Add 'squash and rebase' feature to CEblackst0ne-squash-and-merge-in-gitlab-core-ce

41 files changed, 840 insertions, 64 deletions

diff --git a/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_squash_before_merge.vue b/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_squash_before_merge.vue
index 926a3172412..875c3323dbb 100644
--- a/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_squash_before_merge.vue
+++ b/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_squash_before_merge.vue
@@ -1,15 +1,63 @@
-/*
-The squash-before-merge button is EE only, but it's located right in the middle
-of the readyToMerge state component template.
-
-If we didn't declare this component in CE, we'd need to maintain a separate copy
-of the readyToMergeState template in EE, which is pretty big and likely to change.
-
-Instead, in CE, we declare the component, but it's hidden and is configured to do nothing.
-In EE, the configuration extends this object to add a functioning squash-before-merge
-button.
-*/
-
 <script>
-  export default {};
+import Icon from '~/vue_shared/components/icon.vue';
+import eventHub from '~/vue_merge_request_widget/event_hub';
+import tooltip from '~/vue_shared/directives/tooltip';
+
+export default {
+  components: {
+    Icon,
+  },
+  directives: {
+    tooltip,
+  },
+  props: {
+    mr: {
+      type: Object,
+      required: true,
+    },
+    isMergeButtonDisabled: {
+      type: Boolean,
+      required: true,
+    },
+  },
+  data() {
+    return {
+      squashBeforeMerge: this.mr.squash,
+    };
+  },
+  methods: {
+    updateSquashModel() {
+      eventHub.$emit('MRWidgetUpdateSquash', this.squashBeforeMerge);
+    },
+  },
+};
 </script>
+
+<template>
+  <div class="accept-control inline">
+    <label class="merge-param-checkbox">
+      <input
+        type="checkbox"
+        name="squash"
+        class="qa-squash-checkbox"
+        :disabled="isMergeButtonDisabled"
+        v-model="squashBeforeMerge"
+        @change="updateSquashModel"
+      />
+      {{ __('Squash commits') }}
+    </label>
+    <a
+      :href="mr.squashBeforeMergeHelpPath"
+      data-title="About this feature"
+      data-placement="bottom"
+      target="_blank"
+      rel="noopener noreferrer nofollow"
+      data-container="body"
+      v-tooltip
+    >
+      <icon
+        name="question-o"
+      />
+    </a>
+  </div>
+</template>
diff --git a/app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue b/app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue
index 1d1c8ebc179..3a194320bd8 100644
--- a/app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue
+++ b/app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue
@@ -6,11 +6,13 @@ import MergeRequest from '../../../merge_request';
 import Flash from '../../../flash';
 import statusIcon from '../mr_widget_status_icon.vue';
 import eventHub from '../../event_hub';
+import SquashBeforeMerge from './mr_widget_squash_before_merge.vue';
 
 export default {
   name: 'ReadyToMerge',
   components: {
     statusIcon,
+    'squash-before-merge': SquashBeforeMerge,
   },
   props: {
     mr: { type: Object, required: true },
@@ -101,6 +103,12 @@ export default {
       return enableSquashBeforeMerge && commitsCount > 1;
     },
   },
+  created() {
+    eventHub.$on('MRWidgetUpdateSquash', this.handleUpdateSquash);
+  },
+  beforeDestroy() {
+    eventHub.$off('MRWidgetUpdateSquash', this.handleUpdateSquash);
+  },
   methods: {
     shouldShowMergeControls() {
       return this.mr.isMergeAllowed || this.shouldShowMergeWhenPipelineSucceedsText;
@@ -128,13 +136,9 @@ export default {
         commit_message: this.commitMessage,
         merge_when_pipeline_succeeds: this.setToMergeWhenPipelineSucceeds,
         should_remove_source_branch: this.removeSourceBranch === true,
+        squash: this.mr.squash,
       };
 
-      // Only truthy in EE extension of this component
-      if (this.setAdditionalParams) {
-        this.setAdditionalParams(options);
-      }
-
       this.isMakingRequest = true;
       this.service.merge(options)
         .then(res => res.data)
@@ -154,6 +158,9 @@ export default {
           new Flash('Something went wrong. Please try again.'); // eslint-disable-line
         });
     },
+    handleUpdateSquash(val) {
+      this.mr.squash = val;
+    },
     initiateMergePolling() {
       simplePoll((continuePolling, stopPolling) => {
         this.handleMergePolling(continuePolling, stopPolling);
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 83b7b054e6f..e5b7e1f1c68 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
@@ -15,6 +15,11 @@ export default class MergeRequestStore {
     const currentUser = data.current_user;
     const pipelineStatus = data.pipeline ? data.pipeline.details.status : null;
 
+    this.squash = data.squash;
+    this.squashBeforeMergeHelpPath = this.squashBeforeMergeHelpPath ||
+      data.squash_before_merge_help_path;
+    this.enableSquashBeforeMerge = this.enableSquashBeforeMerge || true;
+
     this.title = data.title;
     this.targetBranch = data.target_branch;
     this.sourceBranch = data.source_branch;
diff --git a/app/controllers/projects/merge_requests/application_controller.rb b/app/controllers/projects/merge_requests/application_controller.rb
index 67d4ea2ca8f..29632bef7e5 100644
--- a/app/controllers/projects/merge_requests/application_controller.rb
+++ b/app/controllers/projects/merge_requests/application_controller.rb
@@ -24,6 +24,7 @@ class Projects::MergeRequests::ApplicationController < Projects::ApplicationCont
       :source_branch,
       :source_project_id,
       :state_event,
+      :squash,
       :target_branch,
       :target_project_id,
       :task_num,
diff --git a/app/controllers/projects/merge_requests_controller.rb b/app/controllers/projects/merge_requests_controller.rb
index 62b739918e6..507a07c6e1b 100644
--- a/app/controllers/projects/merge_requests_controller.rb
+++ b/app/controllers/projects/merge_requests_controller.rb
@@ -253,7 +253,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   end
 
   def merge_params_attributes
-    [:should_remove_source_branch, :commit_message]
+    [:should_remove_source_branch, :commit_message, :squash]
   end
 
   def merge_when_pipeline_succeeds_active?
@@ -282,7 +282,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
 
     return :sha_mismatch if params[:sha] != @merge_request.diff_head_sha
 
-    @merge_request.update(merge_error: nil)
+    @merge_request.update(merge_error: nil, squash: merge_params.fetch(:squash, false))
 
     if params[:merge_when_pipeline_succeeds].present?
       return :failed unless @merge_request.actual_head_pipeline
diff --git a/app/helpers/merge_requests_helper.rb b/app/helpers/merge_requests_helper.rb
index c19c5b9cc82..74251c260f0 100644
--- a/app/helpers/merge_requests_helper.rb
+++ b/app/helpers/merge_requests_helper.rb
@@ -97,8 +97,9 @@ module MergeRequestsHelper
     {
       merge_when_pipeline_succeeds: true,
       should_remove_source_branch: true,
-      sha: merge_request.diff_head_sha
-    }.merge(merge_params_ee(merge_request))
+      sha: merge_request.diff_head_sha,
+      squash: merge_request.squash
+    }
   end
 
   def tab_link_for(merge_request, tab, options = {}, &block)
@@ -149,8 +150,4 @@ module MergeRequestsHelper
       current_user.fork_of(project)
     end
   end
-
-  def merge_params_ee(merge_request)
-    {}
-  end
 end
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 9c4384a6e42..bc97fc3a5d9 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -1140,4 +1140,11 @@ class MergeRequest < ActiveRecord::Base
     maintainer_push_possible? &&
       Ability.allowed?(user, :push_code, source_project)
   end
+
+  def squash_in_progress?
+    # The source project can be deleted
+    return false unless source_project
+
+    source_project.repository.squash_in_progress?(id)
+  end
 end
diff --git a/app/models/repository.rb b/app/models/repository.rb
index 0e1bf11d7c0..6165808cd9a 100644
--- a/app/models/repository.rb
+++ b/app/models/repository.rb
@@ -957,6 +957,14 @@ class Repository
                                        remote_branch: merge_request.target_branch)
   end
 
