Add API support for refreshing merge ref path

Add a merge_requests/:iid/merge_to_ref API which make
use of the groundwork to write merge results into
refs/merge-requests/:iid/merge.

7 files changed, 125 insertions, 52 deletions

diff --git a/app/services/merge_requests/merge_base_service.rb b/app/services/merge_requests/merge_base_service.rb
index 6ac6fcf562a..095bdca5472 100644
--- a/app/services/merge_requests/merge_base_service.rb
+++ b/app/services/merge_requests/merge_base_service.rb
@@ -15,10 +15,7 @@ module MergeRequests
 
     # Overridden in EE.
     def hooks_validation_error(_merge_request)
-<<<<<<< HEAD
       # No-op
-=======
->>>>>>> 89c57ca2673... Support merge to ref for merge-commit and squash
     end
 
     def source
@@ -31,7 +28,6 @@ module MergeRequests
 
     private
 
-<<<<<<< HEAD
     # Overridden in EE.
     def error_check!
       # No-op
@@ -41,8 +37,6 @@ module MergeRequests
       raise MergeError, message
     end
 
-=======
->>>>>>> 89c57ca2673... Support merge to ref for merge-commit and squash
     def handle_merge_error(*args)
       # No-op
     end
diff --git a/app/services/merge_requests/merge_to_ref_service.rb b/app/services/merge_requests/merge_to_ref_service.rb
index 43bb9f3f2f2..586652ae44e 100644
--- a/app/services/merge_requests/merge_to_ref_service.rb
+++ b/app/services/merge_requests/merge_to_ref_service.rb
@@ -14,19 +14,11 @@ module MergeRequests
     def execute(merge_request)
       @merge_request = merge_request
 
-<<<<<<< HEAD
       validate!
 
       commit_id = commit
 
       raise_error('Conflicts detected during merge') unless commit_id
-=======
-      error_check!
-
-      commit_id = commit
-
-      raise MergeError, 'Conflicts detected during merge' unless commit_id
->>>>>>> 89c57ca2673... Support merge to ref for merge-commit and squash
 
       success(commit_id: commit_id)
     rescue MergeError => error
@@ -35,7 +27,6 @@ module MergeRequests
 
     private
 
-<<<<<<< HEAD
     def validate!
       authorization_check!
       error_check!
@@ -48,11 +39,6 @@ module MergeRequests
         if Feature.disabled?(:merge_to_tmp_merge_ref_path, project)
           'Feature is not enabled'
         elsif !merge_method_supported?
-=======
-    def error_check!
-      error =
-        if !merge_method_supported?
->>>>>>> 89c57ca2673... Support merge to ref for merge-commit and squash
           "#{project.human_merge_method} to #{target_ref} is currently not supported."
         elsif !hooks_validation_pass?(merge_request)
           hooks_validation_error(merge_request)
@@ -64,7 +50,6 @@ module MergeRequests
           'No source for merge'
         end
 
-<<<<<<< HEAD
       raise_error(error) if error
     end
 
@@ -72,9 +57,6 @@ module MergeRequests
       unless Ability.allowed?(current_user, :admin_merge_request, project)
         raise_error("You are not allowed to merge to this ref")
       end
-=======
-      raise MergeError, error if error
->>>>>>> 89c57ca2673... Support merge to ref for merge-commit and squash
     end
 
