Fix bundle lock regression when using update and lockfile flags:

- Ref #8917
- ### Problem

  Prior to Bundler 2.5.6, running
  `bundle lock --update foo --lockfile Gemfile_bumped.lock` would
  update only the foo gem and write the lockfile to the
  `Gemfile_bumped.lock`.

  In Bundler 2.5.6 and above running the same command, updates
  absolutely all gems.
  This change is related to #7047

  ### Solution

  We decided to expose the `write_lock` method rather than going
  through a complex deprecation cycle of the `lock` method.

  This commit applies the same business logic as prios to 2.5.6 where,
  we build the definition using the existing lockfile, make changes to
  the definition and dump it into the desired lockfile.

Author: Edouard-chin

bundler/lib/bundler/cli/lock.rb

@@ -35,11 +35,8 @@ def run
         update = { bundler: bundler }
       end
 
-      file = options[:lockfile]
-      file = file ? Pathname.new(file).expand_path : Bundler.default_lockfile
-
       Bundler.settings.temporary(frozen: false) do
-        definition = Bundler.definition(update, file)
+        definition = Bundler.definition(update, Bundler.default_lockfile)
         definition.add_checksums if options["add-checksums"]
 
         Bundler::CLI::Common.configure_gem_version_promoter(definition, options) if options[:update]
@@ -71,8 +68,11 @@ def run
         if print
           puts definition.to_lock
         else
+          file = options[:lockfile]
+          file = file ? Pathname.new(file).expand_path : Bundler.default_lockfile
+
           puts "Writing lockfile to #{file}"
-          definition.lock
+          definition.write_lock(file, false)
         end
       end
 

bundler/lib/bundler/definition.rb

@@ -373,6 +373,44 @@ def lock(file_or_preserve_unknown_sections = false, preserve_unknown_sections_or
       write_lock(target_lockfile, preserve_unknown_sections)
     end
 
+    def write_lock(file, preserve_unknown_sections)
+      return if Definition.no_lock || file.nil?
+
+      contents = to_lock
+
+      # Convert to \r\n if the existing lock has them
+      # i.e., Windows with `git config core.autocrlf=true`
+      contents.gsub!(/\n/, "\r\n") if @lockfile_contents.match?("\r\n")
+
+      if @locked_bundler_version
+        locked_major = @locked_bundler_version.segments.first
+        current_major = bundler_version_to_lock.segments.first
+
+        updating_major = locked_major < current_major
+      end
+
+      preserve_unknown_sections ||= !updating_major && (Bundler.frozen_bundle? || !(unlocking? || @unlocking_bundler))
+
+      if File.exist?(file) && lockfiles_equal?(@lockfile_contents, contents, preserve_unknown_sections)
+        return if Bundler.frozen_bundle?
+        SharedHelpers.filesystem_access(file) { FileUtils.touch(file) }
+        return
+      end
+
+      if Bundler.frozen_bundle?
+        Bundler.ui.error "Cannot write a changed lockfile while frozen."
+        return
+      end
+
+      begin
+        SharedHelpers.filesystem_access(file) do |p|
+          File.open(p, "wb") {|f| f.puts(contents) }
+        end
+      rescue ReadOnlyFileSystemError
+        raise ProductionError, lockfile_changes_summary("file system is read-only")
+      end
+    end
+
     def locked_ruby_version
       return unless ruby_version
       if @unlocking_ruby || !@locked_ruby_version
@@ -573,44 +611,6 @@ def lockfile_exists?
       lockfile && File.exist?(lockfile)
     end
 
-    def write_lock(file, preserve_unknown_sections)
-      return if Definition.no_lock || file.nil?
-
-      contents = to_lock
-
-      # Convert to \r\n if the existing lock has them
-      # i.e., Windows with `git config core.autocrlf=true`
-      contents.gsub!(/\n/, "\r\n") if @lockfile_contents.match?("\r\n")
-
-      if @locked_bundler_version
-        locked_major = @locked_bundler_version.segments.first
-        current_major = bundler_version_to_lock.segments.first
-
-        updating_major = locked_major < current_major
-      end
-
-      preserve_unknown_sections ||= !updating_major && (Bundler.frozen_bundle? || !(unlocking? || @unlocking_bundler))
-
-      if File.exist?(file) && lockfiles_equal?(@lockfile_contents, contents, preserve_unknown_sections)
-        return if Bundler.frozen_bundle?
-        SharedHelpers.filesystem_access(file) { FileUtils.touch(file) }
-        return
-      end
-
-      if Bundler.frozen_bundle?
-        Bundler.ui.error "Cannot write a changed lockfile while frozen."
-        return
-      end
-
-      begin
-        SharedHelpers.filesystem_access(file) do |p|
-          File.open(p, "wb") {|f| f.puts(contents) }
-        end
-      rescue ReadOnlyFileSystemError
-        raise ProductionError, lockfile_changes_summary("file system is read-only")
-      end
-    end
-
     def resolver
       @resolver ||= new_resolver(resolution_base)
     end

bundler/spec/commands/lock_spec.rb

@@ -311,6 +311,44 @@
     expect { read_lockfile }.to raise_error(Errno::ENOENT)
   end
 
+  it "updates a specific gem and write to a custom location" do
+    build_repo4 do
+      build_gem "uri", %w[1.0.2 1.0.3]
+      build_gem "warning", %w[1.4.0 1.5.0]
+    end
+
+    gemfile <<~G
+      source "https://gem.repo4"
+
+      gem "uri"
+      gem "warning"
+    G
+
+    lockfile <<~L
+      GEM
+        remote: https://gem.repo4
+        specs:
+          uri (1.0.2)
+          warning (1.4.0)
+
+      PLATFORMS
+        #{lockfile_platforms}
+
+      DEPENDENCIES
+        uri
+        warning
+
+      BUNDLED WITH
+         #{Bundler::VERSION}
+    L
+
+    bundle "lock --update uri --lockfile=lock"
+
+    lockfile_content = read_lockfile("lock")
+    expect(lockfile_content).to include("uri (1.0.3)")
+    expect(lockfile_content).to include("warning (1.4.0)")
+  end
+
   it "writes to custom location using --lockfile when a default lockfile is present" do
     gemfile_with_rails_weakling_and_foo_from_repo4