[rubygems/rubygems] Fix installing plugins via relative paths

This affected both CLI and Gemfile installs

https://github.com/rubygems/rubygems/commit/a0d101a8df

lib/bundler/plugin/installer.rb

@@ -77,7 +77,7 @@ module Bundler
 
       def install_path(names, version, path)
         source_list = SourceList.new
-        source = source_list.add_path_source({ "path" => path })
+        source = source_list.add_path_source({ "path" => path, "root_path" => SharedHelpers.pwd })
 
         install_all_sources(names, version, source_list, source)
       end

lib/bundler/plugin/installer/path.rb

@@ -5,7 +5,7 @@ module Bundler
     class Installer
       class Path < Bundler::Source::Path
         def root
-          Plugin.root
+          SharedHelpers.in_bundle? ? Bundler.root : Plugin.root
         end
 
         def generate_bin(spec, disable_extensions = false)

spec/bundler/plugins/install_spec.rb

@@ -212,14 +212,42 @@ RSpec.describe "bundler plugin install" do
     end
   end
 
-  it "installs from a path source" do
-    build_lib "path_plugin" do |s|
-      s.write "plugins.rb"
-    end
-    bundle "plugin install path_plugin --path #{lib_path("path_plugin-1.0")}"
+  context "path plugins" do
+    it "installs from a path source" do
+      build_lib "path_plugin" do |s|
+        s.write "plugins.rb"
+      end
+      bundle "plugin install path_plugin --path #{lib_path("path_plugin-1.0")}"
 
-    expect(out).to include("Installed plugin path_plugin")
-    plugin_should_be_installed("path_plugin")
+      expect(out).to include("Installed plugin path_plugin")
+      plugin_should_be_installed("path_plugin")
+    end
+
+    it "installs from a relative path source" do
+      build_lib "path_plugin" do |s|
+        s.write "plugins.rb"
+      end
+      path = lib_path("path_plugin-1.0").relative_path_from(bundled_app)
+      bundle "plugin install path_plugin --path #{path}"
+
+      expect(out).to include("Installed plugin path_plugin")
+      plugin_should_be_installed("path_plugin")
+    end
+
+    it "installs from a relative path source when inside an app" do
+      allow(Bundler::SharedHelpers).to receive(:find_gemfile).and_return(bundled_app_gemfile)
+      gemfile ""
+
+      build_lib "ga-plugin" do |s|
+        s.write "plugins.rb"
+      end
+
+      path = lib_path("ga-plugin-1.0").relative_path_from(bundled_app)
+      bundle "plugin install ga-plugin --path #{path}"
+
+      plugin_should_be_installed("ga-plugin")
+      expect(local_plugin_gem("foo-1.0")).not_to be_directory
+    end
   end
 
   context "Gemfile eval" do
@@ -291,6 +319,21 @@ RSpec.describe "bundler plugin install" do
       plugin_should_be_installed("ga-plugin")
     end
 
+    it "accepts relative path sources" do
+      build_lib "ga-plugin" do |s|
+        s.write "plugins.rb"
+      end
+
+      path = lib_path("ga-plugin-1.0").relative_path_from(bundled_app)
+      install_gemfile <<-G
+        source "#{file_uri_for(gem_repo1)}"
+        plugin 'ga-plugin', :path => "#{path}"
+      G
+
+      expect(out).to include("Installed plugin ga-plugin")
+      plugin_should_be_installed("ga-plugin")
+    end
+
     context "in deployment mode" do
       it "installs plugins" do
         install_gemfile <<-G