     def target_ref
diff --git a/changelogs/unreleased/osw-merge-refs-refreshing-api.yml b/changelogs/unreleased/osw-merge-refs-refreshing-api.yml
new file mode 100644
index 00000000000..095600cd088
--- /dev/null
+++ b/changelogs/unreleased/osw-merge-refs-refreshing-api.yml
@@ -0,0 +1,5 @@
+---
+title: API support for MR merge to temporary merge ref path
+merge_request: 24918
+author:
+type: added
diff --git a/doc/api/merge_requests.md b/doc/api/merge_requests.md
index e176cdffc5f..ed4b6281acc 100644
--- a/doc/api/merge_requests.md
+++ b/doc/api/merge_requests.md
@@ -1101,6 +1101,40 @@ Parameters:
 }
 ```
 
+## Merge to default merge ref path
+
+Merge the changes between the merge request source and target branches into `refs/merge-requests/:iid/merge` 
+ref, of the target project repository. This ref will have the state the target branch would have if
+a regular merge action was taken.
+
+This is not a regular merge action given it doesn't change the merge request state in any manner.
+
+This ref (`refs/merge-requests/:iid/merge`) is **always** overwritten when submitting
+requests to this API, so none of its state is kept or used in the process.
+
+If the merge request has conflicts, is empty or already merged,
+you'll get a `400` and a descriptive error message. If you don't have permissions to do so, 
+you'll get a `403`.
+
+It returns the HEAD commit of `refs/merge-requests/:iid/merge` in the response body in
+case of `200`.
+
+```
+PUT /projects/:id/merge_requests/:merge_request_iid/merge_to_ref
+```
+
+Parameters:
+
+- `id` (required) - The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user
+- `merge_request_iid` (required)            - Internal ID of MR
+- `merge_commit_message` (optional)         - Custom merge commit message
+
+```json
+{
+  "commit_id": "854a3a7a17acbcc0bbbea170986df1eb60435f34"
+}
+```
+
 ## Cancel Merge When Pipeline Succeeds
 
 If you don't have permissions to accept this merge request - you'll get a `401`
diff --git a/lib/api/merge_requests.rb b/lib/api/merge_requests.rb
index 03f6684226f..44f1e81caf2 100644
--- a/lib/api/merge_requests.rb
+++ b/lib/api/merge_requests.rb
@@ -388,6 +388,31 @@ module API
         present merge_request, with: Entities::MergeRequest, current_user: current_user, project: user_project
       end
 
+      desc 'Merge a merge request to its default temporary merge ref path'
+      params do
+        optional :merge_commit_message, type: String, desc: 'Custom merge commit message'
+      end
+      put ':id/merge_requests/:merge_request_iid/merge_to_ref' do
+        merge_request = find_project_merge_request(params[:merge_request_iid])
+
+        authorize! :admin_merge_request, user_project
+
+        merge_params = {
+          commit_message: params[:merge_commit_message]
+        }
+
+        result = ::MergeRequests::MergeToRefService
+          .new(merge_request.target_project, current_user, merge_params)
+          .execute(merge_request)
+
+        if result[:status] == :success
+          present result.slice(:commit_id), 200
+        else
+          http_status = result[:http_status] || 400
+          render_api_error!(result[:message], http_status)
+        end
+      end
+
       desc 'Cancel merge if "Merge When Pipeline Succeeds" is enabled' do
         success Entities::MergeRequest
       end
diff --git a/spec/requests/api/merge_requests_spec.rb b/spec/requests/api/merge_requests_spec.rb
index b4cd3130dc5..273c52bd719 100644
--- a/spec/requests/api/merge_requests_spec.rb
+++ b/spec/requests/api/merge_requests_spec.rb
@@ -1001,6 +1001,67 @@ describe API::MergeRequests do
     end
   end
 
+  describe "PUT /projects/:id/merge_requests/:merge_request_iid/merge_to_ref" do
+    let(:pipeline) { create(:ci_pipeline_without_jobs) }
+    let(:url) do
+      "/projects/#{project.id}/merge_requests/#{merge_request.iid}/merge_to_ref"
+    end
+
+    it 'returns the generated ID from the merge service in case of success' do
+      put api(url, user), params: { merge_commit_message: 'Custom message' }
+
+      commit = project.commit(json_response['commit_id'])
+
+      expect(response).to have_gitlab_http_status(200)
+      expect(json_response['commit_id']).to be_present
+      expect(commit.message).to eq('Custom message')
+    end
+
+    it "returns 400 if branch can't be merged" do
+      merge_request.update!(state: 'merged')
+
+      put api(url, user)
+
