[close #990] Warn on bad shebang line

This article explains binstubs and "shebang" lines: https://devcenter.heroku.com/articles/bad-ruby-binstub-shebang. A relatively common problem when generating a binstub is for it to contain a bad shebang line. Here's an example:

```
#!/usr/bin/env ruby2.5
```

If you've got a binstub with that shebang line in your project and are running on Heroku 18 then your app will be forced to use `ruby2.5` binary no matter what version of Ruby you've specified in the `Gemfile`. This causes very odd behavior and weird, difficult to debug bundler errors. 

This PR does not fix the problem, but will at least warn anyone with a version number in their ruby shebang line.

This behavior was originally:


-    Introduced: #586
-    Rolled back: #623
 

It was rolled back because it would falsely claim that `#!/usr/bin/env bash` was incorrect also it only protected against a limited set of binstubs.

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 * A bug in 2.6.0, 2.6.1, 2.6.3 require a Ruby upgrade, a warning has been added (https://github.com/heroku/heroku-buildpack-ruby/pull/1015)
 * The spring library is now disabled by setting the enviornment variable DISABLE_SPRING=1 (https://github.com/heroku/heroku-buildpack-ruby/pull/1017)
+* Warn when a bad "shebang" line in a binstub is detected (https://github.com/heroku/heroku-buildpack-ruby/pull/1014)
 * Default node version now 12.16.2, yarn is 1.22.4 (https://github.com/heroku/heroku-buildpack-ruby/pull/986)
 * Gracefully handle unrecognised stacks ([#982](https://github.com/heroku/heroku-buildpack-ruby/pull/982))
 

lib/language_pack/helpers/binstub_check.rb

@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+
+# This class is designed to check for binstubs for validity
+#
+# Example:
+#
+#   check = LanguagePack::Helpers::BinstubCheck.new(Dir.pwd, self)
+#   check.call
+class LanguagePack::Helpers::BinstubCheck
+  attr_reader :bad_binstubs
+
+  def initialize(app_root_dir:, warn_object: )
+    @bin_dir = Pathname.new(app_root_dir).join("bin")
+    @warn_object = warn_object
+    @bad_binstubs = []
+  end
+
+  def call
+    return unless @bin_dir.directory?
+
+    @bin_dir.entries.each do |basename|
+      binstub = @bin_dir.join(basename)
+      next unless binstub.file?
+
+      shebang = binstub.open(&:readline)
+
+      if shebang.match?(/^#!\s*\/usr\/bin\/env\s*ruby(\d.*)$/) # https://rubular.com/r/ozbNEPVInc3sSN
+        @bad_binstubs << binstub
+      end
+      rescue EOFError
+    end
+
+    warn unless @bad_binstubs.empty?
+  end
+
+  private def warn
+    message = <<~EOM
+      Improperly formatted binstubs detected in your project
+
+      The following file(s) have appear to contain a problematic "shebang" line
+
+      #{@bad_binstubs.map {|binstub| "  - bin/#{binstub.basename}" }.join("\n")}
+
+      For example bin/#{@bad_binstubs.first.basename} has the shebang line:
+
+      ```
+      #{@bad_binstubs.first.open(&:readline).chomp}
+      ```
+
+      It should be:
+
+      ```
+      #!/usr/bin/env ruby
+      ```
+
+      A malformed shebang line may cause your program to crash.
+
+      For more information about binstubs and "shebang" lines see:
+        https://devcenter.heroku.com/articles/bad-ruby-binstub-shebang
+    EOM
+
+    @warn_object.warn(message, inline: true)
+  end
+end

lib/language_pack/ruby.rb

@@ -10,6 +10,7 @@
 require "language_pack/helpers/yarn_installer"
 require "language_pack/helpers/jvm_installer"
 require "language_pack/helpers/layer"
+require "language_pack/helpers/binstub_check"
 require "language_pack/version"
 
 # base Ruby Language Pack. This is for any base ruby app.
@@ -96,6 +97,7 @@ def compile
       Dir.chdir(build_path)
       remove_vendor_bundle
       warn_bundler_upgrade
+      warn_bad_binstubs
       install_ruby(slug_vendor_ruby, build_ruby_path)
       install_jvm
       setup_language_pack_environment(ruby_layer_path: File.expand_path("."), gem_layer_path: File.expand_path("."))
@@ -121,10 +123,11 @@ def compile
     raise e
   end
 
+
   def build
     new_app?
     remove_vendor_bundle
-
+    warn_bad_binstubs
     ruby_layer = Layer.new(@layer_dir, "ruby", launch: true)
     install_ruby("#{ruby_layer.path}/#{slug_vendor_ruby}")
     ruby_layer.metadata[:version] = ruby_version.version
@@ -174,6 +177,19 @@ def config_detect
 
 private
 
+  # A bad shebang line looks like this:
+  #
+  # ```
+  # #!/usr/bin/env ruby2.5
+  # ```
+  #
+  # Since `ruby2.5` is not a valid binary name
+  #
+  def warn_bad_binstubs
+    check = LanguagePack::Helpers::BinstubCheck.new(app_root_dir: Dir.pwd, warn_object: self)
+    check.call
+  end
+
   def default_malloc_arena_max?
     return true if @metadata.exists?("default_malloc_arena_max")
     return @metadata.touch("default_malloc_arena_max") if new_app?

lib/language_pack/test/ruby.rb

@@ -5,6 +5,7 @@ def compile
       new_app?
       Dir.chdir(build_path)
       remove_vendor_bundle
+      warn_bad_binstubs
       install_ruby(slug_vendor_ruby, build_ruby_path)
       install_jvm
       setup_language_pack_environment(ruby_layer_path: File.expand_path("."), gem_layer_path: File.expand_path("."))

spec/helpers/binstub_check_spec.rb

@@ -0,0 +1,43 @@
+require_relative "../spec_helper.rb"
+
+describe LanguagePack::Helpers::BinstubCheck do
+  it "doesn't error on empty directories" do
+    Dir.mktmpdir do |dir|
+      warn_obj = Object.new
+      check = LanguagePack::Helpers::BinstubCheck.new(app_root_dir: dir, warn_object: warn_obj)
+      check.call
+    end
+  end
+
+  it "checks binstubs and finds bad ones" do
+    Dir.mktmpdir do |dir|
+      bin_dir = Pathname.new(dir).join("bin")
+      bin_dir.mkpath
+
+      # Bad binstub
+      bin_dir.join("bad_binstub_example").write(<<~EOM)
+        #!/usr/bin/env ruby2.5
+
+        nothing else matters
+      EOM
+
+      # Good binstub
+      bin_dir.join("good_binstub_example").write(<<~EOM)
+        #!/usr/bin/env bash
+
+        nothing else matters
+      EOM
+      bin_dir.join("good_binstub_example_two").write("#!/usr/bin/env ruby")
+
+      warn_obj = Object.new
+      def warn_obj.warn(*args, **kwargs); @msg = args.first; end
+      def warn_obj.msg; @msg; end
+
+      check = LanguagePack::Helpers::BinstubCheck.new(app_root_dir: dir, warn_object: warn_obj)
+      check.call
+
+      expect(check.bad_binstubs.count).to eq(1)
+      expect(warn_obj.msg).to include("bin/bad_binstub_example")
+    end
+  end
+end