+  def squash(user, merge_request)
+    raw.squash(user, merge_request.id, branch: merge_request.target_branch,
+                                       start_sha: merge_request.diff_start_sha,
+                                       end_sha: merge_request.diff_head_sha,
+                                       author: merge_request.author,
+                                       message: merge_request.title)
+  end
+
   private
 
   # TODO Generice finder, later split this on finders by Ref or Oid
diff --git a/app/serializers/merge_request_widget_entity.rb b/app/serializers/merge_request_widget_entity.rb
index d0165c148eb..141070aef45 100644
--- a/app/serializers/merge_request_widget_entity.rb
+++ b/app/serializers/merge_request_widget_entity.rb
@@ -10,6 +10,7 @@ class MergeRequestWidgetEntity < IssuableEntity
   expose :merge_when_pipeline_succeeds
   expose :source_branch
   expose :source_project_id
+  expose :squash
   expose :target_branch
   expose :target_project_id
   expose :allow_maintainer_to_push
diff --git a/app/services/merge_requests/merge_service.rb b/app/services/merge_requests/merge_service.rb
index 2209a60a840..126da891c78 100644
--- a/app/services/merge_requests/merge_service.rb
+++ b/app/services/merge_requests/merge_service.rb
@@ -34,6 +34,19 @@ module MergeRequests
       handle_merge_error(log_message: e.message, save_message_on_model: true)
     end
 
+    def source
+      return merge_request.diff_head_sha unless merge_request.squash
+
+      squash_result = ::MergeRequests::SquashService.new(project, current_user, params).execute(merge_request)
+
+      case squash_result[:status]
+      when :success
+        squash_result[:squash_sha]
+      when :error
+        raise ::MergeRequests::MergeService::MergeError, squash_result[:message]
+      end
+    end
+
     private
 
     def error_check!
@@ -116,9 +129,5 @@ module MergeRequests
     def merge_request_info
       merge_request.to_reference(full: true)
     end
-
-    def source
-      @source ||= @merge_request.diff_head_sha
-    end
   end
 end
diff --git a/app/services/merge_requests/squash_service.rb b/app/services/merge_requests/squash_service.rb
new file mode 100644
index 00000000000..a40fb2786bd
--- /dev/null
+++ b/app/services/merge_requests/squash_service.rb
@@ -0,0 +1,28 @@
+module MergeRequests
+  class SquashService < MergeRequests::WorkingCopyBaseService
+    def execute(merge_request)
+      @merge_request = merge_request
+      @repository = target_project.repository
+
+      squash || error('Failed to squash. Should be done manually.')
+    end
+
+    def squash
+      if merge_request.commits_count < 2
+        return success(squash_sha: merge_request.diff_head_sha)
+      end
+
+      if merge_request.squash_in_progress?
+        return error('Squash task canceled: another squash is already in progress.')
+      end
+
+      squash_sha = repository.squash(current_user, merge_request)
+
+      success(squash_sha: squash_sha)
+    rescue => e
+      log_error("Failed to squash merge request #{merge_request.to_reference(full: true)}:")
+      log_error(e.message)
+      false
+    end
+  end
+end
diff --git a/app/views/projects/merge_requests/show.html.haml b/app/views/projects/merge_requests/show.html.haml
index aa3fb623e58..01e38ffee20 100644
--- a/app/views/projects/merge_requests/show.html.haml
+++ b/app/views/projects/merge_requests/show.html.haml
@@ -20,6 +20,8 @@
       window.gl = window.gl || {};
       window.gl.mrWidgetData = #{serialize_issuable(@merge_request, serializer: 'widget')}
 
+      window.gl.mrWidgetData.squash_before_merge_help_path = '#{help_page_path("user/project/merge_requests/squash_and_merge")}';
+
     #js-vue-mr-widget.mr-widget
 
     .content-block.content-block-small.emoji-list-container.js-noteable-awards
diff --git a/app/views/shared/issuable/form/_merge_params.html.haml b/app/views/shared/issuable/form/_merge_params.html.haml
index 1df881e4102..90fbf19e843 100644
--- a/app/views/shared/issuable/form/_merge_params.html.haml
+++ b/app/views/shared/issuable/form/_merge_params.html.haml
@@ -15,3 +15,12 @@
           = hidden_field_tag 'merge_request[force_remove_source_branch]', '0', id: nil
           = check_box_tag 'merge_request[force_remove_source_branch]', '1', issuable.force_remove_source_branch?
           Remove source branch when merge request is accepted.
+
+.form-group
+  .col-sm-10.col-sm-offset-2
+    .checkbox
+      = label_tag 'merge_request[squash]' do
+        = hidden_field_tag 'merge_request[squash]', '0', id: nil
+        = check_box_tag 'merge_request[squash]', '1', issuable.squash
+        Squash commits when merge request is accepted.
+        = link_to 'About this feature', help_page_path('user/project/merge_requests/squash_and_merge')
diff --git a/changelogs/unreleased/blackst0ne-squash-and-merge-in-gitlab-core-ce.yml b/changelogs/unreleased/blackst0ne-squash-and-merge-in-gitlab-core-ce.yml
new file mode 100644
index 00000000000..e603c835b5e
--- /dev/null
+++ b/changelogs/unreleased/blackst0ne-squash-and-merge-in-gitlab-core-ce.yml
@@ -0,0 +1,5 @@
+---
+title: Add `Squash and merge` to GitLab Core (CE)
+merge_request: 18956
+author: "@blackst0ne"
+type: added
diff --git a/db/migrate/20180515005612_add_squash_to_merge_requests.rb b/db/migrate/20180515005612_add_squash_to_merge_requests.rb
new file mode 100644
index 00000000000..f526b45bd4b
--- /dev/null
+++ b/db/migrate/20180515005612_add_squash_to_merge_requests.rb
@@ -0,0 +1,19 @@
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddSquashToMergeRequests < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+  disable_ddl_transaction!
+
+  DOWNTIME = false
+
+  def up
+    unless column_exists?(:merge_requests, :squash)
+      add_column_with_default :merge_requests, :squash, :boolean, default: false, allow_null: false
+    end
+  end
+
+  def down
+    remove_column :merge_requests, :squash if column_exists?(:merge_requests, :squash)
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 884e333874c..ee9c85da7a2 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -1217,6 +1217,7 @@ ActiveRecord::Schema.define(version: 20180521171529) do
     t.integer "latest_merge_request_diff_id"
     t.string "rebase_commit_sha"
     t.boolean "allow_maintainer_to_push"
+    t.boolean "squash", default: false, null: false
   end
 
