Fix max fd calc and add test

src/nix-daemon/nix-daemon.cc

@@ -291,7 +291,7 @@ static int _main(int argc, char * * argv)
                 int from = conn->from.fd;
                 int to = conn->to.fd;
 
-                auto nfds = std::max(from, to) + 1;
+                auto nfds = std::max(from, STDIN_FILENO) + 1;
                 while (true) {
                     fd_set fds;
                     FD_ZERO(&fds);

tests/local.mk

@@ -15,6 +15,7 @@ nix_tests = \
   linux-sandbox.sh \
   build-dry.sh \
   build-remote-input-addressed.sh \
+  ssh-relay.sh \
   nar-access.sh \
   structured-attrs.sh \
   fetchGit.sh \

tests/ssh-relay.sh

@@ -0,0 +1,16 @@
+source common.sh
+
+echo foo > $TEST_ROOT/hello.sh
+
+ssh_localhost=ssh://localhost
+remote_store=?remote-store=$ssh_localhost
+
+store=$ssh_localhost
+
+store+=$remote_store
+store+=$remote_store
+store+=$remote_store
+
+out=$(nix add-to-store --store "$store" $TEST_ROOT/hello.sh)
+
+[ foo = $(< $out) ]