Allow direct get by subject to be all subject based.

[derekcollison]

This avoids marshaling or un-marshaling but also allows subject based permissioning.

Signed-off-by: Derek Collison [email protected]

/cc @nats-io/core

[matthiashanel]

LGTM for the scope of this pr. But I believe that PURGE/DELETE/MSGGET is worth bringing up separately.

[kozlovic]

So what will happen to the other versions of direct get? Meaning as of now, the Go client is using a nats.DirectGet() or nats.DirectGetNext() option with optional sequence number to get a message at a specific sequence number. Will that go away? Since those were added very recently and not yet released, I would like to know if we pull them out, etc..

As for the kv implementation, as I previously said, the KV API has a GetRevision() API that retrieves a key at a given sequence:

	if kv.useDirect {
		_opts[0] = DirectGet()
		opts = _opts[:1]
	}
	if revision == kvLatestRevision {
		m, err = kv.js.GetLastMsg(kv.stream, b.String(), opts...)
	} else {
		m, err = kv.js.GetMsg(kv.stream, revision, opts...)
		// If a sequence was provided, just make sure that the retrieved
		// message subject matches the request.
		if err == nil && m.Subject != b.String() {
			return nil, ErrKeyNotFound
		}
	}

So with the change in the server, I could see how we would modify GetLastMsg() in case of direct to append the subject and not have a payload at all, but what about the other cases and the JSM management get message APIs.

[derekcollison]

The current direct get that takes a request body will continue to work as is today.

I would expect clients to optimize and use this method when they know they are just looking up last value by subject/key as it avoids a marshal and can be subject to security permissions, but the current direct get will continue to work.

[kozlovic]

Ok, so I read that we should try to optimize the clients GetLastMsg() when nats.DirectGet() option is provided to simply add the subject at the end, and not send a marshal request.