Merge branch 'ff_port_from_ee' into 'master'

Move Fast-Forward Merge to CE

See merge request gitlab-org/gitlab-ce!14272

38 files changed, 679 insertions, 95 deletions

diff --git a/app/assets/javascripts/vue_merge_request_widget/components/mr_widget_status_icon.js b/app/assets/javascripts/vue_merge_request_widget/components/mr_widget_status_icon.js
index 703f3a56a34..4998a47b691 100644
--- a/app/assets/javascripts/vue_merge_request_widget/components/mr_widget_status_icon.js
+++ b/app/assets/javascripts/vue_merge_request_widget/components/mr_widget_status_icon.js
@@ -27,7 +27,7 @@ export default {
       <button
         v-if="showDisabledButton"
         type="button"
-        class="btn btn-success btn-sm"
+        class="js-disabled-merge-button btn btn-success btn-sm"
         disabled="true">
         Merge
       </button>
diff --git a/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_conflicts.js b/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_conflicts.js
index f9cb79a0bc1..dc252f8a9b7 100644
--- a/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_conflicts.js
+++ b/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_conflicts.js
@@ -10,27 +10,37 @@ export default {
   },
   template: `
     <div class="mr-widget-body media">
-      <status-icon status="failed" showDisabledButton />
+      <status-icon
+        status="failed"
+        showDisabledButton />
       <div class="media-body space-children">
-        <span class="bold">
-          There are merge conflicts<span v-if="!mr.canMerge">.</span>
-          <span v-if="!mr.canMerge">
-            Resolve these conflicts or ask someone with write access to this repository to merge it locally
-          </span>
+        <span
+          v-if="mr.shouldBeRebased"
+          class="bold">
+          Fast-forward merge is not possible.
+          To merge this request, first rebase locally.
         </span>
-        <a
-          v-if="mr.canMerge && mr.conflictResolutionPath"
-          :href="mr.conflictResolutionPath"
-          class="btn btn-default btn-xs js-resolve-conflicts-button">
-          Resolve conflicts
-        </a>
-        <a
-          v-if="mr.canMerge"
-          class="btn btn-default btn-xs js-merge-locally-button"
-          data-toggle="modal"
-          href="#modal_merge_info">
-          Merge locally
-        </a>
+        <template v-else>
+          <span class="bold">
+            There are merge conflicts<span v-if="!mr.canMerge">.</span>
+            <span v-if="!mr.canMerge">
+              Resolve these conflicts or ask someone with write access to this repository to merge it locally
+            </span>
+          </span>
+          <a
+            v-if="mr.canMerge && mr.conflictResolutionPath"
+            :href="mr.conflictResolutionPath"
+            class="js-resolve-conflicts-button btn btn-default btn-xs">
+            Resolve conflicts
+          </a>
+          <a
+            v-if="mr.canMerge"
+            class="js-merge-locally-button btn btn-default btn-xs"
+            data-toggle="modal"
+            href="#modal_merge_info">
+            Merge locally
+          </a>
+        </template>
       </div>
     </div>
   `,
diff --git a/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_ready_to_merge.js b/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_ready_to_merge.js
index ad709da51ee..f83d3ca00dd 100644
--- a/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_ready_to_merge.js
+++ b/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_ready_to_merge.js
@@ -284,10 +284,16 @@ export default {
                 :mr="mr"
                 :is-merge-button-disabled="isMergeButtonDisabled" />
 
+              <span
+                v-if="mr.ffOnlyEnabled"
+                class="js-fast-forward-message">
+                Fast-forward merge without a merge commit
+              </span>
               <button
+                v-else
                 @click="toggleCommitMessageEditor"
                 :disabled="isMergeButtonDisabled"
-                class="btn btn-default btn-xs"
+                class="js-modify-commit-message-button btn btn-default btn-xs"
                 type="button">
                 Modify commit message
               </button>
diff --git a/app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js b/app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js
index 29464662578..8255fd3e792 100644
--- a/app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js
+++ b/app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js
@@ -57,6 +57,8 @@ export default class MergeRequestStore {
     this.onlyAllowMergeIfPipelineSucceeds = data.only_allow_merge_if_pipeline_succeeds || false;
     this.mergeWhenPipelineSucceeds = data.merge_when_pipeline_succeeds || false;
     this.mergePath = data.merge_path;
+    this.ffOnlyEnabled = data.ff_only_enabled;
+    this.shouldBeRebased = !!data.should_be_rebased;
     this.statusPath = data.status_path;
     this.emailPatchesPath = data.email_patches_path;
     this.plainDiffPath = data.plain_diff_path;
diff --git a/app/controllers/projects_controller.rb b/app/controllers/projects_controller.rb
index b13034d3333..a738ca9f361 100644
--- a/app/controllers/projects_controller.rb
+++ b/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
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 8d9a30397a9..e85b83daf9e 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -524,6 +524,14 @@ class MergeRequest < ActiveRecord::Base
     true
   end
 
+  def ff_merge_possible?
+    project.repository.ancestor?(target_branch_sha, diff_head_sha)
+  end
+
+  def should_be_rebased?
+    project.ff_merge_must_be_possible? && !ff_merge_possible?
+  end
+
   def can_cancel_merge_when_pipeline_succeeds?(current_user)
     can_be_merged_by?(current_user) || self.author == current_user
   end
diff --git a/app/models/project.rb b/app/models/project.rb
index 44d1190cc5b..952e9e22b28 100644
--- a/app/models/project.rb
+++ b/app/models/project.rb
@@ -1566,6 +1566,34 @@ class Project < ActiveRecord::Base
     persisted? && path_changed?
   end
 
