app/services/compare_service.rb


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require 'securerandom'

# Compare 2 branches for one repo or between repositories
# and return Gitlab::CompareResult object that responds to commits and diffs
class CompareService
  def execute(current_user, source_project, source_branch, target_project, target_branch)
    # Try to compare branches to get commits list and diffs
    if target_project == source_project
      Gitlab::CompareResult.new(
        Gitlab::Git::Compare.new(
          target_project.repository.raw_repository,
          target_branch,
          source_branch,
        )
      )
    else
      random_string = SecureRandom.hex
      target_project.repository.fetch_ref(
        source_project.repository.path_to_repo,
        "refs/heads/#{source_branch}",
        "refs/tmp/#{random_string}/head"
      )

      source_sha = source_project.commit(source_branch).sha

      Gitlab::CompareResult.new(
        Gitlab::Git::Compare.new(
          target_project.repository.raw_repository,
          target_branch,
          source_sha,
        )
      )
    end
  end
end