davisp commented on a change in pull request #610: Optimize ddoc cache URL: https://github.com/apache/couchdb/pull/610#discussion_r125986186
########## File path: src/ddoc_cache/src/ddoc_cache_entry.erl ########## @@ -0,0 +1,280 @@ +% 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(ddoc_cache_entry). +-behaviour(gen_server). + + +-export([ + dbname/1, + ddocid/1, + recover/1, + + start_link/1, + shutdown/1, + open/2, + accessed/1, + refresh/1 +]). + +-export([ + init/1, + terminate/2, + handle_call/3, + handle_cast/2, + handle_info/2, + code_change/3 +]). + +-export([ + do_open/1 +]). + + +-include("ddoc_cache.hrl"). + + +-record(st, { + key, + val, + opener, + waiters, + ts +}). + + +dbname({Mod, Arg}) -> + Mod:dbname(Arg). + + +ddocid({Mod, Arg}) -> + Mod:ddocid(Arg). + + +recover({Mod, Arg}) -> + Mod:recover(Arg). + + +start_link(Key) -> + Pid = proc_lib:spawn_link(?MODULE, init, [Key]), + {ok, Pid}. + + +shutdown(Pid) -> + ok = gen_server:call(Pid, shutdown). + + +open(Pid, Key) -> + try + Resp = gen_server:call(Pid, open), + case Resp of + {open_ok, Val} -> + Val; + {open_error, {T, R, S}} -> + erlang:raise(T, R, S) + end + catch exit:_ -> + % Its possible that this process was evicted just + % before we tried talking to it. Just fallback + % to a standard recovery + recover(Key) + end. + + +accessed(Pid) -> + gen_server:cast(Pid, accessed). + + +refresh(Pid) -> + gen_server:cast(Pid, refresh). + + +init(Key) -> + true = ets:update_element(?CACHE, Key, {#entry.pid, self()}), + St = #st{ + key = Key, + opener = spawn_opener(Key), + waiters = [] + }, + ?EVENT(started, Key), + gen_server:enter_loop(?MODULE, [], St). + + +terminate(_Reason, St) -> + #st{ + key = Key, + opener = Pid, + ts = Ts + } = St, + % We may have already deleted our cache entry + % during shutdown + Pattern = #entry{key = Key, pid = self(), _ = '_'}, + CacheMSpec = [{Pattern, [], [true]}], + true = ets:select_delete(?CACHE, CacheMSpec) < 2, + % We may have already deleted our LRU entry + % during shutdown + if Ts == undefined -> ok; true -> + LruMSpec = [{{{Ts, Key, self()}}, [], [true]}], + true = ets:select_delete(?LRU, LruMSpec) < 2 + end, + % Blow away any current opener if it exists + if not is_pid(Pid) -> ok; true -> + catch exit(Pid, kill) + end, + ok. + + +handle_call(open, From, #st{val = undefined} = St) -> + NewSt = St#st{ + waiters = [From | St#st.waiters] + }, + {noreply, NewSt}; + +handle_call(open, _From, St) -> + {reply, St#st.val, St}; + +handle_call(shutdown, _From, St) -> + remove_from_cache(St), + {stop, normal, ok, St}; + +handle_call(Msg, _From, St) -> + {stop, {bad_call, Msg}, {bad_call, Msg}, St}. + + +handle_cast(accessed, St) -> + ?EVENT(accessed, St#st.key), + drain_accessed(), + {noreply, update_lru(St)}; + +handle_cast(refresh, #st{opener = Ref} = St) when is_reference(Ref) -> + #st{ + key = Key + } = St, + erlang:cancel_timer(Ref), + NewSt = St#st{ + opener = spawn_opener(Key) + }, + {noreply, NewSt}; + +handle_cast(refresh, #st{opener = Pid} = St) when is_pid(Pid) -> + catch exit(Pid, kill), + receive + {'DOWN', _, _, Pid, _} -> ok + end, + NewSt = St#st{ + opener = spawn_opener(St#st.key) + }, + {noreply, NewSt}; + +handle_cast(Msg, St) -> + {stop, {bad_cast, Msg}, St}. + + +handle_info({'DOWN', _, _, Pid, Resp}, #st{key = Key, opener = Pid} = St) -> + case Resp of + {open_ok, Key, {ok, Val}} -> + if not is_list(St#st.waiters) -> ok; true -> + respond(St#st.waiters, {open_ok, {ok, Val}}) + end, + update_cache(St, Val), + Msg = {'$gen_cast', refresh}, + Timer = erlang:send_after(?REFRESH_TIMEOUT, self(), Msg), + NewSt = St#st{ + val = {open_ok, {ok, Val}}, + opener = Timer + }, + {noreply, update_lru(NewSt)}; + {Status, Key, Other} -> + NewSt = St#st{ + val = {Status, Other}, + opener = undefined, + waiters = undefined Review comment: No, NewSt has waiters = undefined, the original St record still has the waiters list. This is a switch to know when we're responding vs not. I guess technically we could keep it an empty list and call a function that does a no-op but that seemed slightly weirder than this way as it doesn't show the actual intent so I'd fear it'd get lost in the sands of time. Its also a slightly subtle assumption that once we switch to "actual value inserted in cache" mode that we don't want to handle open calls. ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: [email protected] With regards, Apache Git Services
