Finding a Free Port in Go

I like testing Go HTTP servers by starting a new temporary web server. I usually have 10s-100s of tests, so if these tests are to run in parallel then the web servers need to start on random, free ports.

I prefer keeping codebases to the standard library, so here's the snippet that tends to get reproduced in my projects.

The approach is to first bind to a random open port using the standard library's helper for that. Then retrieve the now-bound port number, close the socket listening on the port and hand the port over to our web server.

This approach does mean there's potential for a race condition between closing the bound port and opening it in the test web server, but every engineering approach has drawbacks and this one's no different.

If you don't want to deal with the potential for a race condition, there are other great approaches to testing http server functionality, such as the httptest library.


import "net"

func newTestServer(t *testing.T) OurCustomServer {
	t.Helper()

	listener, err := net.Listen("tcp", "localhost:0")

	if err != nil {
		t.Fatal(err)
	}

	freePort := listener.Addr().String()
	listener.Close()

	server := newServer(freePort)

	return server
	
}

I like taking some time to write this sort of code out by hand; although ChatGPT can undoubtedly knock this out in seconds, taking the time to write this down and understand it reminds me that Moving Fastest isn't one of my values. I prefer mastery of fundamentals.

A lot of our career is spent trying to achieve the next functionality so that $STARTUP has a chance at winning the market. It's nice sometimes to take a breath and implement something simple and rock solid.