Use correct Bundler version when updating gems

Before, `update-interactive` would update gems using whatever `bundle`
binary was in the PATH. On Rails projects, there is often a binstub for
Bundler located at `bin/bundle`. If this `bin` directory is in the PATH,
then the binstub is executed instead of Bundler being executed directly.

Using the binstub is problematic, because the binstub locks the version
of Bundler to the `BUNDLED WITH` value of the lock file. The binstub
makes an exception for this behavior when running `bin/bundle update`,
so that the latest version of Bundler is used when updating gems.
However, this auto-detection only works when the binstub is invoked
directly in the shell. Since `update-interactive` wraps the invocation
of the update command, the binstub falls back to locking the version of
Bundler at whatever is in the lock file.

As a result, `update-interactive` would not update the lockfile with the
version of Bundler that the user intended.

Fix by using `Gem.bin_path` to get the correct Bundler executable,
rather than using a binstub that might be in the user's PATH.

lib/bundle_update_interactive/bundler_commands.rb

@@ -1,22 +1,31 @@
 # frozen_string_literal: true
 
+require "bundler"
 require "shellwords"
 
 module BundleUpdateInteractive
   module BundlerCommands
     class << self
       def update_gems_conservatively(*gems)
-        system "bundle update --conservative #{gems.flatten.map(&:shellescape).join(' ')}"
+        system "#{bundle_bin.shellescape} update --conservative #{gems.flatten.map(&:shellescape).join(' ')}"
       end
 
       def read_updated_lockfile(*gems)
-        command = ["bundle lock --print"]
+        command = ["#{bundle_bin.shellescape} lock --print"]
         command << "--conservative" if gems.any?
         command << "--update"
         command.push(*gems.flatten.map(&:shellescape))
 
         `#{command.join(" ")}`.tap { raise "bundle lock command failed" unless Process.last_status.success? }
       end
+
+      private
+
+      def bundle_bin
+        Gem.bin_path("bundler", "bundle", Bundler::VERSION)
+      rescue Gem::GemNotFoundException
+        "bundle"
+      end
     end
   end
 end

test/bundle_update_interactive/bundler_commands_test.rb

@@ -1,25 +1,33 @@
 # frozen_string_literal: true
 
 require "test_helper"
+require "bundler"
 
 module BundleUpdateInteractive
   class BundlerCommandsTest < Minitest::Test
+    def setup
+      Gem.stubs(:bin_path).with("bundler", "bundle", Bundler::VERSION).returns("/exe/bundle")
+    end
+
     def test_read_updated_lockfile_runs_bundle_lock_and_captures_output
-      expect_backticks("bundle lock --print --update", captures: "bundler output")
+      expect_backticks("/exe/bundle lock --print --update", captures: "bundler output")
       result = BundlerCommands.read_updated_lockfile
 
       assert_equal "bundler output", result
     end
 
     def test_read_updated_lockfile_runs_bundle_lock_with_specified_gems_conservatively
-      expect_backticks("bundle lock --print --conservative --update actionpack railties", captures: "bundler output")
+      expect_backticks(
+        "/exe/bundle lock --print --conservative --update actionpack railties",
+        captures: "bundler output"
+      )
       result = BundlerCommands.read_updated_lockfile("actionpack", "railties")
 
       assert_equal "bundler output", result
     end
 
     def test_read_updated_lockfile_raises_if_bundler_fails_to_run
-      expect_backticks("bundle lock --print --update", success: false)
+      expect_backticks("/exe/bundle lock --print --update", success: false)
 
       error = assert_raises(RuntimeError) { BundlerCommands.read_updated_lockfile }
       assert_match(/bundle lock command failed/i, error.message)