+  def merge_method
+    if self.merge_requests_ff_only_enabled
+      :ff
+    elsif self.merge_requests_rebase_enabled
+      :rebase_merge
+    else
+      :merge
+    end
+  end
+
+  def merge_method=(method)
+    case method.to_s
+    when "ff"
+      self.merge_requests_ff_only_enabled = true
+      self.merge_requests_rebase_enabled = true
+    when "rebase_merge"
+      self.merge_requests_ff_only_enabled = false
+      self.merge_requests_rebase_enabled = true
+    when "merge"
+      self.merge_requests_ff_only_enabled = false
+      self.merge_requests_rebase_enabled = false
+    end
+  end
+
+  def ff_merge_must_be_possible?
+    self.merge_requests_ff_only_enabled || self.merge_requests_rebase_enabled
+  end
+
   def migrate_to_hashed_storage!
     return if hashed_storage?
 
diff --git a/app/models/repository.rb b/app/models/repository.rb
index a0f57f1e54d..d47dc9a05cd 100644
--- a/app/models/repository.rb
+++ b/app/models/repository.rb
@@ -850,6 +850,25 @@ 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)
diff --git a/app/serializers/merge_request_entity.rb b/app/serializers/merge_request_entity.rb
index 07650ce6f20..7d3c752b8e6 100644
--- a/app/serializers/merge_request_entity.rb
+++ b/app/serializers/merge_request_entity.rb
@@ -13,6 +13,11 @@ class MergeRequestEntity < IssuableEntity
   expose :target_branch
   expose :target_project_id
 
+  expose :should_be_rebased?, as: :should_be_rebased
+  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
diff --git a/app/services/merge_requests/ff_merge_service.rb b/app/services/merge_requests/ff_merge_service.rb
new file mode 100644
index 00000000000..ba6853b835a
--- /dev/null
+++ b/app/services/merge_requests/ff_merge_service.rb
@@ -0,0 +1,24 @@
+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
diff --git a/app/services/merge_requests/merge_service.rb b/app/services/merge_requests/merge_service.rb
index bf26859dd6d..a110abf8256 100644
--- a/app/services/merge_requests/merge_service.rb
+++ b/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?
diff --git a/app/views/projects/_merge_request_fast_forward_settings.html.haml b/app/views/projects/_merge_request_fast_forward_settings.html.haml
new file mode 100644
index 00000000000..9d357293a2f
--- /dev/null
+++ b/app/views/projects/_merge_request_fast_forward_settings.html.haml
@@ -0,0 +1,13 @@
+- form = local_assigns.fetch(:form)
+- project = local_assigns.fetch(:project)
+
+.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.
diff --git a/app/views/projects/_merge_request_rebase_settings.html.haml b/app/views/projects/_merge_request_rebase_settings.html.haml
new file mode 100644
index 00000000000..c52e09573a6
--- /dev/null
+++ b/app/views/projects/_merge_request_rebase_settings.html.haml
@@ -0,0 +1,13 @@
+- form = local_assigns.fetch(:form)
+
+.radio
+  = label_tag :project_merge_method_rebase_merge do
+    = form.radio_button :merge_method, :rebase_merge, class: "js-merge-method-radio"
+    %strong Merge commit with semi-linear history
+    %br
+    %span.descr
+      A merge commit is created for every merge, but merging is only allowed if fast-forward merge is possible.
+      This way you could make sure that if this merge request would build, after merging to target branch it would also build.
+    %br
+    %span.descr
+      When fast-forward merge is not possible, the user must first rebase locally.
diff --git a/app/views/projects/_merge_request_settings.html.haml b/app/views/projects/_merge_request_settings.html.haml
index cc5afa943cf..fd0c419cdac 100644
--- a/app/views/projects/_merge_request_settings.html.haml
+++ b/app/views/projects/_merge_request_settings.html.haml
@@ -1,3 +1,18 @@
 - 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.
+
+  = render 'merge_request_rebase_settings', form: form
+
+  = render 'merge_request_fast_forward_settings', project: @project, form: form
+
 = render 'projects/merge_request_merge_settings', form: form
diff --git a/changelogs/unreleased/ff_port_from_ee.yml b/changelogs/unreleased/ff_port_from_ee.yml
new file mode 100644
index 00000000000..e1cb7804a47
--- /dev/null
+++ b/changelogs/unreleased/ff_port_from_ee.yml
@@ -0,0 +1,5 @@
+---
+title: Move Custom merge methods from EE
+merge_request:
+author:
+type: added
diff --git a/db/migrate/20141126120926_add_merge_request_rebase_enabled_to_projects.rb b/db/migrate/20141126120926_add_merge_request_rebase_enabled_to_projects.rb
new file mode 100644
index 00000000000..3dafdf0fde4
--- /dev/null
+++ b/db/migrate/20141126120926_add_merge_request_rebase_enabled_to_projects.rb
@@ -0,0 +1,17 @@
+# rubocop:disable all
+class AddMergeRequestRebaseEnabledToProjects < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_column_with_default(:projects, :merge_requests_rebase_enabled, :boolean, default: false)
+  end
+
+  def down
+    remove_column(:projects, :merge_requests_rebase_enabled)
+  end
+end
diff --git a/db/migrate/20150827121444_add_fast_forward_option_to_project.rb b/db/migrate/20150827121444_add_fast_forward_option_to_project.rb
new file mode 100644
index 00000000000..014f5b2f372
--- /dev/null
+++ b/db/migrate/20150827121444_add_fast_forward_option_to_project.rb
@@ -0,0 +1,17 @@
+# rubocop:disable all
+class AddFastForwardOptionToProject < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def add
+    add_column_with_default(:projects, :merge_requests_ff_only_enabled, :boolean, default: false)
+  end
+
+  def down
+    remove_column(:projects, :merge_requests_ff_only_enabled)
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index e8e64b9d36b..c238efbedbe 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -1217,6 +1217,8 @@ ActiveRecord::Schema.define(version: 20170928100231) do
     t.integer "storage_version", limit: 2
     t.boolean "resolve_outdated_diff_discussions"
     t.boolean "repository_read_only"
+    t.boolean "merge_requests_ff_only_enabled", default: false
+    t.boolean "merge_requests_rebase_enabled", default: false, null: false
   end
 
