Auto merge of #5719 - bundler:seg-prefer-gems-rb, r=indirect

[2.0] Add a feature flag to prefer gems.rb to Gemfile

With `prefer_gems_rb` set (by default on 2.0), Bundler will prefer a `gems.rb` that is in the same directory as a `Gemfile`

7 files changed, 41 insertions, 9 deletions

diff --git a/lib/bundler.rb b/lib/bundler.rb
index 9bcadc381f..c6d68c49de 100644
--- a/lib/bundler.rb
+++ b/lib/bundler.rb
@@ -207,7 +207,7 @@ module Bundler
 
     def root
       @root ||= begin
-                  default_gemfile.dirname.expand_path
+                  SharedHelpers.root
                 rescue GemfileNotFound
                   bundle_dir = default_bundle_dir
                   raise GemfileNotFound, "Could not locate Gemfile or .bundle/ directory" unless bundle_dir
diff --git a/lib/bundler/feature_flag.rb b/lib/bundler/feature_flag.rb
index f472ba2b7b..34fddb1ecc 100644
--- a/lib/bundler/feature_flag.rb
+++ b/lib/bundler/feature_flag.rb
@@ -19,6 +19,7 @@ module Bundler
     settings_flag(:init_gems_rb) { bundler_2_mode? }
     settings_flag(:only_update_to_newer_versions) { bundler_2_mode? }
     settings_flag(:plugins) { @bundler_version >= Gem::Version.new("1.14") }
+    settings_flag(:prefer_gems_rb) { bundler_2_mode? }
     settings_flag(:update_requires_all_flag) { bundler_2_mode? }
 
     def initialize(bundler_version)
diff --git a/lib/bundler/settings.rb b/lib/bundler/settings.rb
index 34747cbed7..9f4d5b30a0 100644
--- a/lib/bundler/settings.rb
+++ b/lib/bundler/settings.rb
@@ -28,6 +28,7 @@ module Bundler
       no_prune
       only_update_to_newer_versions
       plugins
+      prefer_gems_rb
       silence_root_warning
       update_requires_all_flag
     ].freeze
diff --git a/lib/bundler/shared_helpers.rb b/lib/bundler/shared_helpers.rb
index 593d2c1e06..39f0423f7f 100644
--- a/lib/bundler/shared_helpers.rb
+++ b/lib/bundler/shared_helpers.rb
@@ -19,9 +19,15 @@ end
 
 module Bundler
   module SharedHelpers
-    def default_gemfile
+    def root
       gemfile = find_gemfile
       raise GemfileNotFound, "Could not locate Gemfile" unless gemfile
+      Pathname.new(gemfile).untaint.expand_path.parent
+    end
+
+    def default_gemfile
+      gemfile = find_gemfile(:order_matters)
+      raise GemfileNotFound, "Could not locate Gemfile" unless gemfile
       Pathname.new(gemfile).untaint
     end
 
@@ -137,7 +143,7 @@ module Bundler
     end
 
     def print_major_deprecations!
-      deprecate_gemfile(find_gemfile) if find_gemfile == find_file("Gemfile")
+      deprecate_gemfile(find_gemfile) if find_gemfile(:order_matters) == find_file("Gemfile")
       if RUBY_VERSION < "2"
         major_deprecation("Bundler will only support ruby >= 2.0, you are running #{RUBY_VERSION}")
       end
@@ -172,10 +178,16 @@ module Bundler
 
   private
 
-    def find_gemfile
+    def find_gemfile(order_matters = false)
       given = ENV["BUNDLE_GEMFILE"]
       return given if given && !given.empty?
-      find_file("Gemfile", "gems.rb")
+      names = gemfile_names
+      names.reverse! if order_matters && Bundler.feature_flag.prefer_gems_rb?
+      find_file(*names)
+    end
+
+    def gemfile_names
+      ["Gemfile", "gems.rb"]
     end
 
     def find_file(*names)
@@ -228,7 +240,7 @@ module Bundler
       end
 
       # Set BUNDLE_GEMFILE
-      Bundler::SharedHelpers.set_env "BUNDLE_GEMFILE", find_gemfile.to_s
+      Bundler::SharedHelpers.set_env "BUNDLE_GEMFILE", find_gemfile(:order_matters).to_s
       Bundler::SharedHelpers.set_env "BUNDLER_VERSION", Bundler::VERSION
     end
 
diff --git a/man/bundle-config.ronn b/man/bundle-config.ronn
index d387350f8b..4fa0025815 100644
--- a/man/bundle-config.ronn
+++ b/man/bundle-config.ronn
@@ -223,9 +223,11 @@ learn more about their operation in [bundle install(1)][bundle-install].
    Sets the `--key` parameter for `gem push` when using the `rake release`
    command with a private gemstash server.
 * `error_on_stderr` (`BUNDLE_ERROR_ON_STDERR`)
-   Print Bundler errors to stderr
+   Print Bundler errors to stderr.
 * `init_gems_rb` (`BUNDLE_NEW_GEMFILE_NAME`)
-   generate a new gems.rb on `bundle init` instead of the `Gemfile`
+   Generate a `gems.rb` instead of a `Gemfile` when running `bundle init`.
+* `prefer_gems_rb` (`BUNDLE_PREFER_GEMS_RB`)
+   Prefer `gems.rb` to `Gemfile` when Bundler is searching for a Gemfile.
 * `update_requires_all_flag` (`BUNDLE_UPDATE_REQUIRES_ALL_FLAG`)
    Require passing `--all` to `bundle update` when everything should be updated,
    and disallow passing no options to `bundle update`.
diff --git a/spec/bundler/definition_spec.rb b/spec/bundler/definition_spec.rb
index 4e93d5e01c..cb642da9f7 100644
--- a/spec/bundler/definition_spec.rb
+++ b/spec/bundler/definition_spec.rb
@@ -5,7 +5,7 @@ RSpec.describe Bundler::Definition do
   describe "#lock" do
     before do
       allow(Bundler).to receive(:settings) { Bundler::Settings.new(".") }
-      allow(Bundler).to receive(:default_gemfile) { Pathname.new("Gemfile") }
+      allow(Bundler::SharedHelpers).to receive(:find_gemfile) { Pathname.new("Gemfile") }
       allow(Bundler).to receive(:ui) { double("UI", :info => "", :debug => "") }
     end
     context "when it's not possible to write to the file" do
diff --git a/spec/install/gemfile_spec.rb b/spec/install/gemfile_spec.rb
index 9262dcc92d..d1b1f1e6b8 100644
--- a/spec/install/gemfile_spec.rb
+++ b/spec/install/gemfile_spec.rb
@@ -66,6 +66,22 @@ RSpec.describe "bundle install" do
     end
   end
 
+  context "with prefer_gems_rb set" do
+    before { bundle! "config prefer_gems_rb true" }
+
+    it "prefers gems.rb to Gemfile" do
+      create_file("gems.rb", "gem 'bundler'")
+      create_file("Gemfile", "raise 'wrong Gemfile!'")
+
+      bundle! :install
+
+      expect(bundled_app("gems.rb")).to be_file
+      expect(bundled_app("Gemfile.lock")).not_to be_file
+
+      expect(the_bundle).to include_gem "bundler #{Bundler::VERSION}"
+    end
+  end
+
   context "with engine specified in symbol" do
     it "does not raise any error parsing Gemfile" do
       simulate_ruby_version "2.3.0" do