+      expect(response).to have_gitlab_http_status(400)
+      expect(json_response['message'])
+        .to eq("Merge request is not mergeable to #{merge_request.merge_ref_path}")
+    end
+
+    it 'returns 403 if user has no permissions to merge to the ref' do
+      user2 = create(:user)
+      project.add_reporter(user2)
+
+      put api(url, user2)
+
+      expect(response).to have_gitlab_http_status(403)
+      expect(json_response['message']).to eq('403 Forbidden')
+    end
+
+    it 'returns 404 for an invalid merge request IID' do
+      put api("/projects/#{project.id}/merge_requests/12345/merge_to_ref", user)
+
+      expect(response).to have_gitlab_http_status(404)
+    end
+
+    it "returns 404 if the merge request id is used instead of iid" do
+      put api("/projects/#{project.id}/merge_requests/#{merge_request.id}/merge", user)
+
+      expect(response).to have_gitlab_http_status(404)
+    end
+
+    it "returns 400 when merge method is not supported" do
+      merge_request.project.update!(merge_method: 'ff')
+
+      put api(url, user)
+
+      expected_error =
+        'Fast-forward to refs/merge-requests/1/merge is currently not supported.'
+
+      expect(response).to have_gitlab_http_status(400)
+      expect(json_response['message']).to eq(expected_error)
+    end
+  end
+
   describe "PUT /projects/:id/merge_requests/:merge_request_iid" do
     context "to close a MR" do
       it "returns merge_request" do
diff --git a/spec/services/merge_requests/merge_to_ref_service_spec.rb b/spec/services/merge_requests/merge_to_ref_service_spec.rb
index f13e3524f42..96f2fde7117 100644
--- a/spec/services/merge_requests/merge_to_ref_service_spec.rb
+++ b/spec/services/merge_requests/merge_to_ref_service_spec.rb
@@ -3,7 +3,6 @@
 require 'spec_helper'
 
 describe MergeRequests::MergeToRefService do
-<<<<<<< HEAD
   shared_examples_for 'MergeService for target ref' do
     it 'target_ref has the same state of target branch' do
       repo = merge_request.target_project.repository
@@ -20,8 +19,6 @@ describe MergeRequests::MergeToRefService do
     end
   end
 
-=======
->>>>>>> 89c57ca2673... Support merge to ref for merge-commit and squash
   set(:user) { create(:user) }
   let(:merge_request) { create(:merge_request, :simple) }
   let(:project) { merge_request.project }
@@ -77,7 +74,6 @@ describe MergeRequests::MergeToRefService do
       process_merge_to_ref
     end
 
-<<<<<<< HEAD
     it 'returns error when feature is disabled' do
       stub_feature_flags(merge_to_tmp_merge_ref_path: false)
 
@@ -87,8 +83,6 @@ describe MergeRequests::MergeToRefService do
       expect(result[:message]).to eq('Feature is not enabled')
     end
 
-=======
->>>>>>> 89c57ca2673... Support merge to ref for merge-commit and squash
     it 'returns an error when the failing to process the merge' do
       allow(project.repository).to receive(:merge_to_ref).and_return(nil)
 
@@ -107,25 +101,6 @@ describe MergeRequests::MergeToRefService do
         MergeRequests::MergeService.new(project, user, {})
       end
 
-<<<<<<< HEAD
-=======
-      shared_examples_for 'MergeService for target ref' do
-        it 'target_ref has the same state of target branch' do
-          repo = merge_request.target_project.repository
-
-          process_merge_to_ref
-          merge_service.execute(merge_request)
-
-          ref_commits = repo.commits(merge_request.merge_ref_path, limit: 3)
-          target_branch_commits = repo.commits(merge_request.target_branch, limit: 3)
-
-          ref_commits.zip(target_branch_commits).each do |ref_commit, target_branch_commit|
-            expect(ref_commit.parents).to eq(target_branch_commit.parents)
-          end
-        end
-      end
-
->>>>>>> 89c57ca2673... Support merge to ref for merge-commit and squash
       context 'when merge commit' do
         it_behaves_like 'MergeService for target ref'
       end
@@ -210,7 +185,6 @@ describe MergeRequests::MergeToRefService do
 
       it { expect(todo).not_to be_done }
     end
-<<<<<<< HEAD
 
     it 'returns error when user has no authorization to admin the merge request' do
       unauthorized_user = create(:user)
@@ -223,7 +197,5 @@ describe MergeRequests::MergeToRefService do
       expect(result[:status]).to eq(:error)
       expect(result[:message]).to eq('You are not allowed to merge to this ref')
     end
-=======
->>>>>>> 89c57ca2673... Support merge to ref for merge-commit and squash
   end
 end