   add_index "projects", ["ci_id"], name: "index_projects_on_ci_id", using: :btree
diff --git a/doc/user/project/merge_requests/fast_forward_merge.md b/doc/user/project/merge_requests/fast_forward_merge.md
new file mode 100644
index 00000000000..085170d9f03
--- /dev/null
+++ b/doc/user/project/merge_requests/fast_forward_merge.md
@@ -0,0 +1,35 @@
+# Fast-forward merge requests
+
+Retain a linear Git history and a way to accept merge requests without
+creating merge commits.
+
+## Overview
+
+When the fast-forward merge ([`--ff-only`][ffonly]) setting is enabled, no merge
+commits will be created and all merges are fast-forwarded, which means that
+merging is only allowed if the branch could be fast-forwarded.
+
+When a fast-forward merge is not possible, the user must rebase the branch manually.
+
+## Use cases
+
+Sometimes, a workflow policy might mandate a clean commit history without
+merge commits. In such cases, the fast-forward merge is the perfect candidate.
+
+## Enabling fast-forward merges
+
+1. Navigate to your project's **Settings** and search for the 'Merge method'
+1. Select the **Fast-forward merge** option
+1. Hit **Save changes** for the changes to take effect
+
+Now, when you visit the merge request page, you will be able to accept it
+**only if a fast-forward merge is possible**.
+
+![Fast forward merge request](img/ff_merge_mr.png)
+
+If the target branch is ahead of the source branch, you need to rebase the
+source branch locally before you will be able to do a fast-forward merge.
+
+![Fast forward merge rebase locally](img/ff_merge_rebase_locally.png)
+
+[ffonly]: https://git-scm.com/docs/git-merge#git-merge---ff-only
diff --git a/doc/user/project/merge_requests/img/ff_merge_mr.png b/doc/user/project/merge_requests/img/ff_merge_mr.png
new file mode 100644
index 00000000000..241cc990343
--- /dev/null
+++ b/doc/user/project/merge_requests/img/ff_merge_mr.png
diff --git a/doc/user/project/merge_requests/img/ff_merge_rebase_locally.png b/doc/user/project/merge_requests/img/ff_merge_rebase_locally.png
new file mode 100644
index 00000000000..fb412296efc
--- /dev/null
+++ b/doc/user/project/merge_requests/img/ff_merge_rebase_locally.png
diff --git a/doc/user/project/merge_requests/index.md b/doc/user/project/merge_requests/index.md
index 26c6277d33a..8e081b4f0b8 100644
--- a/doc/user/project/merge_requests/index.md
+++ b/doc/user/project/merge_requests/index.md
@@ -23,12 +23,14 @@ With GitLab merge requests, you can:
 - Organize your issues and merge requests consistently throughout the project with [labels](../../project/labels.md)
 - Add a time estimation and the time spent with that merge request with [Time Tracking](../../../workflow/time_tracking.html#time-tracking)
 - [Resolve merge conflicts from the UI](#resolve-conflicts)
+- Enable [fast-forward merge requests](#fast-forward-merge-requests)
+- Enable [semi-linear history merge requests](#semi-linear-history-merge-requests) as another security layer to guarantee the pipeline is passing in the target branch
+
 
 With **[GitLab Enterprise Edition][ee]**, you can also:
 
 - View the deployment process across projects with [Multi-Project Pipeline Graphs](https://docs.gitlab.com/ee/ci/multi_project_pipeline_graphs.html#multi-project-pipeline-graphs) (available only in GitLab Enterprise Edition Premium)
 - Request [approvals](https://docs.gitlab.com/ee/user/project/merge_requests/merge_request_approvals.html) from your managers (available in GitLab Enterprise Edition Starter)
-- Enable [fast-forward merge requests](https://docs.gitlab.com/ee/user/project/merge_requests/fast_forward_merge.html) (available in GitLab Enterprise Edition Starter)
 - [Squash and merge](https://docs.gitlab.com/ee/user/project/merge_requests/squash_and_merge.html) for a cleaner commit history (available in GitLab Enterprise Edition Starter)
 - Enable [semi-linear history merge requests](https://docs.gitlab.com/ee/user/project/merge_requests/index.html#semi-linear-history-merge-requests) as another security layer to guarantee the pipeline is passing in the target branch (available in GitLab Enterprise Edition Starter)
 - Analise the impact of your changes with [Code Quality reports](https://docs.gitlab.com/ee/user/project/merge_requests/code_quality_diff.html) (available in GitLab Enterprise Edition Starter)
@@ -89,6 +91,22 @@ in a merged merge requests or a commit.
 