   add_index "merge_requests", ["assignee_id"], name: "index_merge_requests_on_assignee_id", using: :btree
diff --git a/doc/api/merge_requests.md b/doc/api/merge_requests.md
index 4e34831422a..8849f490c4f 100644
--- a/doc/api/merge_requests.md
+++ b/doc/api/merge_requests.md
@@ -107,6 +107,7 @@ Parameters:
     "changes_count": "1",
     "should_remove_source_branch": true,
     "force_remove_source_branch": false,
+    "squash": false,
     "web_url": "http://example.com/example/example/merge_requests/1",
     "time_stats": {
       "time_estimate": 0,
@@ -226,6 +227,7 @@ Parameters:
     "changes_count": "1",
     "should_remove_source_branch": true,
     "force_remove_source_branch": false,
+    "squash": false,
     "web_url": "http://example.com/example/example/merge_requests/1",
     "discussion_locked": false,
     "time_stats": {
@@ -305,6 +307,7 @@ Parameters:
   "changes_count": "1",
   "should_remove_source_branch": true,
   "force_remove_source_branch": false,
+  "squash": false,
   "web_url": "http://example.com/example/example/merge_requests/1",
   "discussion_locked": false,
   "time_stats": {
@@ -541,7 +544,8 @@ POST /projects/:id/merge_requests
 | `labels`                   | string  | no       | Labels for MR as a comma-separated list                                         |
 | `milestone_id`             | integer | no       | The global ID of a milestone                                                           |
 | `remove_source_branch`     | boolean | no       | Flag indicating if a merge request should remove the source branch when merging |
-| `allow_maintainer_to_push` | boolean | no       | Whether or not a maintainer of the target project can push to the source branch  |
+| `allow_maintainer_to_push` | boolean | no       | Whether or not a maintainer of the target project can push to the source branch |
+| `squash`                   | boolean | no       | Squash commits into a single commit when merging                                |
 
 ```json
 {
@@ -595,6 +599,7 @@ POST /projects/:id/merge_requests
   "changes_count": "1",
   "should_remove_source_branch": true,
   "force_remove_source_branch": false,
+  "squash": false,
   "web_url": "http://example.com/example/example/merge_requests/1",
   "discussion_locked": false,
   "allow_maintainer_to_push": false,
@@ -627,6 +632,7 @@ PUT /projects/:id/merge_requests/:merge_request_iid
 | `description`              | string  | no       | Description of MR                                                               |
 | `state_event`              | string  | no       | New state (close/reopen)                                                        |
 | `remove_source_branch`     | boolean | no       | Flag indicating if a merge request should remove the source branch when merging |
+| `squash`                   | boolean | no       | Squash commits into a single commit when merging |
 | `discussion_locked`        | boolean | no       | Flag indicating if the merge request's discussion is locked. If the discussion is locked only project members can add, edit or resolve comments. |
 | `allow_maintainer_to_push` | boolean | no       | Whether or not a maintainer of the target project can push to the source branch |
 
@@ -683,6 +689,7 @@ Must include at least one non-required attribute from above.
   "changes_count": "1",
   "should_remove_source_branch": true,
   "force_remove_source_branch": false,
+  "squash": false,
   "web_url": "http://example.com/example/example/merge_requests/1",
   "discussion_locked": false,
   "allow_maintainer_to_push": false,
@@ -790,6 +797,7 @@ Parameters:
   "changes_count": "1",
   "should_remove_source_branch": true,
   "force_remove_source_branch": false,
+  "squash": false,
   "web_url": "http://example.com/example/example/merge_requests/1",
   "discussion_locked": false,
   "time_stats": {
@@ -868,6 +876,7 @@ Parameters:
   "changes_count": "1",
   "should_remove_source_branch": true,
   "force_remove_source_branch": false,
+  "squash": false,
   "web_url": "http://example.com/example/example/merge_requests/1",
   "discussion_locked": false,
   "time_stats": {
@@ -1200,6 +1209,7 @@ Example response:
     "changes_count": "1",
     "should_remove_source_branch": true,
     "force_remove_source_branch": false,
+    "squash": false,
     "web_url": "http://example.com/example/example/merge_requests/1"
   },
   "target_url": "https://gitlab.example.com/gitlab-org/gitlab-ci/merge_requests/7",
diff --git a/doc/user/project/merge_requests/img/squash_edit_form.png b/doc/user/project/merge_requests/img/squash_edit_form.png
new file mode 100644
index 00000000000..496c6f44ea7
--- /dev/null
+++ b/doc/user/project/merge_requests/img/squash_edit_form.png
diff --git a/doc/user/project/merge_requests/img/squash_mr_commits.png b/doc/user/project/merge_requests/img/squash_mr_commits.png
new file mode 100644
index 00000000000..5fc6a8c48bb
--- /dev/null
+++ b/doc/user/project/merge_requests/img/squash_mr_commits.png
diff --git a/doc/user/project/merge_requests/img/squash_mr_widget.png b/doc/user/project/merge_requests/img/squash_mr_widget.png
new file mode 100644
index 00000000000..9cb458b2a35
--- /dev/null
+++ b/doc/user/project/merge_requests/img/squash_mr_widget.png
diff --git a/doc/user/project/merge_requests/img/squash_squashed_commit.png b/doc/user/project/merge_requests/img/squash_squashed_commit.png
new file mode 100644
index 00000000000..0cf5875f82c
--- /dev/null
+++ b/doc/user/project/merge_requests/img/squash_squashed_commit.png
diff --git a/doc/user/project/merge_requests/index.md b/doc/user/project/merge_requests/index.md
index 5932f5a2bc1..b75bcacc9d7 100644
--- a/doc/user/project/merge_requests/index.md
+++ b/doc/user/project/merge_requests/index.md
@@ -29,12 +29,12 @@ With GitLab merge requests, you can:
 - 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
 - [Create new merge requests by email](#create-new-merge-requests-by-email)
 - Allow maintainers of the target project to push directly to the fork by [allowing edits from maintainers](maintainer_access.md)
+- [Squash and merge](squash_and_merge.md) for a cleaner commit history
 
 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) **[PREMIUM]**
 - Request [approvals](https://docs.gitlab.com/ee/user/project/merge_requests/merge_request_approvals.html) from your managers **[STARTER]**
-- [Squash and merge](https://docs.gitlab.com/ee/user/project/merge_requests/squash_and_merge.html) for a cleaner commit history **[STARTER]**
 - Analyze the impact of your changes with [Code Quality reports](https://docs.gitlab.com/ee/user/project/merge_requests/code_quality_diff.html) **[STARTER]**
 
 ## Use cases
@@ -57,7 +57,7 @@ B. Consider you're a web developer writing a webpage for your company's:
 1. Your changes are previewed with [Review Apps](../../../ci/review_apps/index.md)
 1. You request your web designers for their implementation
 1. You request the [approval](https://docs.gitlab.com/ee/user/project/merge_requests/merge_request_approvals.html) from your manager **[STARTER]**
-1. Once approved, your merge request is [squashed and merged](https://docs.gitlab.com/ee/user/project/merge_requests/squash_and_merge.html), and [deployed to staging with GitLab Pages](https://about.gitlab.com/2016/08/26/ci-deployment-and-environments/) (Squash and Merge is available in GitLab Starter)
+1. Once approved, your merge request is [squashed and merged](squash_and_merge.md), and [deployed to staging with GitLab Pages](https://about.gitlab.com/2016/08/26/ci-deployment-and-environments/)
 1. Your production team [cherry picks](#cherry-pick-changes) the merge commit into production
 
 ## Merge requests per project
diff --git a/doc/user/project/merge_requests/squash_and_merge.md b/doc/user/project/merge_requests/squash_and_merge.md
new file mode 100644
index 00000000000..a6efe893853
