Show an error when trying to update bundler in frozen mode

Previously `bundle update --bundler` would succeed but fail to update
the lockfile.

bundler/lib/bundler/definition.rb

@@ -886,7 +886,8 @@ def lockfiles_equal?(current, proposed, preserve_unknown_sections)
       if preserve_unknown_sections
         sections_to_ignore = LockfileParser.sections_to_ignore(@locked_bundler_version)
         sections_to_ignore += LockfileParser.unknown_sections_in_lockfile(current)
-        sections_to_ignore += LockfileParser::ENVIRONMENT_VERSION_SECTIONS
+        sections_to_ignore << LockfileParser::RUBY
+        sections_to_ignore << LockfileParser::BUNDLED unless @unlocking_bundler
         pattern = /#{Regexp.union(sections_to_ignore)}\n(\s{2,}.*\n)+/
         whitespace_cleanup = /\n{2,}/
         current = current.gsub(pattern, "\n").gsub(whitespace_cleanup, "\n\n").strip

bundler/lib/bundler/installer.rb

@@ -90,7 +90,7 @@ def run(options)
 
         Gem::Specification.reset # invalidate gem specification cache so that installed gems are immediately available
 
-        lock unless Bundler.frozen_bundle?
+        lock
         Standalone.new(options[:standalone], @definition).generate if options[:standalone]
       end
     end

bundler/lib/bundler/lockfile_parser.rb

@@ -26,6 +26,7 @@ class LockfileParser
     KNOWN_SECTIONS = SECTIONS_BY_VERSION_INTRODUCED.values.flatten.freeze
 
     ENVIRONMENT_VERSION_SECTIONS = [BUNDLED, RUBY].freeze
+    deprecate_constant(:ENVIRONMENT_VERSION_SECTIONS)
 
     def self.sections_in_lockfile(lockfile_contents)
       lockfile_contents.scan(/^\w[\w ]*$/).uniq

bundler/spec/commands/update_spec.rb

@@ -1451,6 +1451,31 @@
       expect(out).to include("Using bundler 2.3.9")
     end
   end
+
+  it "prints an error when trying to update bundler in frozen mode" do
+    system_gems "bundler-2.3.9"
+
+    gemfile <<~G
+      source "#{file_uri_for(gem_repo2)}"
+    G
+
+    lockfile <<-L
+      GEM
+        remote: #{file_uri_for(gem_repo2)}/
+        specs:
+
+      PLATFORMS
+        ruby
+
+      DEPENDENCIES
+
+      BUNDLED WITH
+         2.1.4
+    L
+
+    bundle "update --bundler=2.3.9", :env => { "BUNDLE_FROZEN" => "true" }
+    expect(err).to include("Cannot write a changed lockfile while frozen")
+  end
 end
 
 # these specs are slow and focus on integration and therefore are not exhaustive. unit specs elsewhere handle that.