dgram: extract cluster lazy loading method to make it testable
There is no need to ensure cluster
loaded in bindServerHandle
because
it's already loaded before calling it.
This PR makes cluster
as an argument to remind developers that cluster
needs to be loaded before calling this function.
Update:
The lazy load parts in bindServerHandle
is unreachable because cluster
has been loaded before calling it. Extract it to a seperated method to make the code testable.
Refs: https://coverage.nodejs.org/coverage-26e318a321a872bc/lib/dgram.js.html#L202