--- /dev/null
+++ b/doc/user/project/merge_requests/squash_and_merge.md
@@ -0,0 +1,80 @@
+# Squash and merge
+
+> [Introduced][ee-1024] in [GitLab Starter][ee] 8.17, and in [GitLab CE][ce] [11.0][ce-18956].
+
+Combine all commits of your merge request into one and retain a clean history.
+
+## Overview
+
+Squashing lets you tidy up the commit history of a branch when accepting a merge
+request. It applies all of the changes in the merge request as a single commit,
+and then merges that commit using the merge method set for the project.
+
+In other words, squashing a merge request turns a long list of commits:
+
+![List of commits from a merge request][mr-commits]
+
+Into a single commit on merge:
+
+![A squashed commit followed by a merge commit][squashed-commit]
+
+The squashed commit's commit message is the merge request title. And note that 
+the squashed commit is still followed by a merge commit, as the merge
+method for this example repository uses a merge commit. Squashing also works
+with the fast-forward merge strategy, see
+[squashing and fast-forward merge](#squash-and-fast-forward-merge) for more
+details.
+
+## Use cases
+
+When working on a feature branch, you sometimes want to commit your current
+progress, but don't really care about the commit messages. Those 'work in
+progress commits' don't necessarily contain important information and as such
+you'd rather not include them in your target branch. 
+
+With squash and merge, when the merge request is ready to be merged,
+all you have to do is enable squashing before you press merge to join
+the commits include in the merge request into a single commit.
+
+This way, the history of your base branch remains clean with
+meaningful commit messages and is simpler to [revert] if necessary.
+
+## Enabling squash for a merge request
+
+Anyone who can create or edit a merge request can choose for it to be squashed
+on the merge request form:
+
+![Squash commits checkbox on edit form][squash-edit-form]
+
+---
+
+This can then be overridden at the time of accepting the merge request:
+
+![Squash commits checkbox on accept merge request form][squash-mr-widget]
+
+## Commit metadata for squashed commits
+
+The squashed commit has the following metadata:
+
+* Message: the title of the merge request.
+* Author: the author of the merge request.
+* Committer: the user who initiated the squash.
+
+## Squash and fast-forward merge
+
+When a project has the [fast-forward merge setting enabled][ff-merge], the merge
+request must be able to be fast-forwarded without squashing in order to squash
+it. This is because squashing is only available when accepting a merge request,
+so a merge request may need to be rebased before squashing, even though
+squashing can itself be considered equivalent to rebasing.
+
+[ee-1024]: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1024
+[ce-18956]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18956
+[mr-commits]: img/squash_mr_commits.png
+[squashed-commit]: img/squash_squashed_commit.png
+[squash-edit-form]: img/squash_edit_form.png
+[squash-mr-widget]: img/squash_mr_widget.png
+[ff-merge]: fast_forward_merge.md#enabling-fast-forward-merges
+[ce]: https://about.gitlab.com/products/
+[ee]: https://about.gitlab.com/products/
+[revert]: revert_changes.md
diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 174c5af91d5..b630fd54354 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -568,6 +568,8 @@ module API
       expose :time_stats, using: 'API::Entities::IssuableTimeStats' do |merge_request|
         merge_request
       end
+
+      expose :squash
     end
 
     class MergeRequest < MergeRequestBasic
diff --git a/lib/api/merge_requests.rb b/lib/api/merge_requests.rb
index bc4df16e3a8..1ba9a09346f 100644
--- a/lib/api/merge_requests.rb
+++ b/lib/api/merge_requests.rb
@@ -10,12 +10,6 @@ module API
     helpers do
       params :optional_params_ee do
       end
-
-      params :merge_params_ee do
-      end
-
-      def update_merge_request_ee(merge_request)
-      end
     end
 
     def self.update_params_at_least_one_of
@@ -29,6 +23,7 @@ module API
         target_branch
         title
         discussion_locked
+        squash
       ]
     end
 
@@ -146,6 +141,7 @@ module API
           optional :labels, type: String, desc: 'Comma-separated list of label names'
           optional :remove_source_branch, type: Boolean, desc: 'Remove source branch when merging'
           optional :allow_maintainer_to_push, type: Boolean, desc: 'Whether a maintainer of the target project can push to the source project'
+          optional :squash, type: Grape::API::Boolean, desc: 'When true, the commits will be squashed into a single commit on merge'
 
           use :optional_params_ee
         end
@@ -308,8 +304,7 @@ module API
         optional :merge_when_pipeline_succeeds, type: Boolean,
                                                 desc: 'When true, this merge request will be merged when the pipeline succeeds'
         optional :sha, type: String, desc: 'When present, must have the HEAD SHA of the source branch'
-
-        use :merge_params_ee
+        optional :squash, type: Grape::API::Boolean, desc: 'When true, the commits will be squashed into a single commit on merge'
       end
       put ':id/merge_requests/:merge_request_iid/merge' do
         Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42317')
@@ -327,7 +322,7 @@ module API
 
         check_sha_param!(params, merge_request)
 
-        update_merge_request_ee(merge_request)
+        merge_request.update(squash: params[:squash]) if params[:squash]
 
         merge_params = {
           commit_message: params[:merge_commit_message],
diff --git a/lib/api/v3/entities.rb b/lib/api/v3/entities.rb
index 68b4d7c3982..28fcf6c6e84 100644
--- a/lib/api/v3/entities.rb
+++ b/lib/api/v3/entities.rb
@@ -134,6 +134,8 @@ module API
         expose :should_remove_source_branch?, as: :should_remove_source_branch
         expose :force_remove_source_branch?, as: :force_remove_source_branch
 
+        expose :squash
+
         expose :web_url do |merge_request, options|
           Gitlab::UrlBuilder.build(merge_request)
         end
diff --git a/lib/api/v3/merge_requests.rb b/lib/api/v3/merge_requests.rb
index 9b0f70e2bfe..af5afd1c334 100644
--- a/lib/api/v3/merge_requests.rb
+++ b/lib/api/v3/merge_requests.rb
@@ -44,6 +44,7 @@ module API
             optional :milestone_id, type: Integer, desc: 'The ID of a milestone to assign the merge request'
             optional :labels, type: String, desc: 'Comma-separated list of label names'
             optional :remove_source_branch, type: Boolean, desc: 'Remove source branch when merging'
+            optional :squash, type: Boolean, desc: 'Squash commits when merging'
           end
         end
 
@@ -166,7 +167,7 @@ module API
             use :optional_params
             at_least_one_of :title, :target_branch, :description, :assignee_id,
                             :milestone_id, :labels, :state_event,
-                            :remove_source_branch
+                            :remove_source_branch, :squash
           end
           put path do
             Gitlab::QueryLimiting.whitelist('https://gitlab.com/gitlab-org/gitlab-ce/issues/42127')
@@ -195,6 +196,7 @@ module API
             optional :merge_when_build_succeeds, type: Boolean,
                                                  desc: 'When true, this merge request will be merged when the build succeeds'
             optional :sha, type: String, desc: 'When present, must have the HEAD SHA of the source branch'
+            optional :squash, type: Boolean, desc: 'When true, the commits will be squashed into a single commit on merge'
           end
           put "#{path}/merge" do
             merge_request = find_project_merge_request(params[:merge_request_id])