 [Learn more about cherry-picking changes.](cherry_pick_changes.md)
 
+## Semi-linear history merge requests
+
+A merge commit is created for every merge, but the branch is only merged if
+a fast-forward merge is possible. This ensures that if the merge request build
+succeeded, the target branch build will also succeed after merging.
+
+Navigate to a project's settings, select the **Merge commit with semi-linear
+history** option under **Merge Requests: Merge method** and save your changes.
+
+## Fast-forward merge requests
+
+If you prefer a linear Git history and a way to accept merge requests without
+creating merge commits, you can configure this on a per-project basis.
+
+[Read more about fast-forward merge requests.](fast_forward_merge.md)
+
 ## Merge when pipeline succeeds
 
 When reviewing a merge request that looks ready to merge but still has one or
@@ -254,4 +272,4 @@ git checkout origin/merge-requests/1
 ```
 
 [protected branches]: ../protected_branches.md
-[ee]: https://about.gitlab.com/gitlab-ee/ "GitLab Enterprise Edition"
\ No newline at end of file
+[ee]: https://about.gitlab.com/gitlab-ee/ "GitLab Enterprise Edition"
diff --git a/doc/user/project/settings/index.md b/doc/user/project/settings/index.md
index 22c343dc027..b7c7a5e5d28 100644
--- a/doc/user/project/settings/index.md
+++ b/doc/user/project/settings/index.md
@@ -23,7 +23,7 @@ Add an [issue description template](../description_templates.md#description-temp
 
 Set up your project's merge request settings:
 
-- Set up the merge request method (merge commit, [fast-forward merge](https://docs.gitlab.com/ee/user/project/merge_requests/fast_forward_merge.html#fast-forward-merge-requests)). _Fast-forward is available in [GitLab Enterprise Edition Starter](https://about.gitlab.com/gitlab-ee/)._
+- Set up the merge request method (merge commit, [fast-forward merge](../merge_requests/fast_forward_merge.html)).
 - Merge request [description templates](../description_templates.md#description-templates).
 - Enable [merge request approvals](https://docs.gitlab.com/ee/user/project/merge_requests/merge_request_approvals.html#merge-request-approvals), _available in [GitLab Enterprise Edition Starter](https://about.gitlab.com/gitlab-ee/)_.
 - Enable [merge only of pipeline succeeds](../merge_requests/merge_when_pipeline_succeeds.md).
diff --git a/doc/workflow/README.md b/doc/workflow/README.md
index 673e08287a3..6b2aba47f54 100644
--- a/doc/workflow/README.md
+++ b/doc/workflow/README.md
@@ -36,6 +36,7 @@
   - [Revert changes in the UI](../user/project/merge_requests/revert_changes.md)
   - [Merge requests versions](../user/project/merge_requests/versions.md)
   - ["Work In Progress" merge requests](../user/project/merge_requests/work_in_progress_merge_requests.md)
+  - [Fast-forward merge requests](../user/project/merge_requests/fast_forward_merge.md)
 - [Manage large binaries with Git LFS](lfs/manage_large_binaries_with_git_lfs.md)
 - [Importing from SVN, GitHub, Bitbucket, etc](importing/README.md)
 - [Todos](todos.md)
diff --git a/features/project/ff_merge_requests.feature b/features/project/ff_merge_requests.feature
new file mode 100644
index 00000000000..995e52f9332
--- /dev/null
+++ b/features/project/ff_merge_requests.feature
@@ -0,0 +1,24 @@
+Feature: Project Ff Merge Requests
+  Background:
+    Given I sign in as a user
+    And I own project "Shop"
+    And project "Shop" have "Bug NS-05" open merge request with diffs inside
+    And merge request "Bug NS-05" is mergeable
+
+  @javascript
+  Scenario: I do ff-only merge for rebased branch
+    Given ff merge enabled
+    And merge request "Bug NS-05" is rebased
+    When I visit merge request page "Bug NS-05"
+    Then I should see ff-only merge button
+    When I accept this merge request
+    Then I should see merged request
+
+  @javascript
+  Scenario: I do ff-only merge for merged branch
+    Given ff merge enabled
+    And merge request "Bug NS-05" merged target
+    When I visit merge request page "Bug NS-05"
+    Then I should see ff-only merge button
+    When I accept this merge request
+    Then I should see merged request
diff --git a/features/steps/project/ff_merge_requests.rb b/features/steps/project/ff_merge_requests.rb
new file mode 100644
index 00000000000..d68fe71e16e
--- /dev/null
+++ b/features/steps/project/ff_merge_requests.rb
@@ -0,0 +1,65 @@
+class Spinach::Features::ProjectFfMergeRequests < Spinach::FeatureSteps
+  include SharedAuthentication
+  include SharedIssuable
+  include SharedProject
+  include SharedNote
+  include SharedPaths
+  include SharedMarkdown
+  include SharedDiffNote
+  include SharedUser
+  include WaitForRequests
+
+  step 'project "Shop" have "Bug NS-05" open merge request with diffs inside' do
+    create(:merge_request_with_diffs,
+           title: "Bug NS-05",
+           source_project: project,
+           target_project: project,
+           author: project.users.first)
+  end
+
+  step 'I should see ff-only merge button' do
+    expect(page).to have_content "Fast-forward merge without a merge commit"
+    expect(page).to have_button 'Merge'
+  end
+
+  step 'merge request "Bug NS-05" is mergeable' do
+    merge_request.mark_as_mergeable
+  end
+
+  step 'I accept this merge request' do
+    page.within '.mr-state-widget' do
+      click_button "Merge"
