Use stable branch for --edge option when possible

[jonathanhefner]

This alleviates the need to update the code when there is a new stable branch (for example, as done in #41454).

[matthewd]

Big +1 to anything that reduces the set of things we need to change during the branch/release process.

@jonathanhefner I think the condition here might want to be something more like Rails.gem_version < Gem::Version.new(Rails.gem_version.segments.first(2).join(".")) (though hopefully there's a nicer way to spell that?)

.. or if we want to really codify [my recollection of] recent practice, + ".0.rc1"

[jonathanhefner]

@matthewd I believe that will give the same result:

ns = %w[0 1 2]
versions = ns.product(ns, [*ns, "0.alpha", "0.rc1", "0.rc2", "0.1"]).
  map { |segments| Gem::Version.new(segments.join(".")) }

versions.all? { |v| v.prerelease? == v < Gem::Version.new(v.segments.first(2).join(".")) }
# => true

I had considered checking for rc vs alpha, but I wasn't sure if an x-y-stable branch was guaranteed to exist immediately after an x.y.0.rc1 release. I thought I remembered a lag in the past, though perhaps not. If not, I can change this to:

version = Rails.gem_version
edge_branch = version < Gem::Version.new("#{version.release}.rc1") ? "main" : [*version.segments.first(2), "stable"].join("-")

[matthewd]

My concern is that 5.2.3.rc2 (or indeed 5.2.3.beta1) should use 5-2-stable, never main -- despite it being a prerelease of 5.2.3, the 5.2 series is established, and by that point it's highly likely main has diverged in notable ways.

Gem::Version.new("5.2.3.rc2").prerelease? # => true
Gem::Version.new("5.2.3.rc2") < Gem::Version.new("5.2") # => false

[jonathanhefner]

That's a good point, though I don't think an x-y-stable branch will exist when an x.y.0.beta1 is released. But perhaps it would be best to just blow up in that case. And likewise for an x.y.0.rc1 release if x-y-stable does not exist. To that end, I've submitted #41837. It changes the condition to target main branch only for alpha versions, and, additionally, to blow up when the targeted stable branch does not exist (rather than continuing on to install webpack).