@@ -211,6 +213,8 @@ module API
               render_api_error!("SHA does not match HEAD of source branch: #{merge_request.diff_head_sha}", 409)
             end
 
+            merge_request.update(squash: params[:squash]) if params[:squash]
+
             merge_params = {
               commit_message: params[:merge_commit_message],
               should_remove_source_branch: params[:should_remove_source_branch]
diff --git a/qa/qa/page/merge_request/show.rb b/qa/qa/page/merge_request/show.rb
index 166861e6c4a..9507f92f4b2 100644
--- a/qa/qa/page/merge_request/show.rb
+++ b/qa/qa/page/merge_request/show.rb
@@ -16,6 +16,10 @@ module QA
           element :no_fast_forward_message, 'Fast-forward merge is not possible'
         end
 
+        view 'app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_squash_before_merge.vue' do
+          element :squash_checkbox
+        end
+
         def rebase!
           click_element :mr_rebase_button
 
@@ -41,6 +45,14 @@ module QA
             has_text?('The changes were merged into')
           end
         end
+
+        def mark_to_squash
+          wait(reload: true) do
+            has_css?(element_selector_css(:squash_checkbox))
+          end
+
+          click_element :squash_checkbox
+        end
       end
     end
   end
diff --git a/qa/qa/specs/features/merge_request/squash_spec.rb b/qa/qa/specs/features/merge_request/squash_spec.rb
new file mode 100644
index 00000000000..dbbdf852a38
--- /dev/null
+++ b/qa/qa/specs/features/merge_request/squash_spec.rb
@@ -0,0 +1,48 @@
+module QA
+  feature 'merge request squash commits', :core do
+    scenario 'when squash commits is marked before merge'  do
+      Runtime::Browser.visit(:gitlab, Page::Main::Login)
+      Page::Main::Login.act { sign_in_using_credentials }
+
+      project = Factory::Resource::Project.fabricate! do |project|
+        project.name = "squash-before-merge"
+      end
+
+      merge_request = Factory::Resource::MergeRequest.fabricate! do |merge_request|
+        merge_request.project = project
+        merge_request.title = 'Squashing commits'
+      end
+
+      Factory::Repository::Push.fabricate! do |push|
+        push.project = project
+        push.commit_message = 'to be squashed'
+        push.branch_name = merge_request.source_branch
+        push.new_branch = false
+        push.file_name = 'other.txt'
+        push.file_content = "Test with unicode characters ❤✓€❄"
+      end
+
+      merge_request.visit!
+
+      Page::MergeRequest::Show.perform do |merge_request_page|
+        merge_request_page.mark_to_squash
+        merge_request_page.merge!
+
+        merge_request.project.visit!
+
+        Git::Repository.perform do |repository|
+          repository.uri = Page::Project::Show.act do
+            choose_repository_clone_http
+            repository_location.uri
+          end
+
+          repository.use_default_credentials
+
+          repository.act { clone }
+
+          expect(repository.commits.size).to eq 3
+        end
+      end
+    end
+  end
+end
diff --git a/spec/controllers/projects/merge_requests_controller_spec.rb b/spec/controllers/projects/merge_requests_controller_spec.rb
index d3042be9e8b..5efaaf2bb3a 100644
--- a/spec/controllers/projects/merge_requests_controller_spec.rb
+++ b/spec/controllers/projects/merge_requests_controller_spec.rb
@@ -275,6 +275,7 @@ describe Projects::MergeRequestsController do
         namespace_id: project.namespace,
         project_id: project,
         id: merge_request.iid,
+        squash: false,
         format: 'json'
       }
     end
@@ -315,8 +316,8 @@ describe Projects::MergeRequestsController do
     end
 
     context 'when the sha parameter matches the source SHA' do
-      def merge_with_sha
-        post :merge, base_params.merge(sha: merge_request.diff_head_sha)
+      def merge_with_sha(params = {})
+        post :merge, base_params.merge(sha: merge_request.diff_head_sha).merge(params)
       end
 
       it 'returns :success' do
@@ -331,6 +332,24 @@ describe Projects::MergeRequestsController do
         merge_with_sha
       end
 
+      context 'when squash is passed as 1' do
+        it 'updates the squash attribute on the MR to true' do
+          merge_request.update(squash: false)
+          merge_with_sha(squash: '1')
+
+          expect(merge_request.reload.squash).to be_truthy
+        end
+      end
+
+      context 'when squash is passed as 0' do
+        it 'updates the squash attribute on the MR to false' do
+          merge_request.update(squash: true)
+          merge_with_sha(squash: '0')
+
+          expect(merge_request.reload.squash).to be_falsey
+        end
+      end
+
       context 'when the pipeline succeeds is passed' do
         let!(:head_pipeline) do
           create(:ci_empty_pipeline, project: project, sha: merge_request.diff_head_sha, ref: merge_request.source_branch, head_pipeline_of: merge_request)
diff --git a/spec/features/merge_requests/user_squashes_merge_request_spec.rb b/spec/features/merge_requests/user_squashes_merge_request_spec.rb
new file mode 100644
index 00000000000..6c952791591
--- /dev/null
+++ b/spec/features/merge_requests/user_squashes_merge_request_spec.rb
@@ -0,0 +1,124 @@
+require 'spec_helper'
+
+feature 'User squashes a merge request', :js do
+  let(:user) { create(:user) }
+  let(:project) { create(:project, :repository) }
+  let(:source_branch) { 'csv' }
+
+  let!(:original_head) { project.repository.commit('master') }
+
+  shared_examples 'squash' do
+    it 'squashes the commits into a single commit, and adds a merge commit' do
+      expect(page).to have_content('Merged')
+
+      latest_master_commits = project.repository.commits_between(original_head.sha, 'master').map(&:raw)
+
+      squash_commit = an_object_having_attributes(sha: a_string_matching(/\h{40}/),
+                                                  message: "Csv\n",
+                                                  author_name: user.name,
+                                                  committer_name: user.name)
+
+      merge_commit = an_object_having_attributes(sha: a_string_matching(/\h{40}/),
+                                                 message: a_string_starting_with("Merge branch 'csv' into 'master'"),
+                                                 author_name: user.name,
+                                                 committer_name: user.name)
+
+      expect(project.repository).not_to be_merged_to_root_ref(source_branch)
+      expect(latest_master_commits).to match([squash_commit, merge_commit])
+    end
+  end
+
+  shared_examples 'no squash' do
+    it 'accepts the merge request without squashing' do
+      expect(page).to have_content('Merged')
+      expect(project.repository).to be_merged_to_root_ref(source_branch)
+    end
+  end
+
+  def accept_mr
+    expect(page).to have_button('Merge')
+
+    uncheck 'Remove source branch'
+    click_on 'Merge'
+  end
+
+  before do
+    # Prevent source branch from being removed so we can use be_merged_to_root_ref
+    # method to check if squash was performed or not
+    allow_any_instance_of(MergeRequest).to receive(:force_remove_source_branch?).and_return(false)
+    project.add_master(user)
+
+    sign_in user
+  end
+
+  context 'when the MR has only one commit' do
+    before do
+      merge_request = create(:merge_request, source_project: project, target_project: project, source_branch: 'master', target_branch: 'branch-merged')
+
+      visit project_merge_request_path(project, merge_request)
+    end
+
+    it 'does not show the squash checkbox' do
+      expect(page).not_to have_field('squash')
+    end
+  end
+
+  context 'when squash is enabled on merge request creation' do
+    before do
+      visit project_new_merge_request_path(project, merge_request: { target_branch: 'master', source_branch: source_branch })
