shutdown existing socket when eaddrinuse hit during socket process restart

Author: andymck

src/grpcbox_socket.erl

@@ -16,70 +16,52 @@
 start_link(Pool, ListenOpts, AcceptorOpts) ->
     gen_server:start_link(?MODULE, [Pool, ListenOpts, AcceptorOpts], []).
 
-
 %% gen_server api
 
 init([Pool, ListenOpts, PoolOpts]) ->
-    %% Trapping exit so can close socket in terminate/2
-    _ = process_flag(trap_exit, true),
     Port = maps:get(port, ListenOpts, 8080),
     IPAddress = maps:get(ip, ListenOpts, {0, 0, 0, 0}),
     AcceptorPoolSize = maps:get(size, PoolOpts, 10),
     SocketOpts = maps:get(socket_options, ListenOpts, [{reuseaddr, true},
                                                        {nodelay, true},
+                                                       {reuseaddr, true},
                                                        {backlog, 32768},
                                                        {keepalive, true}]),
+    %% Trapping exit so can close socket in terminate/2
+    _ = process_flag(trap_exit, true),
     Opts = [{active, false}, {mode, binary}, {packet, raw}, {ip, IPAddress} | SocketOpts],
     case gen_tcp:listen(Port, Opts) of
         {ok, Socket} ->
             %% acceptor could close the socket if there is a problem
             MRef = monitor(port, Socket),
-            {ok, _} = grpcbox_pool:accept_socket(Pool, Socket, AcceptorPoolSize),
+            grpcbox_pool:accept_socket(Pool, Socket, AcceptorPoolSize),
             {ok, {Socket, MRef}};
-        {error, eaddrinuse} ->
+        {error, eaddrinuse} = Error ->
             %% our desired port is already in use
-            %% its likely this grpcbox_socket server is restarting
+            %% its likely this grpcbox_socket server has been killed ( for reason unknown ) and is restarting
             %% previously it would have bound to the port before passing control to our acceptor pool
+            %% the socket remains open
             %% in the restart scenario, the socket process would attempt to bind again
             %% to the port and then stop, the sup would keep restarting it
             %% and we would end up breaching the restart strategy of the parent sup
             %% eventually taking down the entire tree
             %% result of which is we have no active listener and grpcbox is effectively down
-            %% so now if we hit eaddrinuse, we check if our acceptor pool is already the
-            %% controlling process, if so we reuse the port from its state and
-            %% allow grpcbox_socket to start cleanly
+            %% so now if we hit eaddrinuse, we check if our acceptor pool using it
+            %% if so we close the port here and stop this process
+            %% NOTE: issuing stop in init wont trigger terminate and so cant rely on
+            %%       the socket being closed there
+            %% This allows the sup to restart things cleanly
+            %% We could try to reuse the exising port rather than closing it
+            %% but side effects were encountered there, so deliberately avoiding
 
             %% NOTE: acceptor_pool has a grace period for connections before it terminates
             %%       grpcbox_pool sets this to a default of 5 secs
             %%       this needs considered when deciding on related supervisor restart strategies
             %%       AND keep in mind the acceptor pool will continue accepting new connections
             %%       during this grace period
 
-            %% Other possible fixes here include changing the grpcbox_services_sup from its
-            %% rest_for_one to a one_for_all strategy.  This ensures the pool and thus the
-            %% current controlling process of the socket is terminated
-            %% and allows things to restart cleanly if grpcbox_socket dies
-            %% the disadvantage there however is we will drop all existing grpc connections
-
-            %% Another possible fix is to play with the restart strategy intensity and periods
-            %% and ensure the top level sup doesnt get breached but...
-            %% a requirement will be to ensure the grpcbox_service_sup forces a restart
-            %% of grpcbox_pool and therefore the acceptor_pool process
-            %% as only by doing that will be free up the socket and allow grpcbox_socket to rebind
-            %% thus we end up terminating any existing grpc connections
-
-            %% Yet another possible fix is to move the cleanup of closing the socket
-            %% out of grpcbox_socket's terminate and into acceptor_pool's terminate
-            %% that however puts two way co-ordination between two distinct libs
-            %% which is far from ideal and in addition will also result in existing grpc connection
-            %% being dropped
-
-            %% my view is, if at all possible, its better to restart the grpcbox_socket process without
-            %% impacting existing connections, the fix below allows for that, albeit a lil messy
-            %% there is most likely a better solution to all of this, TODO: revisit
-
             %% get the current sockets in use by the acceptor pool
-            %% if one is bound to our target port then reuse
+            %% if one is bound to our target port then close it
             %% need to allow for possibility of multiple services, each with its own socket
             %% so we need to identify our interested socket via port number
             PoolSockets = grpcbox_pool:pool_sockets(Pool),
@@ -89,15 +71,15 @@ init([Pool, ListenOpts, PoolOpts]) ->
                             {ok, Socket};
                         (_, Acc) ->
                             Acc
-                    end, {error, eaddrinuse}, PoolSockets),
+                    end, socket_not_found, PoolSockets),
             case MaybeHaveExistingSocket of
                 {ok, Socket} ->
-                    MRef = monitor(port, Socket),
-                    {ok, {Socket, MRef}};
-                {error, Reason} ->
-                    {stop, Reason}
-            end;
-        {error, Reason}->
+                    gen_tcp:close(Socket);
+                socket_not_found ->
+                    noop
+            end,
+            Error;
+        {error, Reason} ->
             {stop, Reason}
     end.
 
@@ -115,10 +97,11 @@ handle_info(_, State) ->
 code_change(_, State, _) ->
     {ok, State}.
 
-terminate(_, {Socket, MRef}) ->
+terminate(_Reason, {Socket, MRef}) ->
     %% Socket may already be down but need to ensure it is closed to avoid
     %% eaddrinuse error on restart
+    %% this takes care of that, unless of course this process is killed...
     case demonitor(MRef, [flush, info]) of
         true  -> gen_tcp:close(Socket);
         false -> ok
-    end.
+    end.