net: unblock UDP Reads upon Close on plan9, add test

Fixes #72770

Change-Id: I42be7c7349961188f4b5d73287a3550aba323893
Reviewed-on: https://go-review.googlesource.com/c/go/+/656395
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/net/fd_plan9.go

@@ -126,6 +126,11 @@ func (fd *netFD) Close() error {
 			return err
 		}
 	}
+	if fd.net == "udp" {
+		// The following line is required to unblock Reads.
+		// See https://go.dev/issue/72770.
+		fd.SetReadDeadline(time.Now().Add(-time.Hour))
+	}
 	err := fd.ctl.Close()
 	if fd.data != nil {
 		if err1 := fd.data.Close(); err1 != nil && err == nil {

src/net/net_test.go

@@ -508,6 +508,32 @@ func TestCloseUnblocksRead(t *testing.T) {
 	withTCPConnPair(t, client, server)
 }
 
+// Issue 72770: verify that a blocked UDP read is woken up by a Close.
+func TestCloseUnblocksReadUDP(t *testing.T) {
+	t.Parallel()
+	pc, err := ListenPacket("udp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	time.AfterFunc(250*time.Millisecond, func() {
+		t.Logf("closing conn...")
+		pc.Close()
+	})
+	timer := time.AfterFunc(time.Second*10, func() {
+		panic("timeout waiting for Close")
+	})
+	defer timer.Stop()
+
+	n, src, err := pc.(*UDPConn).ReadFromUDPAddrPort([]byte{})
+
+	// Check for n > 0. Checking err == nil alone isn't enough;
+	// on macOS, it returns (n=0, src=0.0.0.0:0, err=nil).
+	if n > 0 {
+		t.Fatalf("unexpected Read success from ReadFromUDPAddrPort; read %d bytes from %v, err=%v", n, src, err)
+	}
+	t.Logf("got expected UDP read error")
+}
+
 // Issue 24808: verify that ECONNRESET is not temporary for read.
 func TestNotTemporaryRead(t *testing.T) {
 	t.Parallel()