From ac2c5ab4dbaf8dc3027c02ad46f6b7781db79d64 Mon Sep 17 00:00:00 2001
From: Alberto Bertogli <albertito@blitiri.com.ar>
Date: Sat, 30 Nov 2019 11:34:14 +0000
Subject: [PATCH] test: Add testlib.GetFreePort function

Some tests require picking ports, and today resort to hard-coding,
which is brittle.

This patch adds a testlib.GetFreePort function to help pick free ports.

It is not totally race-free, but is much better than hard-coding.
---
 internal/testlib/testlib.go      | 12 ++++++++++++
 internal/testlib/testlib_test.go |  7 +++++++
 2 files changed, 19 insertions(+)

diff --git a/internal/testlib/testlib.go b/internal/testlib/testlib.go
index 34299fe..d2bde57 100644
--- a/internal/testlib/testlib.go
+++ b/internal/testlib/testlib.go
@@ -3,6 +3,7 @@ package testlib
 
 import (
 	"io/ioutil"
+	"net"
 	"os"
 	"strings"
 	"testing"
@@ -52,3 +53,14 @@ func Rewrite(t *testing.T, path, contents string) error {
 
 	return err
 }
+
+// GetFreePort returns a free TCP port. This is hacky and not race-free, but
+// it works well enough for testing purposes.
+func GetFreePort() string {
+	l, err := net.Listen("tcp", "localhost:0")
+	if err != nil {
+		panic(err)
+	}
+	defer l.Close()
+	return l.Addr().String()
+}
diff --git a/internal/testlib/testlib_test.go b/internal/testlib/testlib_test.go
index 71b84ef..10c90eb 100644
--- a/internal/testlib/testlib_test.go
+++ b/internal/testlib/testlib_test.go
@@ -76,3 +76,10 @@ func TestRewrite(t *testing.T) {
 		t.Errorf("basic rewrite failed")
 	}
 }
+
+func TestGetFreePort(t *testing.T) {
+	p := GetFreePort()
+	if p == "" {
+		t.Errorf("failed to get free port")
+	}
+}