quic: avoid deadlock in Endpoint.Close

Don't hold Endpoint.connsMu while calling Conn methods that can
indirectly depend on acquiring it.

Also change test cleanup to not wait for connections to drain
when closing a test Endpoint, removing an unnecessary 0.1s delay
in test runtime.

Fixes golang/go#64982.

Change-Id: If336e63b0a7f5b8d2ef63986d36f9ee38a92c477
Reviewed-on: https://go-review.googlesource.com/c/net/+/554695
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/quic/endpoint.go

@@ -103,25 +103,31 @@ func (e *Endpoint) LocalAddr() netip.AddrPort {
 // It waits for the peers of any open connection to acknowledge the connection has been closed.
 func (e *Endpoint) Close(ctx context.Context) error {
 	e.acceptQueue.close(errors.New("endpoint closed"))
+
+	// It isn't safe to call Conn.Abort or conn.exit with connsMu held,
+	// so copy the list of conns.
+	var conns []*Conn
 	e.connsMu.Lock()
 	if !e.closing {
-		e.closing = true
+		e.closing = true // setting e.closing prevents new conns from being created
 		for c := range e.conns {
-			c.Abort(localTransportError{code: errNo})
+			conns = append(conns, c)
 		}
 		if len(e.conns) == 0 {
 			e.udpConn.Close()
 		}
 	}
 	e.connsMu.Unlock()
+
+	for _, c := range conns {
+		c.Abort(localTransportError{code: errNo})
+	}
 	select {
 	case <-e.closec:
 	case <-ctx.Done():
-		e.connsMu.Lock()
-		for c := range e.conns {
+		for _, c := range conns {
 			c.exit()
 		}
-		e.connsMu.Unlock()
 		return ctx.Err()
 	}
 	return nil

internal/quic/endpoint_test.go

@@ -97,7 +97,7 @@ func newLocalEndpoint(t *testing.T, side connSide, conf *Config) *Endpoint {
 		t.Fatal(err)
 	}
 	t.Cleanup(func() {
-		e.Close(context.Background())
+		e.Close(canceledContext())
 	})
 	return e
 }