+    end
+  end
+
+  step 'I should see merged request' do
+    page.within '.status-box' do
+      expect(page).to have_content "Merged"
+      wait_for_requests
+    end
+  end
+
+  step 'ff merge enabled' do
+    project = merge_request.target_project
+    project.merge_requests_ff_only_enabled = true
+    project.save!
+  end
+
+  step 'merge request "Bug NS-05" is rebased' do
+    merge_request.source_branch = 'flatten-dir'
+    merge_request.target_branch = 'improve/awesome'
+    merge_request.reload_diff
+    merge_request.save!
+  end
+
+  step 'merge request "Bug NS-05" merged target' do
+    merge_request.source_branch = 'merged-target'
+    merge_request.target_branch = 'improve/awesome'
+    merge_request.reload_diff
+    merge_request.save!
+  end
+
+  def merge_request
+    @merge_request ||= MergeRequest.find_by!(title: "Bug NS-05")
+  end
+end
diff --git a/spec/controllers/projects_controller_spec.rb b/spec/controllers/projects_controller_spec.rb
index 4459e227fb3..2a91a6613e6 100644
--- a/spec/controllers/projects_controller_spec.rb
+++ b/spec/controllers/projects_controller_spec.rb
@@ -289,6 +289,24 @@ describe ProjectsController do
       end
     end
 
+    it 'updates Fast Forward Merge attributes' do
+      controller.instance_variable_set(:@project, project)
+
+      params = {
+        merge_method: :ff
+      }
+
+      put :update,
+          namespace_id: project.namespace,
+          id: project.id,
+          project: params
+
+      expect(response).to have_http_status(302)
+      params.each do |param, value|
+        expect(project.public_send(param)).to eq(value)
+      end
+    end
+
     def update_project(**parameters)
       put :update,
           namespace_id: project.namespace.path,
diff --git a/spec/features/merge_requests/widget_spec.rb b/spec/features/merge_requests/widget_spec.rb
index 443b596b3c6..791cfa308c3 100644
--- a/spec/features/merge_requests/widget_spec.rb
+++ b/spec/features/merge_requests/widget_spec.rb
@@ -202,6 +202,28 @@ describe 'Merge request', :js do
     end
   end
 
+  context 'view merge request where fast-forward merge is not possible' do
+    before do
+      project.update(merge_requests_ff_only_enabled: true)
+
+      merge_request.update(
+        merge_user: merge_request.author,
+        merge_status: :cannot_be_merged
+      )
+
+      visit project_merge_request_path(project, merge_request)
+    end
+
+    it 'shows information about the merge error' do
+      # Wait for the `ci_status` and `merge_check` requests
+      wait_for_requests
+
+      page.within('.mr-widget-body') do
+        expect(page).to have_content('Fast-forward merge is not possible')
+      end
+    end
+  end
+
   context 'merge error' do
     before do
       allow_any_instance_of(Repository).to receive(:merge).and_return(false)
diff --git a/spec/features/projects/project_settings_spec.rb b/spec/features/projects/project_settings_spec.rb
index 5d77cd1ccd5..06568817757 100644
--- a/spec/features/projects/project_settings_spec.rb
+++ b/spec/features/projects/project_settings_spec.rb
@@ -32,6 +32,32 @@ describe 'Edit Project Settings' do
     end
   end
 
+  describe 'Merge request settings section' do
+    it 'shows "Merge commit" strategy' do
+      visit edit_project_path(project)
+
+      page.within '.merge-requests-feature' do
+        expect(page).to have_content 'Merge commit'
+      end
+    end
+
+    it 'shows "Merge commit with semi-linear history " strategy' do
+      visit edit_project_path(project)
+
+      page.within '.merge-requests-feature' do
+        expect(page).to have_content 'Merge commit with semi-linear history'
+      end
+    end
+
+    it 'shows "Fast-forward merge" strategy' do
+      visit edit_project_path(project)
+
+      page.within '.merge-requests-feature' do
+        expect(page).to have_content 'Fast-forward merge'
+      end
+    end
+  end
+
   describe 'Rename repository section' do
     context 'with invalid characters' do
       it 'shows errors for invalid project path/name' do
diff --git a/spec/fixtures/api/schemas/entities/merge_request.json b/spec/fixtures/api/schemas/entities/merge_request.json
index 1030f323a1f..0796d9b8af9 100644
--- a/spec/fixtures/api/schemas/entities/merge_request.json
+++ b/spec/fixtures/api/schemas/entities/merge_request.json
@@ -96,7 +96,9 @@
     "remove_wip_path": { "type": "string" },
     "commits_count": { "type": "integer" },
     "remove_source_branch": { "type": ["boolean", "null"] },
-    "merge_ongoing": { "type": "boolean" }
+    "merge_ongoing": { "type": "boolean" },
+    "ff_only_enabled": { "type": ["boolean", false] },
+    "should_be_rebased": { "type": "boolean" }
   },
   "additionalProperties": false
 }
diff --git a/spec/javascripts/vue_mr_widget/components/states/mr_widget_conflicts_spec.js b/spec/javascripts/vue_mr_widget/components/states/mr_widget_conflicts_spec.js
index 3b7b7d93662..5d4c7ec09dc 100644
--- a/spec/javascripts/vue_mr_widget/components/states/mr_widget_conflicts_spec.js
+++ b/spec/javascripts/vue_mr_widget/components/states/mr_widget_conflicts_spec.js
@@ -1,20 +1,9 @@
 import Vue from 'vue';
 import conflictsComponent from '~/vue_merge_request_widget/components/states/mr_widget_conflicts';
+import mountComponent from '../../../helpers/vue_mount_component_helper';
 
