start: introduce function-args as alternative to env variable params

the Start function always assumes there are environment variables such
as HOST and PORT on start up. this is not always desirable and
especially makes it hard to run independent tests concurrently.

this commit introduces StartConf, an alternative to Start where the
same settings like host and port are passed in a function argument
instead of the global process environment.

start.go

@@ -11,24 +11,30 @@ import (
 	"github.com/rs/zerolog"
 )
 
+// Settings specify initial startup parameters for a relay server.
+// See StartConf for details.
 type Settings struct {
 	Host string `envconfig:"HOST" default:"0.0.0.0"`
 	Port string `envconfig:"PORT" default:"7447"`
 }
 
-var (
-	s   Settings
-	log = zerolog.New(os.Stderr).Output(zerolog.ConsoleWriter{Out: os.Stderr})
-)
+var log = zerolog.New(os.Stderr).Output(zerolog.ConsoleWriter{Out: os.Stderr})
 
 var Router = mux.NewRouter()
 
+// Start calls StartConf with Settings parsed from the process environment.
 func Start(relay Relay) {
-	// read host/port (implementations can read other stuff on their own if they need)
+	var s Settings
 	if err := envconfig.Process("", &s); err != nil {
 		log.Panic().Err(err).Msg("couldn't process envconfig")
 	}
+	StartConf(s, relay)
+}
 
+// StartConf initalizes the relay and its storage using their respective Init methods,
+// and starts listening for HTTP requests on host:port, as specified in the settings.
+// It never returns until process termination.
+func StartConf(s Settings, relay Relay) {
 	// allow implementations to do initialization stuff
 	if err := relay.Init(); err != nil {
 		Log.Fatal().Err(err).Msg("failed to start")