libnetwork prep for docker port persistence policies.

[bru7us]

This is the libnetwork portion of work required to define port persistence policies (see moby/moby#12622)

Requesting a port without a range will act in the same way as before (docker master does this).
Specifying a range with port "0" will allocate the next available port from that range.
Specifying a range and a non-zero port will first try to allocate the requested port, then fallback to next available in the range if the requested port is already allocated.

Signed-off-by: Dave Russell bru7us@bru7us.com

[bru7us]

@mavenugo, this a minor extension to #285 that just merged today. It enables "preferred" port allocation requests from a range that will support port persistence policies.

All we do here is:

• Add HostPortStart to the PortBinding (so we now have explicit requested port, range start and range end)
• Propagate the added element through all dependent function calls
• Deliver "preferred" port allocation requests from a specified range

This should still be back-compatible with current docker master, but conflicts slightly with PR moby/moby#12927 (@dkjer), which would need to also add the separation of HostPort and HostPortStart if you accept this PR.

[bru7us]

Now that moby/moby#12927 got merged - refactored this one to stay backwards compatible with docker master.

[bru7us]

Hey @mavenugo, was hoping to get some feedback on this part before I rewrite the docker/docker side. I think I'd use slightly different wording for the policies, something like "--port-persistence = [ default | preferred | enforced ]", but apart from that, the concept hasn't really changed much in my mind from where we ended up in moby/moby#12622

[bru7us]

@mavenugo @aboch @mrjana, Any input on this one?

My office had a big outage this morning and had to spend a lot of time reconfiguring our load balancer/proxy after all of our docker hosts restarted. These port persistence policies would have allowed us to auto-recover with no manual input other than restarting the hosts and containers.

Would love to continue pressing forward with this.

[GordonTheTurtle]

@bru7us It has been detected that this issue has not received any activity in over 6 months. Can you please let us know if it is still relevant:

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