[rubygems/rubygems] Fix strange error when running bundle add with frozen mode set

If Gemfile is empty and there's no lockfile (situation after `bundle init`), and
`frozen` is configured, running `bundle add` will result in an strange
error, like this:

```
$ bundle add rake
, but the lockfile can't be updated because frozen mode is set

You have deleted from the Gemfile:
* rake (~> 13.2)

Run `bundle install` elsewhere and add the updated Gemfile to version control.
```

This commit fixes the problem to instead print

https://github.com/rubygems/rubygems/commit/152331a9dc

lib/bundler/definition.rb

@@ -367,6 +367,10 @@ module Bundler
     end
 
     def ensure_equivalent_gemfile_and_lockfile(explicit_flag = false)
+      return unless Bundler.frozen_bundle?
+
+      raise ProductionError, "Frozen mode is set, but there's no lockfile" unless lockfile_exists?
+
       added =   []
       deleted = []
       changed = []

lib/bundler/injector.rb

@@ -23,10 +23,7 @@ module Bundler
     # @param [Pathname] lockfile_path The lockfile in which to inject the new dependency.
     # @return [Array]
     def inject(gemfile_path, lockfile_path)
-      if Bundler.frozen_bundle?
-        # ensure the lock and Gemfile are synced
       Bundler.definition.ensure_equivalent_gemfile_and_lockfile(true)
-      end
 
       # temporarily unfreeze
       Bundler.settings.temporary(deployment: false, frozen: false) do

lib/bundler/installer.rb

@@ -69,9 +69,7 @@ module Bundler
       Bundler.create_bundle_path
 
       ProcessLock.lock do
-        if Bundler.frozen_bundle?
         @definition.ensure_equivalent_gemfile_and_lockfile(options[:deployment])
-        end
 
         if @definition.dependencies.empty?
           Bundler.ui.warn "The Gemfile specifies no dependencies"

lib/bundler/runtime.rb

@@ -10,7 +10,7 @@ module Bundler
     end
 
     def setup(*groups)
-      @definition.ensure_equivalent_gemfile_and_lockfile if Bundler.frozen_bundle?
+      @definition.ensure_equivalent_gemfile_and_lockfile
 
       # Has to happen first
       clean_load_path

spec/bundler/commands/add_spec.rb

@@ -28,6 +28,15 @@ RSpec.describe "bundle add" do
     end
   end
 
+  context "when Gemfile is empty, and frozen mode is set" do
+    it "shows error" do
+      gemfile 'source "https://gem.repo2"'
+      bundle "add bar", raise_on_error: false, env: { "BUNDLE_FROZEN" => "true" }
+
+      expect(err).to include("Frozen mode is set, but there's no lockfile")
+    end
+  end
+
   describe "without version specified" do
     it "version requirement becomes ~> major.minor.patch when resolved version is < 1.0" do
       bundle "add 'bar'"