+const ConflictsComponent = Vue.extend(conflictsComponent);
 const path = '/conflicts';
-const createComponent = () => {
-  const Component = Vue.extend(conflictsComponent);
-
-  return new Component({
-    el: document.createElement('div'),
-    propsData: {
-      mr: {
-        canMerge: true,
-        conflictResolutionPath: path,
-      },
-    },
-  });
-};
 
 describe('MRWidgetConflicts', () => {
   describe('props', () => {
@@ -27,44 +16,90 @@ describe('MRWidgetConflicts', () => {
   });
 
   describe('template', () => {
-    it('should have correct elements', () => {
-      const el = createComponent().$el;
-      const resolveButton = el.querySelector('.js-resolve-conflicts-button');
-      const mergeButton = el.querySelector('.mr-widget-body .btn');
-      const mergeLocallyButton = el.querySelector('.js-merge-locally-button');
-
-      expect(el.textContent).toContain('There are merge conflicts');
-      expect(el.textContent).not.toContain('ask someone with write access');
-      expect(el.querySelector('.btn-success').disabled).toBeTruthy();
-      expect(resolveButton.textContent).toContain('Resolve conflicts');
-      expect(resolveButton.getAttribute('href')).toEqual(path);
-      expect(mergeButton.textContent).toContain('Merge');
-      expect(mergeLocallyButton.textContent).toContain('Merge locally');
+    describe('when allowed to merge', () => {
+      let vm;
+
+      beforeEach(() => {
+        vm = mountComponent(ConflictsComponent, {
+          mr: {
+            canMerge: true,
+            conflictResolutionPath: path,
+          },
+        });
+      });
+
+      afterEach(() => {
+        vm.$destroy();
+      });
+
+      it('should tell you about conflicts without bothering other people', () => {
+        expect(vm.$el.textContent).toContain('There are merge conflicts');
+        expect(vm.$el.textContent).not.toContain('ask someone with write access');
+      });
+
+      it('should allow you to resolve the conflicts', () => {
+        const resolveButton = vm.$el.querySelector('.js-resolve-conflicts-button');
+
+        expect(resolveButton.textContent).toContain('Resolve conflicts');
+        expect(resolveButton.getAttribute('href')).toEqual(path);
+      });
+
+      it('should have merge buttons', () => {
+        const mergeButton = vm.$el.querySelector('.js-disabled-merge-button');
+        const mergeLocallyButton = vm.$el.querySelector('.js-merge-locally-button');
+
+        expect(mergeButton.textContent).toContain('Merge');
+        expect(mergeButton.disabled).toBeTruthy();
+        expect(mergeButton.classList.contains('btn-success')).toEqual(true);
+        expect(mergeLocallyButton.textContent).toContain('Merge locally');
+      });
     });
 
     describe('when user does not have permission to merge', () => {
       let vm;
 
       beforeEach(() => {
-        vm = createComponent();
-        vm.mr.canMerge = false;
+        vm = mountComponent(ConflictsComponent, {
+          mr: {
+            canMerge: false,
+          },
+        });
       });
 
-      it('should show proper message', (done) => {
-        Vue.nextTick(() => {
-          expect(vm.$el.textContent).toContain('ask someone with write access');
-          done();
-        });
+      afterEach(() => {
+        vm.$destroy();
+      });
+
+      it('should show proper message', () => {
+        expect(vm.$el.textContent).toContain('ask someone with write access');
+      });
+
+      it('should not have action buttons', () => {
+        expect(vm.$el.querySelector('.js-disabled-merge-button')).toBeDefined();
+        expect(vm.$el.querySelector('.js-resolve-conflicts-button')).toBeNull();
+        expect(vm.$el.querySelector('.js-merge-locally-button')).toBeNull();
       });
+    });
 
-      it('should not have action buttons', (done) => {
-        Vue.nextTick(() => {
-          expect(vm.$el.querySelectorAll('.btn').length).toBe(1);
-          expect(vm.$el.querySelector('.js-resolve-conflicts-button')).toEqual(null);
-          expect(vm.$el.querySelector('.js-merge-locally-button')).toEqual(null);
-          done();
+    describe('when fast-forward or semi-linear merge enabled', () => {
+      let vm;
+
+      beforeEach(() => {
+        vm = mountComponent(ConflictsComponent, {
+          mr: {
+            shouldBeRebased: true,
+          },
         });
       });
+
+      afterEach(() => {
+        vm.$destroy();
+      });
+
+      it('should tell you to rebase locally', () => {
+        expect(vm.$el.textContent).toContain('Fast-forward merge is not possible.');
+        expect(vm.$el.textContent).toContain('To merge this request, first rebase locally');
+      });
     });
   });
 });
diff --git a/spec/javascripts/vue_mr_widget/components/states/mr_widget_ready_to_merge_spec.js b/spec/javascripts/vue_mr_widget/components/states/mr_widget_ready_to_merge_spec.js
index 03a52f1f91c..2422e844e97 100644
--- a/spec/javascripts/vue_mr_widget/components/states/mr_widget_ready_to_merge_spec.js
+++ b/spec/javascripts/vue_mr_widget/components/states/mr_widget_ready_to_merge_spec.js
@@ -182,36 +182,6 @@ describe('MRWidgetReadyToMerge', () => {
         expect(vm.isMergeButtonDisabled).toBeTruthy();
       });
     });
-
-    describe('Remove source branch checkbox', () => {
-      describe('when user can merge but cannot delete branch', () => {
