Create default merge commit message using customizable template

Use new merge_commit_template field (when set) to create default value
for merge commit message.

Changelog: added

app/models/merge_request.rb

@@ -1317,6 +1317,10 @@ class MergeRequest < ApplicationRecord
   end
 
   def default_merge_commit_message(include_description: false)
+    if self.target_project.merge_commit_template.present? && !include_description
+      return ::Gitlab::MergeRequests::MergeCommitMessage.new(merge_request: self).message
+    end
+
     closes_issues_references = visible_closing_issues_for.map do |issue|
       issue.to_reference(target_project)
     end

lib/gitlab/merge_requests/merge_commit_message.rb

+# frozen_string_literal: true
+module Gitlab
+  module MergeRequests
+    class MergeCommitMessage
+      def initialize(merge_request:)
+        @merge_request = merge_request
+      end
+
+      def message
+        return unless @merge_request.target_project.merge_commit_template.present?
+
+        message = @merge_request.target_project.merge_commit_template
+
+        # Remove placeholders that correspond to empty values and are the last word in the line
+        # along with all whitespace characters preceding them.
+        # This allows us to recreate previous default merge commit message behaviour - we skipped new line character
+        # before empty description and before closed issues when none were present.
+        PLACEHOLDERS.each do |key, value|
+          unless value.call(merge_request).present?
+            message = message.gsub(BLANK_PLACEHOLDERS_REGEXES[key], '')
+          end
+        end
+
+        Gitlab::StringPlaceholderReplacer
+          .replace_string_placeholders(message, PLACEHOLDERS_REGEX) do |key|
+          PLACEHOLDERS[key].call(merge_request)
+        end
+      end
+
+      private
+
+      attr_reader :merge_request
+
+      PLACEHOLDERS = {
+        'source_branch' => ->(merge_request) { merge_request.source_branch.to_s },
+        'target_branch' => ->(merge_request) { merge_request.target_branch.to_s },
+        'title' => ->(merge_request) { merge_request.title },
+        'issues' => ->(merge_request) do
+          return "" if merge_request.visible_closing_issues_for.blank?
+
+          closes_issues_references = merge_request.visible_closing_issues_for.map do |issue|
+            issue.to_reference(merge_request.target_project)
+          end
+          "Closes #{closes_issues_references.to_sentence}"
+        end,
+        'description' => ->(merge_request) { merge_request.description.presence || '' },
+        'reference' => ->(merge_request) { merge_request.to_reference(full: true) }
+      }.freeze
+
+      PLACEHOLDERS_REGEX = Regexp.union(PLACEHOLDERS.keys.map do |key|
+        Regexp.new(Regexp.escape(key))
+      end).freeze
+
+      BLANK_PLACEHOLDERS_REGEXES = (PLACEHOLDERS.map do |key, value|
+        [key, Regexp.new("[\n\r]+%{#{Regexp.escape(key)}}$")]
+      end).to_h.freeze
+    end
+  end
+end

