Synchronous Barrier call holds conn mutex

[nussjustin]

When Barrier is called with no active asynchronous subscriptions, the call will be made synchronously with both nc.mu and nc.subsMu held. Calling any other method on the Conn in the callback will than block the whole connection.

E.g. calling

conn, _ := nats.Connect("nats://127.0.0.1:4222")
conn.Barrier(func() {
	conn.TLSRequired()
})

will block since the callback is called synchronously in Barrier, which holds the two mutexes.

As long as there is at least one asynchronous subscription, the callback will be called later without any mutex held. So this will work:

conn, _ := nats.Connect("nats://127.0.0.1:4222")
conn.Subscribe("nats_is_awesome", func(_ *nats.Msg) {})
conn.Barrier(func() {
	conn.TLSRequired()
})

[kozlovic]

@nussjustin Thanks again for an excellent report (and catch!). I had originally started f as a go-routine but decided at the last minute that there was no need really. However, you are right, in this case I should have released the locks. The PR #346 addresses that. Thanks!