Support Ephemeral ports #5126
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Support Ephemeral ports #5126
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Sealed interfaces are useful in switch exprs to enumerate possible values so that it becomes a compile error to introduce a new server type without implementing an appropriate arm for the switch block.
ServiceAnnouncer requires a non-null String, but an empty server ID is not well-formed so a ServiceAnnouncer shouldn't be created. This was previously handled by MapToolServer being passed null config and the absence of a port to give was sufficient to prevent creating a ServiceAnnouncer with an empty ID, but we can be more defensive about this and check for a null pointer.
This adds extra complexity from the ServiceAnnouncer not always existing but enables later flexibility in supporting binding to an emphemeral port instead of having to preconfigure it.
It is neater to stop announcement, deregister and close UPnP before stopping the server so that discovering clients can't try to connect to an already stopped server though this is mostly just being picky because the human interaction time scales dominate and shut down and deregister are nearly the same time.
If a port number of 0 is chosen then bind() will open a random port in an ephemeral range instead of the provided port number and this port can then be retrieved using getLocalPort(). This enables starting servers on arbitrary ports instead of preconfigured ones. Since only direct connections need to know the port number connections using the registry or LAN can use an ephemeral port number and the connection information tab can display the port direct connections should use. This requires a router with functioning UPnP instead of port forwarding though so is not univerally useful.
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Identify the Bug or Feature request
Closes #5125
Depends on #5124 or #5131
Description of the Change
This relaxes the validation of the server port field to allow 0, changes code that used the configured port for sending to the registry or advertising with service discovery to use the value from the bound socket, and ensures they are configured after the socket has been bound so the port can be correctly determined.
Possible Drawbacks
If a user specifies it as 0 without using UPnP then connections will probably fail and those error messages are less informative than having picked an invalid port.
Release Notes
This change is