spec/lib/gitlab/merge_requests/merge_commit_message_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::MergeRequests::MergeCommitMessage do
+  let(:merge_commit_template) { nil }
+  let(:project) { create(:project, :public, :repository, merge_commit_template: merge_commit_template) }
+  let(:user) { project.creator }
+  let(:merge_request_description) { "Merge Request Description\nNext line" }
+  let(:merge_request_title) { 'Bugfix' }
+  let(:merge_request) do
+    create(
+      :merge_request,
+      :simple,
+      source_project: project,
+      target_project: project,
+      author: user,
+      description: merge_request_description,
+      title: merge_request_title
+    )
+  end
+
+  subject { described_class.new(merge_request: merge_request) }
+
+  it 'returns nil when template is not set in target project' do
+    expect(subject.message).to be_nil
+  end
+
+  context 'when project has custom merge commit template' do
+    let(:merge_commit_template) { <<~MSG.rstrip }
+      %{title}
+
+      See merge request %{reference}
+    MSG
+
+    it 'uses custom template' do
+      expect(subject.message).to eq <<~MSG.rstrip
+        Bugfix
+
+        See merge request #{merge_request.to_reference(full: true)}
+      MSG
+    end
+  end
+
+  context 'when project has merge commit template with closed issues' do
+    let(:merge_commit_template) { <<~MSG.rstrip }
+      Merge branch '%{source_branch}' into '%{target_branch}'
+
+      %{title}
+
+      %{issues}
+
+      See merge request %{reference}
+    MSG
+
+    it 'omits issues and new lines when no issues are mentioned in description' do
+      expect(subject.message).to eq <<~MSG.rstrip
+        Merge branch 'feature' into 'master'
+
+        Bugfix
+
+        See merge request #{merge_request.to_reference(full: true)}
+      MSG
+    end
+
+    context 'when MR closes issues' do
+      let(:issue_1) { create(:issue, project: project) }
+      let(:issue_2) { create(:issue, project: project) }
+      let(:merge_request_description) { "Description\n\nclosing #{issue_1.to_reference}, #{issue_2.to_reference}" }
+
+      it 'includes them and keeps new line characters' do
+        expect(subject.message).to eq <<~MSG.rstrip
+          Merge branch 'feature' into 'master'
+
+          Bugfix
+
+          Closes #{issue_1.to_reference} and #{issue_2.to_reference}
+
+          See merge request #{merge_request.to_reference(full: true)}
+        MSG
+      end
+    end
+  end
+
+  context 'when project has merge commit template with description' do
+    let(:merge_commit_template) { <<~MSG.rstrip }
+      Merge branch '%{source_branch}' into '%{target_branch}'
+
+      %{title}
+
+      %{description}
+
+      See merge request %{reference}
+    MSG
+
+    it 'uses template' do
+      expect(subject.message).to eq <<~MSG.rstrip
+        Merge branch 'feature' into 'master'
+
+        Bugfix
+
+        Merge Request Description
+        Next line
+
+        See merge request #{merge_request.to_reference(full: true)}
+      MSG
+    end
+
+    context 'when description is empty string' do
+      let(:merge_request_description) { '' }
+
+      it 'skips description placeholder and removes new line characters before it' do
+        expect(subject.message).to eq <<~MSG.rstrip
+          Merge branch 'feature' into 'master'
+
+          Bugfix
+
+          See merge request #{merge_request.to_reference(full: true)}
+        MSG
+      end
+    end
+
+    context 'when description is nil' do
+      let(:merge_request_description) { nil }
+
+      it 'skips description placeholder and removes new line characters before it' do
+        expect(subject.message).to eq <<~MSG.rstrip
+          Merge branch 'feature' into 'master'
+
+          Bugfix
+
+          See merge request #{merge_request.to_reference(full: true)}
+        MSG
+      end
+    end
+
+    context 'when description is blank string' do
+      let(:merge_request_description) { "\n\r  \n" }
+
+      it 'skips description placeholder and removes new line characters before it' do
+        expect(subject.message).to eq <<~MSG.rstrip
+          Merge branch 'feature' into 'master'
+
+          Bugfix
+
+          See merge request #{merge_request.to_reference(full: true)}
+        MSG
+      end
+    end
+  end
+
+  context 'when custom merge commit template contains placeholder in the middle or beginning of the line' do
+    let(:merge_commit_template) { <<~MSG.rstrip }
+      Merge branch '%{source_branch}' into '%{target_branch}'
+
+      %{description} %{title}
+
+      See merge request %{reference}
+    MSG
+
+    it 'uses custom template' do
+      expect(subject.message).to eq <<~MSG.rstrip
+        Merge branch 'feature' into 'master'
+
+        Merge Request Description
+        Next line Bugfix
+
+        See merge request #{merge_request.to_reference(full: true)}
+      MSG
+    end
+
+    context 'when description is empty string' do
+      let(:merge_request_description) { '' }
+
+      it 'does not remove new line characters before empty placeholder' do
+        expect(subject.message).to eq <<~MSG.rstrip
+          Merge branch 'feature' into 'master'
+
+           Bugfix
+
+          See merge request #{merge_request.to_reference(full: true)}
+        MSG
+      end
+    end
+  end
+end

spec/models/merge_request_spec.rb

@@ -1638,6 +1638,22 @@ RSpec.describe MergeRequest, factory_default: :keep do
       expect(request.default_merge_commit_message)
         .not_to match("By removing all code\n\n")
     end
+
+    it 'uses template from target project' do
+      request = build(:merge_request, title: 'Fix everything')
+      subject.target_project.merge_commit_template = '%{title}'
+
+      expect(request.default_merge_commit_message)
+        .to eq('Fix everything')
+    end
+
+    it 'ignores template when include_description is true' do
+      request = build(:merge_request, title: 'Fix everything')
+      subject.target_project.merge_commit_template = '%{title}'
+
+      expect(request.default_merge_commit_message(include_description: true))
+        .to match("See merge request #{request.to_reference(full: true)}")
+    end
   end
 
   describe "#auto_merge_strategy" do