-        it('isRemoveSourceBranchButtonDisabled should be true', () => {
-          expect(vm.isRemoveSourceBranchButtonDisabled).toBe(true);
-        });
-
-        it('should be disabled in the rendered output', () => {
-          const checkboxElement = vm.$el.querySelector('#remove-source-branch-input');
-          expect(checkboxElement.getAttribute('disabled')).toBe('disabled');
-        });
-      });
-
-      describe('when user can merge and can delete branch', () => {
-        beforeEach(() => {
-          this.customVm = createComponent({
-            mr: { canRemoveSourceBranch: true },
-          });
-        });
-
-        it('isRemoveSourceBranchButtonDisabled should be false', () => {
-          expect(this.customVm.isRemoveSourceBranchButtonDisabled).toBe(false);
-        });
-
-        it('should be enabled in rendered output', () => {
-          const checkboxElement = this.customVm.$el.querySelector('#remove-source-branch-input');
-          expect(checkboxElement.getAttribute('disabled')).toBeNull();
-        });
-      });
-    });
   });
 
   describe('methods', () => {
@@ -467,4 +437,54 @@ describe('MRWidgetReadyToMerge', () => {
       });
     });
   });
+
+  describe('Remove source branch checkbox', () => {
+    describe('when user can merge but cannot delete branch', () => {
+      it('isRemoveSourceBranchButtonDisabled should be true', () => {
+        expect(vm.isRemoveSourceBranchButtonDisabled).toBe(true);
+      });
+
+      it('should be disabled in the rendered output', () => {
+        const checkboxElement = vm.$el.querySelector('#remove-source-branch-input');
+        expect(checkboxElement.getAttribute('disabled')).toBe('disabled');
+      });
+    });
+
+    describe('when user can merge and can delete branch', () => {
+      beforeEach(() => {
+        this.customVm = createComponent({
+          mr: { canRemoveSourceBranch: true },
+        });
+      });
+
+      it('isRemoveSourceBranchButtonDisabled should be false', () => {
+        expect(this.customVm.isRemoveSourceBranchButtonDisabled).toBe(false);
+      });
+
+      it('should be enabled in rendered output', () => {
+        const checkboxElement = this.customVm.$el.querySelector('#remove-source-branch-input');
+        expect(checkboxElement.getAttribute('disabled')).toBeNull();
+      });
+    });
+  });
+
+  describe('Commit message area', () => {
+    it('when using merge commits, should show "Modify commit message" button', () => {
+      const customVm = createComponent({
+        mr: { ffOnlyEnabled: false },
+      });
+
+      expect(customVm.$el.querySelector('.js-fast-forward-message')).toBeNull();
+      expect(customVm.$el.querySelector('.js-modify-commit-message-button')).toBeDefined();
+    });
+
+    it('when fast-forward merge is enabled, only show fast-forward message', () => {
+      const customVm = createComponent({
+        mr: { ffOnlyEnabled: true },
+      });
+
+      expect(customVm.$el.querySelector('.js-fast-forward-message')).toBeDefined();
+      expect(customVm.$el.querySelector('.js-modify-commit-message-button')).toBeNull();
+    });
+  });
 });
diff --git a/spec/lib/gitlab/import_export/safe_model_attributes.yml b/spec/lib/gitlab/import_export/safe_model_attributes.yml
index 899d17d97c2..7268226112c 100644
--- a/spec/lib/gitlab/import_export/safe_model_attributes.yml
+++ b/spec/lib/gitlab/import_export/safe_model_attributes.yml
@@ -412,6 +412,8 @@ Project:
 - last_repository_updated_at
 - ci_config_path
 - delete_error
+- merge_requests_ff_only_enabled
+- merge_requests_rebase_enabled
 Author:
 - name
 ProjectFeature:
diff --git a/spec/models/project_spec.rb b/spec/models/project_spec.rb
index c94481d53c3..176bb568cbe 100644
--- a/spec/models/project_spec.rb
+++ b/spec/models/project_spec.rb
@@ -408,6 +408,18 @@ describe Project do
     end
   end
 
+  describe '#merge_method' do
+    it 'returns "ff" merge_method when ff is enabled' do
+      project = build(:project, merge_requests_ff_only_enabled: true)
+      expect(project.merge_method).to be :ff
+    end
+
+    it 'returns "merge" merge_method when ff is disabled' do
+      project = build(:project, merge_requests_ff_only_enabled: false)
+      expect(project.merge_method).to be :merge
+    end
+  end
+
   describe '#repository_storage_path' do
     let(:project) { create(:project, repository_storage: 'custom') }
 
diff --git a/spec/models/repository_spec.rb b/spec/models/repository_spec.rb
index 157a10edd73..8a4dcdc311e 100644
--- a/spec/models/repository_spec.rb
+++ b/spec/models/repository_spec.rb
@@ -1345,6 +1345,34 @@ describe Repository do
     end
   end
 
+  describe '#ff_merge' do
+    before do
+      repository.add_branch(user, 'ff-target', 'feature~5')
+    end
+
+    it 'merges the code and return the commit id' do
+      merge_request = create(:merge_request, source_branch: 'feature', target_branch: 'ff-target', source_project: project)
+      merge_commit_id = repository.ff_merge(user,
+                                            merge_request.diff_head_sha,
+                                            merge_request.target_branch,
+                                            merge_request: merge_request)
+      merge_commit = repository.commit(merge_commit_id)
+
+      expect(merge_commit).to be_present
