fix double-sync

if the preamble already exists, injecting the preamble would evaluate
the gemfile, and recursively load the plugin (and plugins.rb).
avoid the problems of that by only adding our hooks in multilock.rb,
which will only be required once

.rspec

@@ -0,0 +1 @@
+--require spec_helper

lib/bundler/multilock.rb

@@ -146,6 +146,7 @@ def after_install_all(install: true)
         require "tempfile"
         require_relative "multilock/lockfile_generator"
 
+        Bundler.ui.debug("Syncing to alternate lockfiles")
         Bundler.ui.info ""
 
         default_lockfile_contents = Bundler.default_lockfile.read.freeze
@@ -321,6 +322,8 @@ def loaded?
 
       # @!visibility private
       def inject_preamble
+        Bundler.ui.debug("Injecting multilock preamble")
+
         minor_version = Gem::Version.new(::Bundler::Multilock::VERSION).segments[0..1].join(".")
         bundle_preamble1_match = %(plugin "bundler-multilock")
         bundle_preamble1 = <<~RUBY
@@ -448,3 +451,30 @@ def write_lockfile(lockfile_definition, lockfile, install:, dependency_changes:
 end
 
 Bundler::Multilock.inject_preamble unless Bundler::Multilock.loaded?
+
+# this is terrible, but we can't prepend into these modules because we only load
+# _inside_ of the CLI commands already running
+if defined?(Bundler::CLI::Check)
+  require_relative "lib/bundler/multilock/check"
+  at_exit do
+    next unless $!.nil?
+    next if $!.is_a?(SystemExit) && !$!.success?
+
+    next if Bundler::Multilock::Check.run
+
+    Bundler.ui.warn("You can attempt to fix by running `bundle install`")
+    exit 1
+  end
+end
+if defined?(Bundler::CLI::Lock)
+  at_exit do
+    next unless $!.nil?
+    next if $!.is_a?(SystemExit) && !$!.success?
+
+    Bundler::Multilock.after_install_all(install: false)
+  end
+end
+
+Bundler::Plugin.add_hook(Bundler::Plugin::Events::GEM_AFTER_INSTALL_ALL) do |_|
+  Bundler::Multilock.after_install_all
+end

plugins.rb

@@ -19,30 +19,3 @@
 #
 
 require_relative "lib/bundler/multilock"
-
-# this is terrible, but we can't prepend into these modules because we only load
-# _inside_ of the CLI commands already running
-if defined?(Bundler::CLI::Check)
-  require_relative "lib/bundler/multilock/check"
-  at_exit do
-    next unless $!.nil?
-    next if $!.is_a?(SystemExit) && !$!.success?
-
-    next if Bundler::Multilock::Check.run
-
-    Bundler.ui.warn("You can attempt to fix by running `bundle install`")
-    exit 1
-  end
-end
-if defined?(Bundler::CLI::Lock)
-  at_exit do
-    next unless $!.nil?
-    next if $!.is_a?(SystemExit) && !$!.success?
-
-    Bundler::Multilock.after_install_all(install: false)
-  end
-end
-
-Bundler::Plugin.add_hook(Bundler::Plugin::Events::GEM_AFTER_INSTALL_ALL) do |_|
-  Bundler::Multilock.after_install_all
-end

spec/bundler/multilock_spec.rb

@@ -192,7 +192,7 @@
 
       gem "concurrent-ruby", "1.2.2"
     RUBY
-      create_local_gem("test_local", "")
+      create_local_gem("test_local")
 
       invoke_bundler("install")
 
@@ -611,11 +611,32 @@
     end
   end
 
+  it "only syncs once per lockfile" do
+    with_gemfile(<<~RUBY) do
+      gemspec
+
+      lockfile("rails-7.0", default: true) do
+        gem "activesupport", "~> 7.0.0"
+      end
+    RUBY
+      create_local_gem("test", subdirectory: false)
+      invoke_bundler("install")
+      output = invoke_bundler("install", env: { "DEBUG" => "1" })
+
+      expect(output.split("\n").grep(/Syncing to alternate lockfiles/).length).to be 1
+    end
+  end
+
   private
 
-  def create_local_gem(name, content)
-    FileUtils.mkdir_p(name)
-    File.write("#{name}/#{name}.gemspec", <<~RUBY)
+  def create_local_gem(name, content = "", subdirectory: true)
+    if subdirectory
+      FileUtils.mkdir_p(name)
+      subdirectory = "#{name}/"
+    else
+      subdirectory = nil
+    end
+    File.write("#{subdirectory}#{name}.gemspec", <<~RUBY)
       Gem::Specification.new do |spec|
         spec.name          = #{name.inspect}
         spec.version       = "0.0.1"
@@ -626,6 +647,8 @@ def create_local_gem(name, content)
       end
     RUBY
 
+    return unless subdirectory
+
     File.write("#{name}/Gemfile", <<~RUBY)
       source "https://rubygems.org"
 

spec/spec_helper.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "debug"
+
 RSpec.configure do |config|
   config.expect_with :rspec do |c|
     c.max_formatted_output_length = nil