[close #1025] Remove default bin/rake binstub

[schneems]

After v216 was released there were several failures reported in #1025 and #1005 these are both related to bin/ being put first on the path consistently in build and run time.

After investigation, it was uncovered that bin/rake was a file we're putting in that location if the customer does not check in a bin/rake binstub. This file is generated by compiling a Ruby binary:

$ curl -O https://heroku-buildpack-ruby.s3.amazonaws.com/heroku-18/ruby-2.7.1.tgz
$ tar -xzf ruby-2.7.1.tgz bin/
$ cat bin/rake
#!/bin/sh
# -*- ruby -*-
# This file was generated by RubyGems.
#
# The application 'rake' is installed as part of a gem, and
# this file is here to facilitate running it.
#
_=_\
=begin
bindir="${0%/*}"
exec "$bindir/ruby" "-x" "$0" "$@"
=end
#!/usr/bin/env ruby

require 'rubygems'

version = ">= 0.a"

str = ARGV.first
if str
  str = str.b[/\A_(.*)_\z/, 1]
  if str and Gem::Version.correct?(str)
    version = str
    ARGV.shift
  end
end

if Gem.respond_to?(:activate_bin_path)
load Gem.activate_bin_path('rake', 'rake', version)
else
gem "rake", version
load Gem.bin_path("rake", "rake", version)
end

v216 release caused two related issues due to having `bin/rake first on the PATH.

Bad shebang lines

Issue #1005 is related to having bad shebang lines:

$ cat bin/rake | head -n 1
#!/app/vendor/ruby-2.3.8/bin/ruby

This was fixed by modifying our compilation code in: heroku/docker-heroku-ruby-builder#5

Bundler not loaded

Issue #1029 occurs because the code in bin/rake that is generated from compiling Ruby is not bundler aware. It does not load bundler. As a result:

• Git based gems do not work with the Ruby compiled bin/rake
• Referencing code from bundler like Bundler.setup does not work because it's not yet required

This behavior is described in detail in this comment: #1025 (comment)

The short version is: Without putting bin/ first on the path at build time then the binstub generated by bundler in the location vendor/bundle/bin/ takes precedence and this code (since it is generated by bundler) is bundler aware. When bin/ was moved to be first in the path, this behavior broke.

Fix implementation

This fix for #1029 works by not putting the Ruby compiled bin/rake on in the customer's bin/ folder. We keep bin/ first in the path so if a customer is checking in a binstub their binstub will be used. If they are not checking in a binstub then the bundler generated version of vendor/bundle/bin/ will be used.

This fix also would have superseded the work to fix #1005 and not required re-compiling dozens of binaries, but this bug was found and diagnosed after #1005 was reported.

Known risks

The one risk with this approach is that anyone relying on running rake assets:precompile at build time but they're not using Rake in their Gemfile will no longer work. I believe this is not a common case, and it's best practice to have a specific version of Rake in the Gemfile.lock.