Auto merge of #6201 - jetthoughts:binstub-use-gemfile-from-env, r=indirect

Setup custom Gemfile path before loading bundler for binstubs

### What was the end-user problem that led to this PR?

While you have several gemfiles: `Gemfile` and `Gemfile.tools`.
and generates binstubs for gems from second gemfile: `BUNDLE_GEMFILE=Gemfile.tools bundle binstubs rubocop`
when you invoke those bin `bin/rubocop`
then you see error like:

```bash
/usr/local/opt/rbenv/versions/2.4.2/lib/ruby/gems/2.4.0/gems/bundler-1.16.0/lib/bundler/rubygems_integration.rb:458:in `block in replace_bin_path': can't find executable rubocop for gem rubocop. rubocop is not currently included in the bundle, perhaps you meant to add it to your Gemfile? (Gem::Exception)
	from /usr/local/opt/rbenv/versions/2.4.2/lib/ruby/gems/2.4.0/gems/bundler-1.16.0/lib/bundler/rubygems_integration.rb:489:in `block in replace_bin_path'
	from bin/rubocop:21:in `<main>'
```

### What was your diagnosis of the problem?

When you have generated `bin/bundler` by rails or by `bundler` it has setup of `BUNDLE_GEMFILE` by default as `Gemfile` or by gemfile which has been setup on `bundle binstub bundler`.

So your binstub for rubocop could not change it.

### What is your fix for the problem, implemented in this PR?

I propose to use`BUNDLE_GEMFILE` from gem's binstub over bundler's binstub version

### Why did you choose this fix out of the possible options?

This was default behavior before #5878 introduced. Just added some fix to related PR.

2 files changed, 32 insertions, 3 deletions

diff --git a/lib/bundler/templates/Executable b/lib/bundler/templates/Executable
index 9289debc26..4e35e586db 100755
--- a/lib/bundler/templates/Executable
+++ b/lib/bundler/templates/Executable
@@ -8,13 +8,13 @@
 # this file is here to facilitate running it.
 #
 
-bundle_binstub = File.expand_path("../bundle", __FILE__)
-load(bundle_binstub) if File.file?(bundle_binstub)
-
 require "pathname"
 ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../<%= relative_gemfile_path %>",
   Pathname.new(__FILE__).realpath)
 
+bundle_binstub = File.expand_path("../bundle", __FILE__)
+load(bundle_binstub) if File.file?(bundle_binstub)
+
 require "rubygems"
 require "bundler/setup"
 
diff --git a/spec/runtime/executable_spec.rb b/spec/runtime/executable_spec.rb
index 388ee049d0..dcee234e15 100644
--- a/spec/runtime/executable_spec.rb
+++ b/spec/runtime/executable_spec.rb
@@ -158,4 +158,33 @@ RSpec.describe "Running bin/* commands" do
 
     expect(bundled_app("bin/rackup").read).to_not eq("OMG")
   end
+
+  it "use BUNDLE_GEMFILE gemfile for binstub" do
+    # context with bin/bunlder w/ default Gemfile
+    bundle! "binstubs bundler"
+
+    # generate other Gemfile with executable gem
+    build_repo2 do
+      build_gem("bindir") {|s| s.executables = "foo" }
+    end
+
+    create_file("OtherGemfile", <<-G)
+      source "file://#{gem_repo2}"
+      gem 'bindir'
+    G
+
+    # generate binstub for executable from non default Gemfile (other then bin/bundler version)
+    ENV["BUNDLE_GEMFILE"] = "OtherGemfile"
+    bundle "install"
+    bundle! "binstubs bindir"
+
+    # remove user settings
+    ENV["BUNDLE_GEMFILE"] = nil
+
+    # run binstub for non default Gemfile
+    gembin "foo"
+
+    expect(exitstatus).to eq(0) if exitstatus
+    expect(out).to eq("1.0")
+  end
 end