Auto merge of #5956 - bundler:seg-bundle-update-bundler-no-reresolve, r=indirect

[Update] Avoid a normal update when running with only --bundler

### What was the end-user problem that led to this PR?

The problem was `bundle update --bundler` went through the full update/resolution path when really we just want to pin to a different bundler version.

### What was your diagnosis of the problem?

My diagnosis was we should just update the lockfile, without threading the rest of the process like an update.

This implements what @indirect requested in https://github.com/rubygems/rubygems/pull/1977#issuecomment-322852799.

4 files changed, 34 insertions, 4 deletions

diff --git a/lib/bundler/cli/update.rb b/lib/bundler/cli/update.rb
index 26ee192aa9..b57b82ecc1 100644
--- a/lib/bundler/cli/update.rb
+++ b/lib/bundler/cli/update.rb
@@ -45,7 +45,8 @@ module Bundler
         end
 
         Bundler.definition(:gems => gems, :sources => sources, :ruby => options[:ruby],
-                           :lock_shared_dependencies => options[:conservative])
+                           :lock_shared_dependencies => options[:conservative],
+                           :bundler => options[:bundler])
       end
 
       Bundler::CLI::Common.configure_gem_version_promoter(Bundler.definition, options)
diff --git a/lib/bundler/definition.rb b/lib/bundler/definition.rb
index fea66f4245..1385ff2a94 100644
--- a/lib/bundler/definition.rb
+++ b/lib/bundler/definition.rb
@@ -55,7 +55,15 @@ module Bundler
     # @param ruby_version [Bundler::RubyVersion, nil] Requested Ruby Version
     # @param optional_groups [Array(String)] A list of optional groups
     def initialize(lockfile, dependencies, sources, unlock, ruby_version = nil, optional_groups = [], gemfiles = [])
-      @unlocking = unlock == true || !unlock.empty?
+      if [true, false].include?(unlock)
+        @unlocking_bundler = false
+        @unlocking = unlock
+      else
+        unlock = unlock.dup
+        @unlocking_bundler = unlock.delete(:bundler)
+        unlock.delete_if {|_k, v| Array(v).empty? }
+        @unlocking = !unlock.empty?
+      end
 
       @dependencies    = dependencies
       @sources         = sources
@@ -326,7 +334,7 @@ module Bundler
         end
       end
 
-      preserve_unknown_sections ||= !updating_major && (Bundler.frozen? || !unlocking?)
+      preserve_unknown_sections ||= !updating_major && (Bundler.frozen? || !(unlocking? || @unlocking_bundler))
       return if lockfiles_equal?(@lockfile_contents, contents, preserve_unknown_sections)
 
       if Bundler.frozen?
diff --git a/lib/bundler/installer.rb b/lib/bundler/installer.rb
index 18debb72fe..2bb405bc7a 100644
--- a/lib/bundler/installer.rb
+++ b/lib/bundler/installer.rb
@@ -266,7 +266,7 @@ module Bundler
 
     # returns whether or not a re-resolve was needed
     def resolve_if_needed(options)
-      if !options["update"] && !options["force"] && !Bundler.settings[:inline] && Bundler.default_lockfile.file?
+      if !@definition.unlocking? && !options["force"] && !Bundler.settings[:inline] && Bundler.default_lockfile.file?
         return false if @definition.nothing_changed? && !@definition.missing_specs?
       end
 
diff --git a/spec/commands/update_spec.rb b/spec/commands/update_spec.rb
index 2e6abfa9a5..a488be536f 100644
--- a/spec/commands/update_spec.rb
+++ b/spec/commands/update_spec.rb
@@ -629,6 +629,27 @@ RSpec.describe "bundle update --ruby" do
   end
 end
 
+RSpec.describe "bundle update --bundler" do
+  it "updates the bundler version in the lockfile without re-resolving" do
+    build_repo4 do
+      build_gem "rack", "1.0"
+    end
+
+    install_gemfile! <<-G
+      source "file:#{gem_repo4}"
+      gem "rack"
+    G
+    lockfile lockfile.sub(Bundler::VERSION, "1.0.0")
+
+    FileUtils.rm_r gem_repo4
+
+    bundle! :update, :bundler => true, :verbose => true
+    expect(the_bundle).to include_gem "rack 1.0"
+
+    expect(the_bundle.locked_gems.bundler_version).to eq v(Bundler::VERSION)
+  end
+end
+
 # these specs are slow and focus on integration and therefore are not exhaustive. unit specs elsewhere handle that.
 RSpec.describe "bundle update conservative" do
   context "patch and minor options" do