+      check 'merge_request[squash]'
+      click_on 'Submit merge request'
+      wait_for_requests
+    end
+
+    it 'shows the squash checkbox as checked' do
+      expect(page).to have_checked_field('squash')
+    end
+
+    context 'when accepting with squash checked' do
+      before do
+        accept_mr
+      end
+
+      include_examples 'squash'
+    end
+
+    context 'when accepting and unchecking squash' do
+      before do
+        uncheck 'squash'
+        accept_mr
+      end
+
+      include_examples 'no squash'
+    end
+  end
+
+  context 'when squash is not enabled on merge request creation' do
+    before do
+      visit project_new_merge_request_path(project, merge_request: { target_branch: 'master', source_branch: source_branch })
+      click_on 'Submit merge request'
+      wait_for_requests
+    end
+
+    it 'shows the squash checkbox as unchecked' do
+      expect(page).to have_unchecked_field('squash')
+    end
+
+    context 'when accepting and checking squash' do
+      before do
+        check 'squash'
+        accept_mr
+      end
+
+      include_examples 'squash'
+    end
+
+    context 'when accepting with squash unchecked' do
+      before do
+        accept_mr
+      end
+
+      include_examples 'no squash'
+    end
+  end
+end
diff --git a/spec/fixtures/api/schemas/entities/merge_request_widget.json b/spec/fixtures/api/schemas/entities/merge_request_widget.json
index 233102c4314..7be8c9e3e67 100644
--- a/spec/fixtures/api/schemas/entities/merge_request_widget.json
+++ b/spec/fixtures/api/schemas/entities/merge_request_widget.json
@@ -112,7 +112,8 @@
     "rebase_commit_sha": { "type": ["string", "null"] },
     "rebase_in_progress": { "type": "boolean" },
     "can_push_to_source_branch": { "type": "boolean" },
-    "rebase_path": { "type": ["string", "null"] }
+    "rebase_path": { "type": ["string", "null"] },
+    "squash": { "type": "boolean" }
   },
   "additionalProperties": false
 }
diff --git a/spec/fixtures/api/schemas/public_api/v3/merge_requests.json b/spec/fixtures/api/schemas/public_api/v3/merge_requests.json
index b5c74bcc26e..9af7a8c9f4f 100644
--- a/spec/fixtures/api/schemas/public_api/v3/merge_requests.json
+++ b/spec/fixtures/api/schemas/public_api/v3/merge_requests.json
@@ -73,7 +73,8 @@
       "should_remove_source_branch": { "type": ["boolean", "null"] },
       "force_remove_source_branch": { "type": ["boolean", "null"] },
       "web_url": { "type": "uri" },
-      "subscribed": { "type": ["boolean"] }
+      "subscribed": { "type": ["boolean"] },
+      "squash": { "type": "boolean" }
     },
     "required": [
       "id", "iid", "project_id", "title", "description",
@@ -83,7 +84,7 @@
       "labels", "work_in_progress", "milestone", "merge_when_build_succeeds",
       "merge_status", "sha", "merge_commit_sha", "user_notes_count",
       "should_remove_source_branch", "force_remove_source_branch",
-      "web_url", "subscribed"
+      "web_url", "subscribed", "squash"
     ],
     "additionalProperties": false
   }
diff --git a/spec/fixtures/api/schemas/public_api/v4/merge_requests.json b/spec/fixtures/api/schemas/public_api/v4/merge_requests.json
index 0dc2eabec5d..f97461ce9cc 100644
--- a/spec/fixtures/api/schemas/public_api/v4/merge_requests.json
+++ b/spec/fixtures/api/schemas/public_api/v4/merge_requests.json
@@ -75,6 +75,7 @@
       "force_remove_source_branch": { "type": ["boolean", "null"] },
       "discussion_locked": { "type": ["boolean", "null"] },
       "web_url": { "type": "uri" },
+      "squash": { "type": "boolean" },
       "time_stats": {
         "time_estimate": { "type": "integer" },
         "total_time_spent": { "type": "integer" },
@@ -91,7 +92,7 @@
       "labels", "work_in_progress", "milestone", "merge_when_pipeline_succeeds",
       "merge_status", "sha", "merge_commit_sha", "user_notes_count",
       "should_remove_source_branch", "force_remove_source_branch",
-      "web_url"
+      "web_url", "squash"
     ],
     "additionalProperties": false
   }
diff --git a/spec/lib/gitlab/import_export/safe_model_attributes.yml b/spec/lib/gitlab/import_export/safe_model_attributes.yml
index 62da967cf96..3d5271cd030 100644
--- a/spec/lib/gitlab/import_export/safe_model_attributes.yml
+++ b/spec/lib/gitlab/import_export/safe_model_attributes.yml
@@ -165,6 +165,7 @@ MergeRequest:
 - approvals_before_merge
 - rebase_commit_sha
 - time_estimate
+- squash
 - last_edited_at
 - last_edited_by_id
 - head_pipeline_id
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index 92e33a64d26..9ffa91fc265 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -14,6 +14,65 @@ describe MergeRequest do
     it { is_expected.to have_many(:merge_request_diffs) }
   end
 
+  describe '#squash_in_progress?' do
+    shared_examples 'checking whether a squash is in progress' do
+      let(:repo_path) { subject.source_project.repository.path }
+      let(:squash_path) { File.join(repo_path, "gitlab-worktree", "squash-#{subject.id}") }
+
+      before do
+        system(*%W(#{Gitlab.config.git.bin_path} -C #{repo_path} worktree add --detach #{squash_path} master))
+      end
+
+      it 'returns true when there is a current squash directory' do
+        expect(subject.squash_in_progress?).to be_truthy
+      end
+
+      it 'returns false when there is no squash directory' do
+        FileUtils.rm_rf(squash_path)
+
+        expect(subject.squash_in_progress?).to be_falsey
+      end
+
+      it 'returns false when the squash directory has expired' do
+        time = 20.minutes.ago.to_time
+        File.utime(time, time, squash_path)
+
+        expect(subject.squash_in_progress?).to be_falsey
+      end
+
+      it 'returns false when the source project has been removed' do
+        allow(subject).to receive(:source_project).and_return(nil)
+
+        expect(subject.squash_in_progress?).to be_falsey
+      end
+    end
+
+    context 'when Gitaly squash_in_progress is enabled' do
+      it_behaves_like 'checking whether a squash is in progress'
+    end
+
+    context 'when Gitaly squash_in_progress is disabled', :disable_gitaly do
+      it_behaves_like 'checking whether a squash is in progress'
+    end
+  end
+
+  describe '#squash?' do
+    let(:merge_request) { build(:merge_request, squash: squash) }
+    subject { merge_request.squash? }
+
+    context 'disabled in database' do
+      let(:squash) { false }
+
+      it { is_expected.to be_falsy }
+    end
+
+    context 'enabled in database' do
+      let(:squash) { true }
+
+      it { is_expected.to be_truthy }
+    end
+  end
+
   describe 'modules' do
     subject { described_class }
 
diff --git a/spec/requests/api/merge_requests_spec.rb b/spec/requests/api/merge_requests_spec.rb
index 1eeeb4f1045..6b91e48ae6a 100644
--- a/spec/requests/api/merge_requests_spec.rb
+++ b/spec/requests/api/merge_requests_spec.rb
@@ -263,6 +263,7 @@ describe API::MergeRequests do
         expect(json_response.first['sha']).to eq(merge_request_merged.diff_head_sha)
         expect(json_response.first['merge_commit_sha']).not_to be_nil
         expect(json_response.first['merge_commit_sha']).to eq(merge_request_merged.merge_commit_sha)
+        expect(json_response.first['squash']).to eq(merge_request_merged.squash)
       end
 
       it "returns an array of all merge_requests using simple mode" do
@@ -671,12 +672,14 @@ describe API::MergeRequests do
              target_branch: 'master',
              author: user,
              labels: 'label, label2',
-             milestone_id: milestone.id
+             milestone_id: milestone.id,
+             squash: true
 
         expect(response).to have_gitlab_http_status(201)
         expect(json_response['title']).to eq('Test merge_request')
         expect(json_response['labels']).to eq(%w(label label2))
         expect(json_response['milestone']['id']).to eq(milestone.id)
