Cluster 2.0 - step 6 : Add disconnect methods to graceful shutdown

[AndreasMadsen]

It is important that a worker can die graceful so no connected clients are interrupted. This patch adds:

• worker.disconnect() stop server from accepting new connections, and close the IPC channel when all connection are ended. The documentation include an example on how to make a timeout.
• cluster.disconnect() run worker.disconnect() and all living workers and then close the handlers create in master allowing the cluster to die graceful.
• worker.on('disconnect') emits when IPC channel is closed.
• cluster.on('disconnect') same as worker.on('disconnect') just global this follow the existing pattern.

There was an related issue to this: #2088

This is about the merging in the good parts from the cluster 2.0 ( see #2038 ) pull request

[AndreasMadsen]

I'm afraide this this need a better testcase.

[AndreasMadsen]

@isaacs this is ready for review and thoughts

[AndreasMadsen]

Another approach to cluster.disconnect() graceful kill the cluster would be to make a cluster.close() function there will simply close all handlers when all workers are disconnected. The difference is that userland will need to manually call worker.disconnect() on all workers. This is much like the net.Server.close() where userland will need to end all connections before the server handler is closed.

However net.Server.close() do also stop the server from accepting new connections, if cluster.close() should do that too there wouldn't be any difference between cluster.disconnect() and cluster.close().

Somehow I needed to write this message too conclude that cluster.disconnect() is the best approach.

[AndreasMadsen]

@isaacs I have now rebased onto master and resolved all conflicts, the commits has also been squashed.

[isaacs]

Landed on master. Thanks!