {error, Reason} ->
+            {reply, {error, Reason}, State}
+        end
+    catch {unknown_query_language, _} ->
+        {reply, {unknown_query_language, Lang}, State};
+    error:Reason ->
+        ?LOG_ERROR("~p ~p ~p", [?MODULE, Reason, erlang:get_stacktrace()]),
+        {reply, {error, Reason}, State}
+    end;
+
+handle_call({get_proc, Lang}, {Client, _}, State) ->
+    try get_procs(State#state.tab, Lang) of
+    [Proc0|_] ->
+        Proc = Proc0#proc{client = erlang:monitor(process, Client)},
+        ets:insert(State#state.tab, Proc),
+        {reply, {ok, Proc, get_query_server_config()}, State}
+    catch {unknown_query_language, _} ->
+        {reply, {unknown_query_language, Lang}, State};
+    error:Reason ->
+        ?LOG_ERROR("~p ~p ~p", [?MODULE, Reason, erlang:get_stacktrace()]),
+        {reply, {error, Reason}, State}
+    end;
+
+handle_call({ret_proc, #proc{client=Ref, pid=Pid} = Proc}, _From, State) ->
+    erlang:demonitor(Ref, [flush]),
+    % We need to check if the process is alive here, as the client could be
+    % handing us a #proc{} with a dead one.  We would have already removed the
+    % #proc{} from our own table, so the alternative is to do a lookup in the
+    % table before the insert.  Don't know which approach is cheaper.
+    case is_process_alive(Pid) of true ->
+        maybe_reuse_proc(State#state.tab, Proc);
+    false -> ok end,
+    {reply, true, State};
+
+handle_call(_Call, _From, State) ->
+    {reply, ignored, State}.
+
+handle_cast(_Msg, State) ->
+    {noreply, State}.
+
+handle_info({'EXIT', Pid, Reason}, State) ->
+    ?LOG_INFO("~p ~p died ~p", [?MODULE, Pid, Reason]),
+    ets:delete(State#state.tab, Pid),
+    {noreply, State};
+
+handle_info({'DOWN', Ref, _, _, _Reason}, State) ->
+    case ets:match_object(State#state.tab, #proc{client=Ref, _='_'}) of
+    [] ->
+        ok;
+    [#proc{pid = Pid} = Proc] ->
+        case is_process_alive(Pid) of true ->
+            maybe_reuse_proc(State#state.tab, Proc);
+        false -> ok end
+    end,
+    {noreply, State};
+
+handle_info(_Msg, State) ->
+    {noreply, State}.
+
+terminate(_Reason, #state{tab=Tab}) ->
+    ets:foldl(fun(#proc{pid=P}, _) -> couch_util:shutdown_sync(P) end, 0, Tab),
+    ok.
+
+code_change(_OldVsn, State, _Extra) ->
+    {ok, State}.
+
+maybe_reuse_proc(Tab, #proc{pid = Pid} = Proc) ->
+    Limit = couch_config:get("query_server_config", "os_process_soft_limit", "100"),
+    case ets:info(Tab, size) > list_to_integer(Limit) of
+    true ->
+        ets:delete(Tab, Pid),
+        unlink(Pid),
+        exit(Pid, kill);
+    false ->
+        garbage_collect(Pid),
+        ets:insert(Tab, Proc#proc{client=nil})
+    end.
+
+get_procs(Tab, Lang) when is_binary(Lang) ->
+    get_procs(Tab, binary_to_list(Lang));
+get_procs(Tab, Lang) when is_list(Lang) ->
+    case ets:match_object(Tab, #proc{lang=Lang, client=nil, _='_'}) of
+    [] ->
+        {ok, NewProc} = new_proc(Lang), % check OS process limit
+        [NewProc];
+    Procs ->
+        Procs
+    end.
+
+new_proc(Lang) when is_binary(Lang) ->
+    new_proc(binary_to_list(Lang));
+new_proc(Lang) when is_list(Lang) ->
+    case couch_config:get("query_servers", Lang) of
+    undefined ->
+        case couch_config:get("native_query_servers", Lang) of
+        undefined ->
+            throw({unknown_query_language, Lang});
+        SpecStr ->
+            {ok, {M,F,A}} = couch_util:parse_term(SpecStr),
+            {ok, Pid} = apply(M, F, A),
+            make_proc(Pid, Lang, M)
+        end;
+    Command ->
+        {ok, Pid} = couch_os_process:start_link(Command),
+        make_proc(Pid, Lang, couch_os_process)
+    end.
+
+make_proc(Pid, Lang, Mod) ->
+    Proc = #proc{
+        lang = Lang,
+        pid = Pid,
+        prompt_fun = {Mod, prompt},
+        set_timeout_fun = {Mod, set_timeout},
+        stop_fun = {Mod, stop}
+    },
+    {ok, Proc}.
+
+get_query_server_config() ->
+    Limit = couch_config:get("query_server_config", "reduce_limit", "true"),
+    {[{<<"reduce_limit">>, list_to_atom(Limit)}]}.
+
+proc_with_ddoc(DDoc, DDocKey, Procs) ->
+    Filter = fun(#proc{ddoc_keys=Keys}) -> not lists:member(DDocKey, Keys) end,
+    case lists:dropwhile(Filter, Procs) of
+    [DDocProc|_] ->
+        {ok, DDocProc};
+    [] ->
+        teach_any_proc(DDoc, DDocKey, Procs)
+    end.
+
+teach_any_proc(DDoc, DDocKey, [Proc|Rest]) ->
+    try
+        teach_ddoc(DDoc, DDocKey, Proc)
+    catch _:_ ->
+        teach_any_proc(DDoc, DDocKey, Rest)
+    end;
+teach_any_proc(_, _, []) ->
+    {error, noproc}.
+
+teach_ddoc(DDoc, {DDocId, _Rev}=DDocKey, #proc{ddoc_keys=Keys}=Proc) ->
+    % send ddoc over the wire
+    % we only share the rev with the client we know to update code
+    % but it only keeps the latest copy, per each ddoc, around.
+    true = couch_query_servers:proc_prompt(Proc, [<<"ddoc">>, <<"new">>,
+        DDocId, couch_doc:to_json_obj(DDoc, [])]),
+    % we should remove any other ddocs keys for this docid
+    % because the query server overwrites without the rev
+    Keys2 = [{D,R} || {D,R} <- Keys, D /= DDocId],
+    % add ddoc to the proc
+    {ok, Proc#proc{ddoc_keys=[DDocKey|Keys2]}}.