Add server side middleware support

doc/tutorial.md

@@ -811,7 +811,12 @@ On the server side, a message that is sent to the client can be compressed
 by applying `grpc:set_compression/2` on the stream, similar to the way
 metadata can be added using `grpc:set_headers/2` or `grpc:set_trailers/2`.
 
+## Server Middleware
+
+You can implement middleware as described in https://ninenines.eu/docs/en/cowboy/1.0/guide/middlewares/. The list of middlewares in order of execution will then need to be specified in the `middlewares` option when starting the server. The list must include `cowboy_router` and `cowboy_handler`.
+
 TODO: 
 -  Timeout.
 -  Description and improvement of the actual implementation of error
    handling.
+-  Client side middleware support

examples/route_guide/server/route_guide_logging_middleware.erl

@@ -0,0 +1,10 @@
+%% Middleware example that logs to disk
+-module(route_guide_middleware).
+-behaviour(cowboy_middleware).
+
+-export([execute/2]).
+
+execute(Req, Env) ->
+    LoggingData = "here are some logs",
+    ok = file:write_file("route_guide_middleware_log", LoggingData),
+	{ok, Req, Env}.

src/grpc_server.erl

@@ -46,6 +46,7 @@
 start(Name, Transport, Port, Services, Options) ->
     {ok, _Started} = application:ensure_all_started(grpc),
     AuthFun = get_authfun(Transport, Options),
+    Middlewares = get_middlewares(Options),
     %% All requests are dispatched to this same module (?MODULE),
     %% which means that `init/2` below will be called for each
     %% request.
@@ -57,7 +58,8 @@ start(Name, Transport, Port, Services, Options) ->
     ProtocolOpts = #{env => #{dispatch => Dispatch},
                      %% inactivity_timeout => infinity,
                      stream_handlers => [grpc_stream_handler,
-                                         cowboy_stream_h]},
+                                         cowboy_stream_h],
+                     middlewares => Middlewares},
     case Transport of
         tcp ->
             cowboy:start_clear(Name, [{port, Port}], ProtocolOpts);
@@ -146,6 +148,15 @@ get_authfun(ssl, Options) ->
 get_authfun(_, _) ->
     undefined.
 
+get_middlewares(Options) ->
+    case proplists:get_value(middlewares, Options) of
+        undefined ->
+            %% defaulting to default cowboy middlewares
+            [cowboy_router, cowboy_handler];
+        Middlewares ->
+            Middlewares
+    end.
+
 authenticate(Req, #{auth_fun := AuthFun}) when is_function(AuthFun) ->
     case cowboy_req:cert(Req) of
         undefined ->