Auto merge of #4643 - bundler:seg-postit-trampoline, r=indirect

[bundle] Automatically trampoline to postit

- [x] Specs

~~Except on bundle exec, since that would be too slow~~

~~If you want, I guess we could vendor postit and manually setup the `LOAD_PATH` and `-r` the full path to the vendored postit exe. Your call.~~ Done this

\c @indirect

4 files changed, 141 insertions, 2 deletions

diff --git a/spec/bundler/shared_helpers_spec.rb b/spec/bundler/shared_helpers_spec.rb
index 98f7994f..2ad0c98d 100644
--- a/spec/bundler/shared_helpers_spec.rb
+++ b/spec/bundler/shared_helpers_spec.rb
@@ -222,7 +222,7 @@ describe Bundler::SharedHelpers do
     shared_examples_for "ENV['RUBYOPT'] gets set correctly" do
       it "ensures -rbundler/setup is at the beginning of ENV['RUBYOPT']" do
         subject.set_bundle_environment
-        expect(ENV["RUBYOPT"].split(" ").first).to include("-rbundler/setup")
+        expect(ENV["RUBYOPT"].split(" ")).to start_with("-rbundler/setup")
       end
     end
 
diff --git a/spec/other/trampoline_spec.rb b/spec/other/trampoline_spec.rb
new file mode 100644
index 00000000..640c6085
--- /dev/null
+++ b/spec/other/trampoline_spec.rb
@@ -0,0 +1,137 @@
+# frozen_string_literal: true
+require "spec_helper"
+
+describe "bundler version trampolining" do
+  before do
+    ENV["BUNDLE_DISABLE_POSTIT"] = nil
+    FileUtils.rm_rf(system_gem_path)
+    FileUtils.cp_r(base_system_gems, system_gem_path)
+  end
+
+  context "version guessing" do
+    shared_examples_for "guesses" do |version|
+      it "guesses the correct bundler version" do
+        bundle! "--version"
+        expect(out).to eq("Bundler version #{version}")
+
+        if bundled_app("Gemfile").file?
+          bundle! "exec ruby -e 'puts Bundler::VERSION'"
+          expect(out).to eq(version)
+        end
+      end
+    end
+
+    context "with a lockfile" do
+      before do
+        install_gemfile ""
+        lockfile lockfile.sub(Bundler::VERSION, "1.12.0")
+      end
+
+      include_examples "guesses", "1.12.0"
+    end
+
+    context "with BUNDLER_VERSION" do
+      before do
+        ENV["BUNDLER_VERSION"] = "1.12.0"
+      end
+
+      context "with a lockfile" do
+        before { install_gemfile "" }
+        include_examples "guesses", "1.12.0"
+      end
+
+      context "without a lockfile" do
+        include_examples "guesses", "1.12.0"
+      end
+    end
+
+    context "with no hints" do
+      include_examples "guesses", Bundler::VERSION
+    end
+
+    context "with a gemfile and no lockfile" do
+      before do
+        gemfile ""
+      end
+
+      include_examples "guesses", Bundler::VERSION
+    end
+  end
+
+  context "installing missing bundler versions", :realworld => true do
+    before do
+      ENV["BUNDLER_VERSION"] = "1.12.3"
+      if Bundler::RubygemsIntegration.provides?("< 2.6.4")
+        # necessary since we intall with 2.6.4 but the specs can run against
+        # older versions that match againt the "gem" invocation
+        %w(bundle bundler).each do |exe|
+          system_gem_path.join("bin", exe).open("a") do |f|
+            f << %(\ngem "bundler", ">= 0.a"\n)
+          end
+        end
+      end
+    end
+
+    it "guesses & installs the correct bundler version" do
+      expect(system_gem_path.join("gems", "bundler-1.12.3")).not_to exist
+      bundle! "--version"
+      expect(out).to eq("Bundler version 1.12.3")
+      expect(system_gem_path.join("gems", "bundler-1.12.3")).to exist
+    end
+
+    it "fails gracefully when installing the bundler fails" do
+      ENV["BUNDLER_VERSION"] = "9999"
+      bundle "--version", :expect_err => true
+      expect(err).to start_with(<<-E.strip)
+Installing the inferred bundler version (= 9999) failed.
+If you'd like to update to the current bundler version (1.12.5) in this project, run `bundle update --bundler`.
+The error was:
+      E
+    end
+  end
+
+  context "bundle update --bundler" do
+    before do
+      simulate_bundler_version("1.11.1") do
+        install_gemfile ""
+      end
+    end
+
+    it "updates to the specified version" do
+      # HACK: since no released bundler version actually supports this feature!
+      bundle "update --bundler=1.12.0", :expect_err => true
+      expect(out).to include("Unknown switches '--bundler=1.12.0'")
+    end
+
+    it "updates to the specified (running) version" do
+      # HACK: since no released bundler version actually supports this feature!
+      bundle! "update --bundler=#{Bundler::VERSION}"
+      bundle! "--version"
+      expect(out).to eq("Bundler version #{Bundler::VERSION}")
+    end
+
+    it "updates to the running version" do
+      # HACK: since no released bundler version actually supports this feature!
+      bundle! "update --bundler"
+      bundle! "--version"
+      expect(out).to eq("Bundler version #{Bundler::VERSION}")
+    end
+  end
+
+  context "-rbundler/setup" do
+    before do
+      simulate_bundler_version("1.12.0") do
+        install_gemfile ""
+      end
+    end
+
+    it "uses the locked version" do
+      ruby! <<-R
+        require "bundler/setup"
+        puts Bundler::VERSION
+      R
+      expect(err).to be_empty
+      expect(out).to eq("1.12.0")
+    end
+  end
+end
diff --git a/spec/support/helpers.rb b/spec/support/helpers.rb
index ff41dfbf..704e2140 100644
--- a/spec/support/helpers.rb
+++ b/spec/support/helpers.rb
@@ -12,6 +12,7 @@ module Spec
       end
       FileUtils.mkdir_p(tmp)
       FileUtils.mkdir_p(home)
+      ENV["BUNDLE_DISABLE_POSTIT"] = "1"
       Bundler.send(:remove_instance_variable, :@settings) if Bundler.send(:instance_variable_defined?, :@settings)
     end
 
diff --git a/spec/support/rubygems_ext.rb b/spec/support/rubygems_ext.rb
index 64cc4144..95f596ea 100644
--- a/spec/support/rubygems_ext.rb
+++ b/spec/support/rubygems_ext.rb
@@ -11,7 +11,8 @@ module Spec
         # Rake version has to be consistent for tests to pass
         "rake" => "10.0.2",
         # 3.0.0 breaks 1.9.2 specs
-        "builder" => "2.1.2"
+        "builder" => "2.1.2",
+        "bundler" => "1.12.0",
       }
       # ruby-graphviz is used by the viz tests
       deps["ruby-graphviz"] = nil if RUBY_VERSION >= "1.9.3"