What is the purpose of dialer's control function?

I am currently studying network programming in Go.

Currently, a timeout test using the deadline of the context is in progress by myself.

package socket

import (
	"context"
	"net"
	"syscall"
	"testing"
	"time"
)

func TestDialContext(t *testing.T) {
	deadline := time.Now().Add(time.Second * 5)
	ctx, cancelFunc := context.WithDeadline(context.Background(), deadline)
	defer cancelFunc()

	var dialer net.Dialer
	dialer.Control = func(_, _ string, _ syscall.RawConn) error {
		time.Sleep(4 * time.Second)
		return nil
	}
	conn, err := dialer.DialContext(ctx, "tcp", "10.0.0.0:http")
	if err == nil {
		conn.Close()
		t.Fatal("expected no connection")
	}
	if nErr, ok := err.(net.Error); ok && nErr.Timeout() {
		t.Logf("expected timeout error: %v", err)
	} else {
		t.Errorf("expected timeout error; actual: %v", err)
	}
	if ctx.Err() != context.DeadlineExceeded {
		t.Errorf("expected deadline exceeded; actual: %v", ctx.Err())
	}
}

As you can see from the code, I tried to connect using a context with a 5-second deadline.

In addition, by using the dialer's control function when requested, the control function is called immediately after being connected (actually right before being connected), so that a nil error is returned after 4 seconds.

By my intent, this test should fail. The reason is that the control function returns a nil error before the deadline of 5 seconds is reached.

But the test succeeds as below.

=== RUN   TestDialContext
    dial_context_test.go:27: expected timeout error: dial tcp 10.0.0.0:80: i/o timeout
--- PASS: TestDialContext (5.01s)
PASS

What exactly is the purpose of the control function? Is the error returned by the control function unrelated to the error returned by the net.Dial() function? Is the test using the control function like the code I wrote meaningless?

Thanks for reading this long question!

> What exactly is the purpose of the control function?

To give the dialing code a chance to call functions on the socket's underlying file descriptor — e.g. one of the syscall.Setsockopt functions — after the socket is created and before the connection is established.

> Is the error returned by the control function unrelated to the error returned by the net.Dial() function?

If the control function returns an error, then dialing fails with that error. If the control function returns a nil error, then dialing continues as usual.

> Is the test using the control function like the code I wrote meaningless?

Pretty much. Your control function sleeps 4 seconds but otherwise doesn't do anything.