1 files changed, 115 insertions, 0 deletions
diff --git a/app/services/repositories/changelog_service.rb b/app/services/repositories/changelog_service.rb
new file mode 100644
index 00000000000..96a63865a49
--- /dev/null
+++ b/app/services/repositories/changelog_service.rb
@@ -0,0 +1,115 @@
+# frozen_string_literal: true
+
+module Repositories
+  # A service class for generating a changelog section.
+  class ChangelogService
+    DEFAULT_TRAILER = 'Changelog'
+    DEFAULT_FILE = 'CHANGELOG.md'
+
+    # The `project` specifies the `Project` to generate the changelog section
+    # for.
+    #
+    # The `user` argument specifies a `User` to use for committing the changes
+    # to the Git repository.
+    #
+    # The `version` arguments must be a version `String` using semantic
+    # versioning as the format.
+    #
+    # The arguments `from` and `to` must specify a Git ref or SHA to use for
+    # fetching the commits to include in the changelog. The SHA/ref set in the
+    # `from` argument isn't included in the list.
+    #
+    # The `date` argument specifies the date of the release, and defaults to the
+    # current time/date.
+    #
+    # The `branch` argument specifies the branch to commit the changes to. The
+    # branch must already exist.
+    #
+    # The `trailer` argument is the Git trailer to use for determining what
+    # commits to include in the changelog.
+    #
+    # The `file` arguments specifies the name/path of the file to commit the
+    # changes to. If the file doesn't exist, it's created automatically.
+    #
+    # The `message` argument specifies the commit message to use when committing
+    # the changelog changes.
+    #
+    # rubocop: disable Metrics/ParameterLists
+    def initialize(
+      project,
+      user,
+      version:,
+      to:,
+      from: nil,
+      date: DateTime.now,
+      branch: project.default_branch_or_master,
+      trailer: DEFAULT_TRAILER,
+      file: DEFAULT_FILE,
+      message: "Add changelog for version #{version}"
+    )
+      @project = project
+      @user = user
+      @version = version
+      @from = from
+      @to = to
+      @date = date
+      @branch = branch
+      @trailer = trailer
+      @file = file
+      @message = message
+    end
+    # rubocop: enable Metrics/ParameterLists
+
+    def execute
+      from = start_of_commit_range
+
+      # For every entry we want to only include the merge request that
+      # originally introduced the commit, which is the oldest merge request that
+      # contains the commit. We fetch there merge requests in batches, reducing
+      # the number of SQL queries needed to get this data.
+      mrs_finder = MergeRequests::OldestPerCommitFinder.new(@project)
+      config = Gitlab::Changelog::Config.from_git(@project)
+      release = Gitlab::Changelog::Release
+        .new(version: @version, date: @date, config: config)
+
+      commits =
+        CommitsWithTrailerFinder.new(project: @project, from: from, to: @to)
+
+      commits.each_page(@trailer) do |page|
+        mrs = mrs_finder.execute(page)
+
+        # Preload the authors. This ensures we only need a single SQL query per
+        # batch of commits, instead of needing a query for every commit.
+        page.each(&:lazy_author)
+
+        page.each do |commit|
+          release.add_entry(
+            title: commit.title,
+            commit: commit,
+            category: commit.trailers.fetch(@trailer),
+            author: commit.author,
+            merge_request: mrs[commit.id]
+          )
+        end
+      end
+
+      Gitlab::Changelog::Committer
+        .new(@project, @user)
+        .commit(release: release, file: @file, branch: @branch, message: @message)
+    end
+
+    def start_of_commit_range
+      return @from if @from
+
+      if (prev_tag = PreviousTagFinder.new(@project).execute(@version))
+        return prev_tag.target_commit.id
+      end
+
+      raise(
+        Gitlab::Changelog::Error,
+        'The commit start range is unspecified, and no previous tag ' \
+          'could be found to use instead'
+      )
+    end
+  end
+end