{control,net}: close idle connections of custom transports

I noticed a few places with custom http.Transport where we are not
closing idle connections when transport is no longer used.

Updates tailscale/corp#21609

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

control/controlclient/direct.go

@@ -333,6 +333,9 @@ func (c *Direct) Close() error {
 		}
 	}
 	c.noiseClient = nil
+	if tr, ok := c.httpc.Transport.(*http.Transport); ok {
+		tr.CloseIdleConnections()
+	}
 	return nil
 }
 

net/dnsfallback/dnsfallback.go

@@ -281,6 +281,7 @@ type nameIP struct {
 func bootstrapDNSMap(ctx context.Context, serverName string, serverIP netip.Addr, queryName string, logf logger.Logf, ht *health.Tracker, netMon *netmon.Monitor) (dnsMap, error) {
 	dialer := netns.NewDialer(logf, netMon)
 	tr := http.DefaultTransport.(*http.Transport).Clone()
+	tr.DisableKeepAlives = true // This transport is meant to be used once.
 	tr.Proxy = tshttpproxy.ProxyFromEnvironment
 	tr.DialContext = func(ctx context.Context, netw, addr string) (net.Conn, error) {
 		return dialer.DialContext(ctx, "tcp", net.JoinHostPort(serverIP.String(), "443"))

net/tsdial/tsdial.go

@@ -166,6 +166,7 @@ func (d *Dialer) Close() error {
 		c.Close()
 	}
 	d.activeSysConns = nil
+	d.PeerAPITransport().CloseIdleConnections()
 	return nil
 }