server: enable custom listener API

server/run_option.go

@@ -2,11 +2,14 @@ package server
 
 import (
 	"context"
+	"net"
 )
 
 // RunOptions holds a set of configurations for Server.Run().
 type RunOptions struct {
 	gracefullCtx context.Context
+
+	listener net.Listener
 }
 
 // RunOption is an option of Server.Run().
@@ -19,3 +22,10 @@ func WithGracefullContext(ctx context.Context) RunOption {
 		options.gracefullCtx = ctx
 	}
 }
+
+// WithListener allows the caller to override the default listener
+func WithListener(l net.Listener) RunOption {
+	return func(options *RunOptions) {
+		options.listener = l
+	}
+}

server/server.go

@@ -117,9 +117,13 @@ func (server *Server) Run(ctx context.Context, options ...RunOption) error {
 		log.Printf("Port number configured to `0`, choosing a random port")
 	}
 	hostPort := net.JoinHostPort(server.options.Address, server.options.Port)
-	listener, err := net.Listen("tcp", hostPort)
-	if err != nil {
-		return errors.Wrapf(err, "failed to listen at `%s`", hostPort)
+
+	listener := opts.listener
+	if listener == nil {
+		listener, err = net.Listen("tcp", hostPort)
+		if err != nil {
+			return errors.Wrapf(err, "failed to listen at `%s`", hostPort)
+		}
 	}
 
 	scheme := "http"