NXD: Apply patches for MR with more than one commits will ff_merge

If MR has more than one commit, it's possible to do fast-forward merge when
the MR branch is rebased.

app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue

@@ -159,6 +159,13 @@ export default {
 
       return this.mr.canRemoveSourceBranch;
     },
+    ffMergePossible() {
+      if (this.glFeatures.mergeRequestWidgetGraphql) {
+        return this.stateData.ffMergePossible;
+      }
+
+      return this.mr.ffMergePossible;
+    },
     commits() {
       if (this.glFeatures.mergeRequestWidgetGraphql) {
         return this.state.commitsWithoutMergeCommits.nodes;
@@ -175,10 +182,16 @@ export default {
     },
     ApplyPatchButtonText() {
 
-      return __('Apply Patch ');
+      if (this.mr.commitsCount === 1) {
+        return __('Apply Patch ');
+      }
+      return __('Fast-forward Merge ');
     },
-    isApplyPatchAllowed() {
-      return this.mr.commitsCount !== 1 || this.isMergeButtonDisabled;
+    isApplyPatchDisabled() {
+      if (this.mr.commitsCount === 1) {
+        return this.isMergeButtonDisabled;
+      }
+      return !this.ffMergePossible || this.isMergeButtonDisabled;
     },
     preferredAutoMergeStrategy() {
       if (this.glFeatures.mergeRequestWidgetGraphql) {
@@ -539,7 +552,7 @@ export default {
                 class="patch-merge-request accept-merge-request"
                 data-testid="apply-button"
                 :variant="mergeButtonVariant"
-                :disabled="isApplyPatchAllowed"
+                :disabled="isApplyPatchDisabled"
                 :loading="isMakingRequest"
                 data-qa-selector="apply_patch_button"
                 @click="handleMergeButtonClick(false, false, true)"

app/assets/javascripts/vue_merge_request_widget/queries/get_state.query.graphql

@@ -16,6 +16,7 @@ query getState($projectPath: ID!, $iid: String!) {
         status
         warnings
       }
+      ffMergePossible
       shouldBeRebased
       sourceBranchExists
       state

app/assets/javascripts/vue_merge_request_widget/queries/states/ready_to_merge.fragment.graphql

@@ -17,6 +17,7 @@ fragment ReadyToMerge on Project {
     mergeWhenPipelineSucceeds
     commitCount
     diffHeadSha
+    ffMergePossible
     userPermissions {
       removeSourceBranch
     }

app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js

@@ -144,6 +144,7 @@ export default class MergeRequestStore {
       this.projectArchived = data.project_archived;
       this.isSHAMismatch = this.sha !== data.diff_head_sha;
       this.shouldBeRebased = Boolean(data.should_be_rebased);
+      this.ffMergePossible = Boolean(data.ff_merge_possible);
       this.workInProgress = data.work_in_progress;
     }
 
@@ -187,6 +188,7 @@ export default class MergeRequestStore {
     this.isPipelineFailed = this.ciStatus === 'failed' || this.ciStatus === 'canceled';
     this.isSHAMismatch = this.sha !== mergeRequest.diffHeadSha;
     this.shouldBeRebased = mergeRequest.shouldBeRebased;
+    this.ffMergePossible = mergeRequest.ffMergePossible;
     this.workInProgress = mergeRequest.workInProgress;
     this.mergeRequestState = mergeRequest.state;
 

app/graphql/mutations/merge_requests/accept.rb

@@ -40,6 +40,9 @@ module Mutations
                required: false,
                default_value: false,
                description: 'Squash commits on the source branch before merge.'
+      argument :apply_patch, ::GraphQL::BOOLEAN_TYPE,
+               required: false,
+               description: 'Merge with apply patche button enabled.'
 
       def resolve(project_path:, iid:, **args)
         Gitlab::QueryLimiting.disable!('https://gitlab.com/gitlab-org/gitlab/-/issues/4796')

app/graphql/types/merge_request_type.rb

@@ -91,6 +91,8 @@ module Types
           description: 'Indicates if members of the target project can push to the fork.'
     field :should_be_rebased, GraphQL::BOOLEAN_TYPE, method: :should_be_rebased?, null: false, calls_gitaly: true,
           description: 'Indicates if the merge request will be rebased.'
+    field :ff_merge_possible, GraphQL::BOOLEAN_TYPE, method: :ff_merge_possible?, null: false, calls_gitaly: true,
+          description: 'Indicates if the ff merge is possible.'
     field :rebase_commit_sha, GraphQL::STRING_TYPE, null: true,
           description: 'Rebase commit SHA of the merge request.'
     field :rebase_in_progress, GraphQL::BOOLEAN_TYPE, method: :rebase_in_progress?, null: false, calls_gitaly: true,

app/models/merge_request.rb

@@ -116,6 +116,7 @@ class MergeRequest < ApplicationRecord
   has_many :reviews, inverse_of: :merge_request
 
   KNOWN_MERGE_PARAMS = [
+    :apply_patch,
     :auto_merge_strategy,
     :should_remove_source_branch,
     :force_remove_source_branch,

app/services/merge_requests/merge_service.rb

@@ -86,21 +86,31 @@ module MergeRequests
       log_info("Git merge started on JID #{merge_jid}")
 
       mr_1patch = merge_request.commits.size == 1
-      do_apply = params[:apply_patches] || mr_1patch
-      do_merge = !do_apply
-      patch = merge_request.commits.reverse.first
-
-      commit_id = do_merge ? try_merge \
-          : repository.cherry_pick(current_user, repository.commit(patch.sha), merge_request.target_branch, commit_message,
-            start_branch_name: nil, start_project: project, dry_run: false)
+      ff_merged = false
+
+      if params[:apply_patch]
+        if mr_1patch
+          patch = merge_request.commits.reverse.first
+          commit_id = repository.cherry_pick(current_user, repository.commit(patch.sha), merge_request.target_branch,
+                        commit_message, start_branch_name: nil, start_project: project, dry_run: false)
+        else
+          # ff_merge
+          ff_merged = true
+          try_ff_merge
+        end
+      else
+        commit_id = try_merge
+      end
 
       if commit_id
         log_info("Git merge finished on JID #{merge_jid} commit #{commit_id}")
+        merge_request.update!(merge_commit_sha: commit_id)
+      elsif ff_merged
+        log_info("Git merge finished on JID #{merge_jid}")
       else
         raise_error(GENERIC_ERROR_MESSAGE)
       end
 
-      merge_request.update!(merge_commit_sha: commit_id)
     ensure
       merge_request.update_and_mark_in_progress_merge_commit_sha(nil)
     end
@@ -117,6 +127,20 @@ module MergeRequests
       raise_error(GENERIC_ERROR_MESSAGE)
     end
 
+    def try_ff_merge
+      repository.ff_merge(current_user,
+                          source,
+                          merge_request.target_branch,
+                          merge_request: merge_request)
+
+    rescue Gitlab::Git::PreReceiveError => e
+      raise MergeError,
+            "Something went wrong during merge pre-receive hook. #{e.message}".strip
+    rescue StandardError => e
+      handle_merge_error(log_message: e.message)
+      raise_error(GENERIC_ERROR_MESSAGE)
+    end
+
     def after_merge
       log_info("Post merge started on JID #{merge_jid} with state #{state}")
       MergeRequests::PostMergeService.new(project: project, current_user: current_user).execute(merge_request)