+        expect(json_response['squash']).to be_truthy
         expect(json_response['force_remove_source_branch']).to be_falsy
       end
 
@@ -965,6 +968,14 @@ describe API::MergeRequests do
       expect(response).to have_gitlab_http_status(200)
     end
 
+    it "updates the MR's squash attribute" do
+      expect do
+        put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/merge", user), squash: true
+      end.to change { merge_request.reload.squash }
+
+      expect(response).to have_gitlab_http_status(200)
+    end
+
     it "enables merge when pipeline succeeds if the pipeline is active" do
       allow_any_instance_of(MergeRequest).to receive(:head_pipeline).and_return(pipeline)
       allow(pipeline).to receive(:active?).and_return(true)
@@ -1029,6 +1040,13 @@ describe API::MergeRequests do
       expect(json_response['milestone']['id']).to eq(milestone.id)
     end
 
+    it "updates squash and returns merge_request" do
+      put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}", user), squash: true
+
+      expect(response).to have_gitlab_http_status(200)
+      expect(json_response['squash']).to be_truthy
+    end
+
     it "returns merge_request with renamed target_branch" do
       put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}", user), target_branch: "wiki"
       expect(response).to have_gitlab_http_status(200)
diff --git a/spec/requests/api/v3/merge_requests_spec.rb b/spec/requests/api/v3/merge_requests_spec.rb
index be70cb24dce..79a16fbd1b0 100644
--- a/spec/requests/api/v3/merge_requests_spec.rb
+++ b/spec/requests/api/v3/merge_requests_spec.rb
@@ -40,6 +40,7 @@ describe API::MergeRequests do
         expect(json_response.first['sha']).to eq(merge_request_merged.diff_head_sha)
         expect(json_response.first['merge_commit_sha']).not_to be_nil
         expect(json_response.first['merge_commit_sha']).to eq(merge_request_merged.merge_commit_sha)
+        expect(json_response.first['squash']).to eq(merge_request_merged.squash)
       end
 
       it "returns an array of all merge_requests" do
@@ -241,13 +242,15 @@ describe API::MergeRequests do
              author: user,
              labels: 'label, label2',
              milestone_id: milestone.id,
-             remove_source_branch: true
+             remove_source_branch: true,
+             squash: true
 
         expect(response).to have_gitlab_http_status(201)
         expect(json_response['title']).to eq('Test merge_request')
         expect(json_response['labels']).to eq(%w(label label2))
         expect(json_response['milestone']['id']).to eq(milestone.id)
         expect(json_response['force_remove_source_branch']).to be_truthy
+        expect(json_response['squash']).to be_truthy
       end
 
       it "returns 422 when source_branch equals target_branch" do
@@ -489,6 +492,14 @@ describe API::MergeRequests do
       expect(response).to have_gitlab_http_status(200)
     end
 
+    it "updates the MR's squash attribute" do
+      expect do
+        put v3_api("/projects/#{project.id}/merge_requests/#{merge_request.id}/merge", user), squash: true
+      end.to change { merge_request.reload.squash }
+
+      expect(response).to have_gitlab_http_status(200)
+    end
+
     it "enables merge when pipeline succeeds if the pipeline is active" do
       allow_any_instance_of(MergeRequest).to receive(:head_pipeline).and_return(pipeline)
       allow(pipeline).to receive(:active?).and_return(true)
@@ -529,6 +540,13 @@ describe API::MergeRequests do
       expect(json_response['milestone']['id']).to eq(milestone.id)
     end
 
+    it "updates squash and returns merge_request" do
+      put v3_api("/projects/#{project.id}/merge_requests/#{merge_request.id}", user), squash: true
+
+      expect(response).to have_gitlab_http_status(200)
+      expect(json_response['squash']).to be_truthy
+    end
+
     it "returns merge_request with renamed target_branch" do
       put v3_api("/projects/#{project.id}/merge_requests/#{merge_request.id}", user), target_branch: "wiki"
       expect(response).to have_gitlab_http_status(200)
diff --git a/spec/services/merge_requests/merge_service_spec.rb b/spec/services/merge_requests/merge_service_spec.rb
index e8568bf8bb3..dc30a9bccc1 100644
--- a/spec/services/merge_requests/merge_service_spec.rb
+++ b/spec/services/merge_requests/merge_service_spec.rb
@@ -249,24 +249,58 @@ describe MergeRequests::MergeService do
         expect(Rails.logger).to have_received(:error).with(a_string_matching(error_message))
       end
 
-      context "when fast-forward merge is not allowed" do
+      context 'when squashing' do
         before do
-          allow_any_instance_of(Repository).to receive(:ancestor?).and_return(nil)
+          merge_request.update!(source_branch: 'master', target_branch: 'feature')
         end
 
-        %w(semi-linear ff).each do |merge_method|
-          it "logs and saves error if merge is #{merge_method} only" do
-            merge_method = 'rebase_merge' if merge_method == 'semi-linear'
-            merge_request.project.update(merge_method: merge_method)
-            error_message = 'Only fast-forward merge is allowed for your project. Please update your source branch'
-            allow(service).to receive(:execute_hooks)
+        it 'logs and saves error if there is an error when squashing' do
+          error_message = 'Failed to squash. Should be done manually'
 
-            service.execute(merge_request)
+          allow_any_instance_of(MergeRequests::SquashService).to receive(:squash).and_return(nil)
+          merge_request.update(squash: true)
+
+          service.execute(merge_request)
+
+          expect(merge_request).to be_open
+          expect(merge_request.merge_commit_sha).to be_nil
+          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 a squash in progress' do
+          error_message = 'another squash is already in progress'
+
+          allow_any_instance_of(MergeRequest).to receive(:squash_in_progress?).and_return(true)
+          merge_request.update(squash: true)
+
+          service.execute(merge_request)
 
-            expect(merge_request).to be_open
-            expect(merge_request.merge_commit_sha).to be_nil
-            expect(merge_request.merge_error).to include(error_message)
-            expect(Rails.logger).to have_received(:error).with(a_string_matching(error_message))
+          expect(merge_request).to be_open
+          expect(merge_request.merge_commit_sha).to be_nil
+          expect(merge_request.merge_error).to include(error_message)
+          expect(Rails.logger).to have_received(:error).with(a_string_matching(error_message))
+        end
+
+        context "when fast-forward merge is not allowed" do
+          before do
+            allow_any_instance_of(Repository).to receive(:ancestor?).and_return(nil)
+          end
+
+          %w(semi-linear ff).each do |merge_method|
+            it "logs and saves error if merge is #{merge_method} only" do
+              merge_method = 'rebase_merge' if merge_method == 'semi-linear'
+              merge_request.project.update(merge_method: merge_method)
+              error_message = 'Only fast-forward merge is allowed for your project. Please update your source branch'
+              allow(service).to receive(:execute_hooks)
+
+              service.execute(merge_request)
+
+              expect(merge_request).to be_open
+              expect(merge_request.merge_commit_sha).to be_nil
+              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/services/merge_requests/squash_service_spec.rb b/spec/services/merge_requests/squash_service_spec.rb
new file mode 100644
index 00000000000..bd884787425
--- /dev/null
+++ b/spec/services/merge_requests/squash_service_spec.rb
@@ -0,0 +1,199 @@
+require 'spec_helper'
+
+describe MergeRequests::SquashService do
