Drop Ruby 2.3 support.

[casperisfine]

Ref: #402
Ref: https://bugs.ruby-lang.org/issues/10222
Ref: ruby/ruby@5754f15
Ref: ruby/ruby@b6d3927

Up to Ruby 2.3, require would resolve symlinks, but require_relative wouldn't:

require 'fileutils'
FileUtils.mkdir_p("realpath")
File.write("realpath/a.rb", "p :a_loaded")
File.symlink("realpath", "symlink") rescue nil

$LOAD_PATH.unshift(File.realpath(__dir__) + "/symlink")
require "a.rb" # load symlink/a.rb in 2.3 and older, load realpath/a.rb on 2.4 and newer
require_relative "realpath/a.rb" # noop on 2.4+

This would easily cause double loading issue when require and require_relative were mixed, but was fixed in 2.4 (https://bugs.ruby-lang.org/issues/10222).

The problem is that Bootsnap kinda negated this fix, because realpath() wouldn't be applied to absolute paths:

require 'fileutils'
FileUtils.mkdir_p("realpath")
File.write("realpath/a.rb", "p :a_loaded")
File.symlink("realpath", "symlink") rescue nil

$LOAD_PATH.unshift(File.realpath(__dir__) + "/symlink")
require File.expand_path("symlink/a.rb") # load symlink/a.rb in 3.0 and older, load realpath/a.rb on 3.1 and newer
require_relative "realpath/a.rb" # noop on 3.1+

And for performance reasons, Bootsnap tried really hard not to call realpath, as it's a syscall, instead it used expand_path, which is entirely in use space and doesn't reach to the file system. So if you had a symlink in $LOAD_PATH, bootsnap would perpetuate this bug, which led to the addition of #136.

This was ultimately fixed in Ruby 3.1 (https://bugs.ruby-lang.org/issues/17885), now realpath is applied even on absolute paths.

Solution

While realpath is indeed expensive, I think the performance impact is ok if we only call it for $LOAD_PATH members, rather than for all requirable files. So if you have X gems, it's going to be more or less X realpath calls.

It would stay a problem if a gem actually contained symlinks and used require_relative, but it's quite the stretch, and with 3.1 now handling it, it's not worth keeping such workaround.

#402