net: TCPListener fails to return errors after Close on js/wasm

[bcmills]

The test in https://go.dev/play/p/TO_F52CqMt8 consistently passes locally.
However, it consistently fails under js/wasm (CC @neelance):

$ GOOS=js GOARCH=wasm go test -v
=== RUN   TestTCPListenAfterClose
    listener_test.go:51: after l.Close(), l.Accept() = _, <nil>
        want use of closed network connection
--- FAIL: TestTCPListenAfterClose (0.01s)
FAIL

As far as I can tell this violates the net.Listener interface, which requires (emphasis mine):

Close closes the listener.
Any blocked Accept operations will be unblocked and return errors.

I noticed this via #22926.

package main

import (
	"context"
	"errors"
	"net"
	"sync"
	"testing"
	"time"
)

func TestTCPListenAfterClose(t *testing.T) {
	l, err := net.Listen("tcp", "127.0.0.1:0")
	if err != nil {
		t.Fatal(err)
	}

	var wg sync.WaitGroup
	ctx, cancel := context.WithCancel(context.Background())

	d := &net.Dialer{}
	for n := 2; n > 0; n-- {
		wg.Add(1)
		go func() {
			defer wg.Done()

			c, err := d.DialContext(ctx, l.Addr().Network(), l.Addr().String())
			if err == nil {
				<-ctx.Done()
				c.Close()
			}
		}()
	}

	c, err := l.Accept()
	if err == nil {
		c.Close()
	} else {
		t.Error(err)
	}
	time.Sleep(10 * time.Millisecond)
	cancel()
	wg.Wait()
	l.Close()

	c, err = l.Accept()
	if !errors.Is(err, net.ErrClosed) {
		if err == nil {
			c.Close()
		}
		t.Errorf("after l.Close(), l.Accept() = _, %v\nwant %v", err, net.ErrClosed)
	}
}

[gopherbot]

Change https://golang.org/cl/372714 mentions this issue: netutil: make LimitListener tests more robust

[bcmills]

From further testing in CL 372714, it appears that the client ends of the buffered connections are not closed until the server ends have been returned (and closed) by Accept.

If the server closes the listener without subsequently draining the Accept queue, that can cause the client sides of those pending connections to hang indefinitely.

[gopherbot]

Change https://go.dev/cl/526117 mentions this issue: net: enable most tests on wasip1 and js

[gopherbot]

Change https://go.dev/cl/557176 mentions this issue: net: remove an unused sync.Map in the fake net implementation