+      expect(repository.blob_at(merge_commit.id, 'files/ruby/feature.rb')).to be_present
+    end
+
+    it 'sets the `in_progress_merge_commit_sha` flag for the given merge request' do
+      merge_request = create(:merge_request, source_branch: 'feature', target_branch: 'ff-target', source_project: project)
+      merge_commit_id = repository.ff_merge(user,
+                                            merge_request.diff_head_sha,
+                                            merge_request.target_branch,
+                                            merge_request: merge_request)
+
+      expect(merge_request.in_progress_merge_commit_sha).to eq(merge_commit_id)
+    end
+  end
+
   describe '#revert' do
     let(:new_image_commit) { repository.commit('33f3729a45c02fc67d00adb1b8bca394b0e761d9') }
     let(:update_image_commit) { repository.commit('2f63565e7aac07bcdadb654e253078b727143ec4') }
diff --git a/spec/serializers/merge_request_entity_spec.rb b/spec/serializers/merge_request_entity_spec.rb
index a2fd5b7daae..4288955ddbc 100644
--- a/spec/serializers/merge_request_entity_spec.rb
+++ b/spec/serializers/merge_request_entity_spec.rb
@@ -47,7 +47,8 @@ describe MergeRequestEntity do
                                :cancel_merge_when_pipeline_succeeds_path,
                                :create_issue_to_resolve_discussions_path,
                                :source_branch_path, :target_branch_commits_path,
-                               :target_branch_tree_path, :commits_count, :merge_ongoing)
+                               :target_branch_tree_path, :commits_count, :merge_ongoing,
+                               :ff_only_enabled)
   end
 
   it 'has email_patches_path' do
diff --git a/spec/services/merge_requests/ff_merge_service_spec.rb b/spec/services/merge_requests/ff_merge_service_spec.rb
new file mode 100644
index 00000000000..aaabf3ed2b0
--- /dev/null
+++ b/spec/services/merge_requests/ff_merge_service_spec.rb
@@ -0,0 +1,84 @@
+require 'spec_helper'
+
+describe MergeRequests::FfMergeService do
+  let(:user) { create(:user) }
+  let(:user2) { create(:user) }
+  let(:merge_request) do
+    create(:merge_request,
+           source_branch: 'flatten-dir',
+           target_branch: 'improve/awesome',
+           assignee: user2)
+  end
+  let(:project) { merge_request.project }
+
+  before do
+    project.team << [user, :master]
+    project.team << [user2, :developer]
+  end
+
+  describe '#execute' do
+    context 'valid params' do
+      let(:service) { described_class.new(project, user, {}) }
+
+      before do
+        allow(service).to receive(:execute_hooks)
+
+        perform_enqueued_jobs do
+          service.execute(merge_request)
+        end
+      end
+
+      it "does not create merge commit" do
+        source_branch_sha = merge_request.source_project.repository.commit(merge_request.source_branch).sha
+        target_branch_sha = merge_request.target_project.repository.commit(merge_request.target_branch).sha
+        expect(source_branch_sha).to eq(target_branch_sha)
+      end
+
+      it { expect(merge_request).to be_valid }
+      it { expect(merge_request).to be_merged }
+
+      it 'sends email to user2 about merge of new merge_request' do
+        email = ActionMailer::Base.deliveries.last
+        expect(email.to.first).to eq(user2.email)
+        expect(email.subject).to include(merge_request.title)
+      end
+
+      it 'creates system note about merge_request merge' do
+        note = merge_request.notes.last
+        expect(note.note).to include 'merged'
+      end
+    end
+
+    context "error handling" do
+      let(:service) { described_class.new(project, user, commit_message: 'Awesome message') }
+
+      before do
+        allow(Rails.logger).to receive(:error)
+      end
+
+      it 'logs and saves error if there is an exception' do
+        error_message = 'error message'
+
+        allow(service).to receive(:repository).and_raise("error message")
+        allow(service).to receive(:execute_hooks)
+
+        service.execute(merge_request)
+
+        expect(merge_request.merge_error).to include(error_message)
+        expect(Rails.logger).to have_received(:error).with(a_string_matching(error_message))
+      end
+
+      it 'logs and saves error if there is an PreReceiveError exception' do
+        error_message = 'error message'
+
+        allow(service).to receive(:repository).and_raise(Gitlab::Git::HooksService::PreReceiveError, error_message)
+        allow(service).to receive(:execute_hooks)
+
+        service.execute(merge_request)
+
+        expect(merge_request.merge_error).to include(error_message)
+        expect(Rails.logger).to have_received(:error).with(a_string_matching(error_message))
+      end
+    end
+  end
+end
diff --git a/spec/support/test_env.rb b/spec/support/test_env.rb
index b4e8b5ea67b..79395f4c564 100644
--- a/spec/support/test_env.rb
+++ b/spec/support/test_env.rb
@@ -17,6 +17,7 @@ module TestEnv
     'feature_conflict'                   => 'bb5206f',
     'fix'                                => '48f0be4',
     'improve/awesome'                    => '5937ac0',
+    'merged-target'                      => '21751bf',
     'markdown'                           => '0ed8c6c',
     'lfs'                                => 'be93687',
     'master'                             => 'b83d6e3',