Adding backpressure & timeout protection

Adding backpressure to return_member operations by converting
those messages from cast to call. This prevents a pool from losing
members faster than it can receive them by slowing down the callers.

Also lengthened all timeouts to infinty to allow performance to slow
degrade instead of going out with a bang when the gen_server message
times out.

src/pooler.erl

@@ -120,7 +120,7 @@ stop() ->
 %%
 -spec take_member() -> pid() | error_no_members.
 take_member() ->
-    gen_server:call(?SERVER, take_member).
+    gen_server:call(?SERVER, take_member, infinity).
 
 %% @doc Obtain exclusive access to a member from `PoolName'.
 %%
@@ -128,7 +128,7 @@ take_member() ->
 %%
 -spec take_member(string()) -> pid() | error_no_members | error_no_pool.
 take_member(PoolName) when is_list(PoolName) ->
-    gen_server:call(?SERVER, {take_member, PoolName}).
+    gen_server:call(?SERVER, {take_member, PoolName}, infinity).
 
 %% @doc Return a member to the pool so it can be reused.
 %%
@@ -138,8 +138,7 @@ take_member(PoolName) when is_list(PoolName) ->
 -spec return_member(pid() | error_no_members, ok | fail) -> ok.
 return_member(Pid, Status) when is_pid(Pid) andalso
                                 (Status =:= ok orelse Status =:= fail) ->
-    CPid = self(),
-    gen_server:cast(?SERVER, {return_member, Pid, Status, CPid}),
+    gen_server:call(?SERVER, {return_member, Pid, Status}, infinity),
     ok;
 return_member(error_no_members, _) ->
     ok.
@@ -148,8 +147,7 @@ return_member(error_no_members, _) ->
 %%
 -spec return_member(pid() | error_no_members) -> ok.
 return_member(Pid) when is_pid(Pid) ->
-    CPid = self(),
-    gen_server:cast(?SERVER, {return_member, Pid, ok, CPid}),
+    gen_server:call(?SERVER, {return_member, Pid, ok}, infinity),
     ok;
 return_member(error_no_members) ->
     ok.
@@ -220,6 +218,8 @@ handle_call(take_member, {CPid, _Tag},
 handle_call({take_member, PoolName}, {CPid, _Tag}, #state{} = State) ->
     {Member, NewState} = take_member(PoolName, CPid, State),
     {reply, Member, NewState};
+handle_call({return_member, Pid, Status}, {CPid, _Tag}, State) ->
+    {reply, ok, do_return_member(Pid, Status, State)};
 handle_call(stop, _From, State) ->
     {stop, normal, stop_ok, State};
 handle_call(pool_stats, _From, State) ->
@@ -228,8 +228,6 @@ handle_call(_Request, _From, State) ->
     {noreply, State}.
 
 -spec handle_cast(_,_) -> {'noreply', _}.
-handle_cast({return_member, Pid, Status, _CPid}, State) ->
-    {noreply, do_return_member(Pid, Status, State)};
 handle_cast(_Msg, State) ->
     {noreply, State}.