+  let(:service) { described_class.new(project, user, {}) }
+  let(:user) { project.owner }
+  let(:project) { create(:project, :repository) }
+  let(:repository) { project.repository.raw }
+  let(:log_error) { "Failed to squash merge request #{merge_request.to_reference(full: true)}:" }
+  let(:squash_dir_path) do
+    File.join(Gitlab.config.shared.path, 'tmp/squash', repository.gl_repository, merge_request.id.to_s)
+  end
+  let(:merge_request_with_one_commit) do
+    create(:merge_request,
+           source_branch: 'feature', source_project: project,
+           target_branch: 'master', target_project: project)
+  end
+
+  let(:merge_request_with_only_new_files) do
+    create(:merge_request,
+           source_branch: 'video', source_project: project,
+           target_branch: 'master', target_project: project)
+  end
+
+  let(:merge_request_with_large_files) do
+    create(:merge_request,
+           source_branch: 'squash-large-files', source_project: project,
+           target_branch: 'master', target_project: project)
+  end
+
+  shared_examples 'the squash succeeds' do
+    it 'returns the squashed commit SHA' do
+      result = service.execute(merge_request)
+
+      expect(result).to match(status: :success, squash_sha: a_string_matching(/\h{40}/))
+      expect(result[:squash_sha]).not_to eq(merge_request.diff_head_sha)
+    end
+
+    it 'cleans up the temporary directory' do
+      service.execute(merge_request)
+
+      expect(File.exist?(squash_dir_path)).to be(false)
+    end
+
+    it 'does not keep the branch push event' do
+      expect { service.execute(merge_request) }.not_to change { Event.count }
+    end
+
+    context 'the squashed commit' do
+      let(:squash_sha) { service.execute(merge_request)[:squash_sha] }
+      let(:squash_commit) { project.repository.commit(squash_sha) }
+
+      it 'copies the author info and message from the merge request' do
+        expect(squash_commit.author_name).to eq(merge_request.author.name)
+        expect(squash_commit.author_email).to eq(merge_request.author.email)
+
+        # Commit messages have a trailing newline, but titles don't.
+        expect(squash_commit.message.chomp).to eq(merge_request.title)
+      end
+
+      it 'sets the current user as the committer' do
+        expect(squash_commit.committer_name).to eq(user.name.chomp('.'))
+        expect(squash_commit.committer_email).to eq(user.email)
+      end
+
+      it 'has the same diff as the merge request, but a different SHA' do
+        rugged = project.repository.rugged
+        mr_diff = rugged.diff(merge_request.diff_base_sha, merge_request.diff_head_sha)
+        squash_diff = rugged.diff(merge_request.diff_start_sha, squash_sha)
+
+        expect(squash_diff.patch.length).to eq(mr_diff.patch.length)
+        expect(squash_commit.sha).not_to eq(merge_request.diff_head_sha)
+      end
+    end
+  end
+
+  describe '#execute' do
+    context 'when there is only one commit in the merge request' do
+      it 'returns that commit SHA' do
+        result = service.execute(merge_request_with_one_commit)
+
+        expect(result).to match(status: :success, squash_sha: merge_request_with_one_commit.diff_head_sha)
+      end
+
+      it 'does not perform any git actions' do
+        expect(repository).not_to receive(:popen)
+
+        service.execute(merge_request_with_one_commit)
+      end
+    end
+
+    context 'when squashing only new files' do
+      let(:merge_request) { merge_request_with_only_new_files }
+
+      include_examples 'the squash succeeds'
+    end
+
+    context 'when squashing with files too large to display' do
+      let(:merge_request) { merge_request_with_large_files }
+
+      include_examples 'the squash succeeds'
+    end
+
+    context 'git errors' do
+      let(:merge_request) { merge_request_with_only_new_files }
+      let(:error) { 'A test error' }
+
+      context 'with gitaly enabled' do
+        before do
+          allow(repository.gitaly_operation_client).to receive(:user_squash)
+            .and_raise(Gitlab::Git::Repository::GitError, error)
+        end
+
+        it 'logs the stage and output' do
+          expect(service).to receive(:log_error).with(log_error)
+          expect(service).to receive(:log_error).with(error)
+
+          service.execute(merge_request)
+        end
+
+        it 'returns an error' do
+          expect(service.execute(merge_request)).to match(status: :error,
+                                                          message: a_string_including('squash'))
+        end
+      end
+
+      context 'with Gitaly disabled', :skip_gitaly_mock do
+        stages = {
+          'add worktree for squash' => 'worktree',
+          'configure sparse checkout' => 'config',
+          'get files in diff' => 'diff --name-only',
+          'check out target branch' => 'checkout',
+          'apply patch' => 'diff --binary',
+          'commit squashed changes' => 'commit',
+          'get SHA of squashed commit' => 'rev-parse'
+        }
+
+        stages.each do |stage, command|
+          context "when the #{stage} stage fails" do
+            before do
+              git_command = a_collection_containing_exactly(
+                a_string_starting_with("#{Gitlab.config.git.bin_path} #{command}")
+              ).or(
+                a_collection_starting_with([Gitlab.config.git.bin_path] + command.split)
+              )
+
+              allow(repository).to receive(:popen).and_return(['', 0])
+              allow(repository).to receive(:popen).with(git_command, anything, anything, anything).and_return([error, 1])
+            end
+
+            it 'logs the stage and output' do
+              expect(service).to receive(:log_error).with(log_error)
+              expect(service).to receive(:log_error).with(error)
+
+              service.execute(merge_request)
+            end
+
+            it 'returns an error' do
+              expect(service.execute(merge_request)).to match(status: :error,
+                                                              message: a_string_including('squash'))
+            end
+
+            it 'cleans up the temporary directory' do
+              expect(File.exist?(squash_dir_path)).to be(false)
+
+              service.execute(merge_request)
+            end
+          end
+        end
+      end
+    end
+
+    context 'when any other exception is thrown' do
+      let(:merge_request) { merge_request_with_only_new_files }
+      let(:error) { 'A test error' }
+
+      before do
+        allow(merge_request).to receive(:commits_count).and_raise(error)
+      end
+
+      it 'logs the MR reference and exception' do
+        expect(service).to receive(:log_error).with(a_string_including("#{project.full_path}#{merge_request.to_reference}"))
+        expect(service).to receive(:log_error).with(error)
+
+        service.execute(merge_request)
+      end
+
+      it 'returns an error' do
+        expect(service.execute(merge_request)).to match(status: :error,
+                                                        message: a_string_including('squash'))
+      end
+
+      it 'cleans up the temporary directory' do
+        service.execute(merge_request)
+
+        expect(File.exist?(squash_dir_path)).to be(false)
+      end
+    end
+  end
+end
diff --git a/spec/support/helpers/test_env.rb b/spec/support/helpers/test_env.rb
index 57aa07cf4fa..1fef50a52ec 100644
--- a/spec/support/helpers/test_env.rb
+++ b/spec/support/helpers/test_env.rb
@@ -47,6 +47,7 @@ module TestEnv
     'v1.1.0'                             => 'b83d6e3',
     'add-ipython-files'                  => '93ee732',
     'add-pdf-file'                       => 'e774ebd',
+    'squash-large-files'                 => '54cec52',
     'add-pdf-text-binary'                => '79faa7b',
     'add_images_and_changes'             => '010d106'
   }.freeze