Fast forward merge: basic implemenation[ci skip]

app/controllers/projects_controller.rb

@@ -344,6 +344,7 @@ class ProjectsController < Projects::ApplicationController
       :tag_list,
       :visibility_level,
       :template_name,
+      :merge_method,
       project_feature_attributes: %i[
         builds_access_level

app/models/project.rb

@@ -1557,6 +1557,23 @@ class Project < ActiveRecord::Base
     persisted? && path_changed?
   end
+  def merge_method
+    if self.merge_requests_ff_only_enabled
+      :ff
+    else
+      :merge
+    end
+  end
+
+  def merge_method=(method)
+    self.merge_requests_ff_only_enabled = method.to_s == "ff"
+  end
+
+  def ff_merge_must_be_possible?
+    self.merge_requests_ff_only_enabled
+  end
+  # alias_method :merge_requests_ff_only_enabled?, :merge_requests_ff_only_enabled
+
   private
   def storage

app/models/repository.rb

@@ -854,6 +854,25 @@ class Repository
     end
   end
+  def ff_merge(user, source, target_branch, merge_request: nil)
+    our_commit = rugged.branches[target_branch].target
+    their_commit =
+      if source.is_a?(Gitlab::Git::Commit)
+        source.raw_commit
+      else
+        rugged.lookup(source)
+      end
+
+    raise 'Invalid merge target' if our_commit.nil?
+    raise 'Invalid merge source' if their_commit.nil?
+
+    with_branch(user, target_branch) do |start_commit|
+      merge_request&.update(in_progress_merge_commit_sha: their_commit.oid)
+
+      their_commit.oid
+    end
+  end
+
   def revert(
     user, commit, branch_name, message,
     start_branch_name: nil, start_project: project)

app/serializers/merge_request_entity.rb

@@ -13,6 +13,11 @@ class MergeRequestEntity < IssuableEntity
   expose :target_branch
   expose :target_project_id
+  expose :ff_only_enabled do |merge_request|
+    merge_request.project.merge_requests_ff_only_enabled
+  end
+
+
   # Events
   expose :merge_event, using: EventEntity
   expose :closed_event, using: EventEntity

app/services/merge_requests/ff_merge_service.rb

+module MergeRequests
+  # MergeService class
+  #
+  # Do git fast-forward merge and in case of success
+  # mark merge request as merged and execute all hooks and notifications
+  # Executed when you do fast-forward merge via GitLab UI
+  #
+  class FfMergeService < MergeRequests::MergeService
+    private
+
+    def commit
+      repository.ff_merge(current_user,
+                          source,
+                          merge_request.target_branch,
+                          merge_request: merge_request)
+    rescue Gitlab::Git::HooksService::PreReceiveError => e
+      raise MergeError, e.message
+    rescue StandardError => e
+      raise MergeError, "Something went wrong during merge: #{e.message}"
+    ensure
+      merge_request.update(in_progress_merge_commit_sha: nil)
+    end
+  end
+end

app/services/merge_requests/merge_service.rb

@@ -11,6 +11,11 @@ module MergeRequests
     attr_reader :merge_request, :source
     def execute(merge_request)
+      if project.merge_requests_ff_only_enabled && !self.is_a?(FfMergeService)
+        FfMergeService.new(project, current_user, params).execute(merge_request)
+        return
+      end
+
       @merge_request = merge_request
       unless @merge_request.mergeable?

app/views/projects/_merge_request_merge_settings.html.haml

 - form = local_assigns.fetch(:form)
+.form-group
+  = label_tag :merge_method_merge, class: 'label-light' do
+    Merge method
+  .radio
+    = label_tag :project_merge_method_merge do
+      = form.radio_button :merge_method, :merge, class: "js-merge-method-radio"
+      %strong Merge commit
+      %br
+      %span.descr
+        A merge commit is created for every merge, and merging is allowed as long as there are no conflicts.
+
+  .radio
+    = label_tag :project_merge_method_ff do
+      = form.radio_button :merge_method, :ff, class: "js-merge-method-radio"
+      %strong Fast-forward merge
+      %br
+      %span.descr
+        No merge commits are created and all merges are fast-forwarded, which means that merging is only allowed if the branch could be fast-forwarded.
+      %br
+      %span.descr
+        When fast-forward merge is not possible, the user must first rebase locally.
+
 .form-group
   .checkbox.builds-feature{ class: ("hidden" if @project && @project.project_feature.send(:builds_access_level) == 0) }
     = form.label :only_allow_merge_if_pipeline_succeeds do

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -412,6 +412,7 @@ Project:
 - last_repository_updated_at
 - ci_config_path
 - delete_error
+- merge_requests_ff_only_enabled
 Author:
 - name
 ProjectFeature: