support unix domain sockets as host in grpc_client

README.md

@@ -277,6 +277,17 @@ If no channel is specified in the options to a rpc call the `default_channel` is
 {client, #{channels => [{default_channel, [{http, "localhost", 8080, []}], #{}}]}}
 ```
 
+Unix sockets (UDS) may also be used with the same notation that is defined in `gen_tcp`. Considerations:
+* for UDS, only the `http` scheme is permitted
+* the port must strictly be `0`
+* only available on POSIX operating systems
+* abstract UDS are only available on Linux, and such sockets' names must start with a zero byte
+```
+{client, #{channels => [{default_channel, [{http, {local, "/path/to/unix/socket_name"}, 0, []}], #{}}]}}
+%% or to use an abstract Unix socket:
+%% {client, #{channels => [{default_channel, [{http, {local,  [0 | "socket_name"]}, 0, []}], #{}}]}}
+```
+
 The empty map at the end can contain configuration for the load balancing algorithm, interceptors, statistics handling and compression:
 
 ```

src/grpcbox.app.src

@@ -9,7 +9,7 @@
                  acceptor_pool,
                  gproc,
                  ctx]},
-  {env, [{client, #{channels => [%% {default_channel, round_robin, [{http, "localhost", 8080, []}], #{}}
+  {env, [{client, #{channels => [%% {default_channel, [{http, "localhost", 8080, []}], #{}}
                                 ]}},
 
          {grpc_opts, #{service_protos => [],

src/grpcbox_subchannel.erl

@@ -48,6 +48,24 @@
                              endpoint=Endpoint,
                              channel=Channel}}.
 
+%% In case of unix socket transport
+%% (defined as tuple {local, _UnixPath} in gen_tcp),
+%% there is no standard on what the authority field value
+%% should be, as HTTP/2 over UDS is not formally specified.
+%% To follow other gRPC implementations' behavior,
+%% the "localhost" value is used.
+info_map({Scheme, {local, _UnixPath} = Host, Port, _, _}, Encoding, StatsHandler) ->
+    case {Scheme, Port} of
+        %% The ssl layer is not functional over unix sockets currently,
+        %% and the port is strictly required to be 0 by gen_tcp.
+        {http, 0} ->
+            #{authority => <<"localhost">>,
+              scheme => <<"http">>,
+              encoding => Encoding,
+              stats_handler => StatsHandler};
+        _ ->
+            error({badarg, [Scheme, Host, Port]})
+    end;
 info_map({http, Host, 80, _, _}, Encoding, StatsHandler) ->
     #{authority => list_to_binary(Host),
       scheme => <<"http">>,