Allow configuring default priority value at midpoint of range

[tkoar]

Summary

This PR Adds a configuration value (.assign_at_midpoint) to Delayed::Priority

The intent is to allow users to change the default priority of a job (i.e. when using a named priority) to be the middle of the configured priority ranges -- derived from the default or custom priority names -- as opposed to the starting value of the range.

By allowing a default value that is in the middle of the range, users will be able to bump the priority of some jobs such that they remain in their assigned queue but take priority over jobs of the same named priority.

Note: since the last priority uses a range that is infinite, it's not possible to derive the midpoint. So, for now, I've defaulted to adding 5, since that matches our defaulted priority values.

[smudge]

Yup, this makes sense. At some point we should consider making this the default, but opt-in is good for now.

[smudge]

I was thinking about what it meant to always call to_i on other, and also what it means to always respond with an integer. For example:

Priority.new(1) + Priority.new(10)

I would expect that to return Priority.new(11), not 11.

And also these examples:

Priority.new(10) + 1

Should also probably return Priority.new(11).

But I'm not sure what this should return:

Priority.new(10) + 0.9

(Should it round to Priority.new(11), or raise an error?)

Maybe we don't need to handle that last case for now.

So I think that leaves us with this:

Suggested change

	def -(other)
	to_i - other.to_i
	end
	def +(other)
	to_i + other.to_i
	end
	def -(other)
	new(to_i - other.to_i)
	end
	def +(other)
	new(to_i + other.to_i)
	end

Though we could do what we do in <=> and cast other to_i only if it's a priority:

other = other.to_i if other.is_a?(self.class)

WDYT?

[tkoar]

Oh yeah that's a good call! We should return a Priority instance, since that is the caller.

Hmm yeah the math complexities are interesting. I would say we probably kick the can on dealing with floats? Right now the initialize method (and the mid_point method) will ensure all instances have integer values.

[smudge]

domain LGTM && platform LGTM