Have the zookeeper backend use the host port for the service paths, allow publishing services if the base service path already exists, and allow publishing into the root of zookeeper.

[jmeichle]

This PR solves three problems with the initially committed zookeeper backend.

Problem 1: Znode name for a registrator service is published as the PrivatePort not the PublicPort (or service.Port). This was a mistake when originally writing this and is a pretty bad flaw. The reason this is severe is because we cannot run more than a single container with the same service name and private port, since they cannot both be published in zookeeper.

A quick example, running this ubuntu container:

docker run -p 80 -it ubuntu:14.04 sleep 36000

with registrator configured for zookeeper://zk:2181/services publishes this in zookeeper:

--/services/ubuntu has 1 children
----/services/ubuntu/80 data: {"Name":"ubuntu","IP":"","PublicPort":32771,"PrivatePort":80,"ContainerID":"a9147e4a0901","Tags":[],"Attrs":{}}

The path is the base chroot (/services), the service name (ubuntu), then the "PrivatePort" (which is service.Origin.ExposedPort in registrator). This needs to be service.Port.

Problem 2: registrator cannot publish into zookeeper without a chroot style base path. When running this docker-compose.yml:

zookeeper:
  image: garland/zookeeper
  ports:
   - "2181:2181"
   - "2888:2888"
   - "3888:3888"
  environment:
   - SERVCE_2181_name=zookeeper
   - SERVCE_2888_name=zookeeper
   - SERVCE_3888_name=zookeeper
registrator:
  image: gliderlabs/registrator:master
  volumes:
   - /var/run/docker.sock:/tmp/docker.sock
  command: zookeeper://zk:2181/
  links:
   - zookeeper:zk
ubuntu:
  image: ubuntu:14.04
  ports:
   - "80"
  environment:
   - SERVICE_80_name=ubuntu
  command: sleep 36000

I get this error

registrator_1 | 2016/03/12 21:15:19 Listening for Docker events ...
registrator_1 | 2016/03/12 21:15:19 Syncing services on 3 containers
zookeeper_1   | 2016-03-12 21:15:19,122 [myid:] - INFO  [ProcessThread(sid:0 cport:-1)::PrepRequestProcessor@358] - Invalid path //ubuntu with session 0x1536caef2f60000
zookeeper_1   | 2016-03-12 21:15:19,123 [myid:] - INFO  [ProcessThread(sid:0 cport:-1)::PrepRequestProcessor@645] - Got user-level KeeperException when processing sessionid:0x1536caef2f60000 type:create cxid:0x4 zxid:0x2 txntype:-1 reqpath:n/a Error Path://ubuntu Error:KeeperErrorCode = BadArguments for //ubuntu
registrator_1 | 2016/03/12 21:15:19 zookeeper: failed to create base service node:  zk: unknown error
registrator_1 | 2016/03/12 21:15:19 added: afe6f01c1095 d3aafb922ff2:zkservicescompose_ubuntu_1:80
registrator_1 | 2016/03/12 21:15:19 ignored: d3aafb922ff2 no published ports

Registrator is attempting create the service node at //ubuntu instead of /ubuntu and fails.

Problem 3: The only the first container for a service name is published

This is due to the zookeeper plugin checking if the 'base path' in zookeeper exists (chroot + service name). If it didn't exist, zookeeper creates the node, and then continues to create the service port entries in zookeeper under the service name path. However, the nested logic prevented any publishing from occuring if the base service name path existed. This is incorrect because if the path for the service name exists, registrator should still attempt to publish services for this name.

This PR addresses all three problems. Quick example here for the above docker-compose.yml which was tweaked to run a second ubuntu container (with the same SERVICE_80_name) and modified for a test build of registrator that this PR is for:

jmeichle@jkm-ThinkPad-T440s:~/registrator$ zk-tool list /ubuntu
--/ubuntu data: 
--/ubuntu has 2 children
----/ubuntu/32785 data: {"Name":"ubuntu","IP":"","PublicPort":32785,"PrivatePort":80,"ContainerID":"911beae425bc","Tags":[],"Attrs":{}}
----/ubuntu/32784 data: {"Name":"ubuntu","IP":"","PublicPort":32784,"PrivatePort":80,"ContainerID":"c5c212cb508c","Tags":[],"Attrs":{}}

The service name is now published correctly into the root of zookeeper (/ubuntu) the service znode's name is now the PublicPort instead of the PrivatePort, and we see the services from both containers sharing the name 'ubuntu'. This enables me to run multiple containers for the same service name, after fixing the fact the second container wouldn't publish at all, and the path would have a collision since it used the PrivatePort.

[jmeichle]

@progrium ping

[jmeichle]

This addresses problem 2