Support merge to ref for merge-commit and squash

Adds the ground work for writing into
the merge ref refs/merge-requests/:iid/merge the
merge result between source and target branches of
a MR, without further side-effects such as
mailing, MR updates and target branch changes.

1 files changed, 53 insertions, 0 deletions

diff --git a/app/services/merge_requests/merge_base_service.rb b/app/services/merge_requests/merge_base_service.rb
new file mode 100644
index 00000000000..4c5a0d96957
--- /dev/null
+++ b/app/services/merge_requests/merge_base_service.rb
@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+
+module MergeRequests
+  class MergeBaseService < MergeRequests::BaseService
+    include Gitlab::Utils::StrongMemoize
+
+    MergeError = Class.new(StandardError)
+
+    attr_reader :merge_request
+
+    # Overridden in EE.
+    def hooks_validation_pass?(_merge_request)
+      true
+    end
+
+    # Overridden in EE.
+    def hooks_validation_error(_merge_request)
+    end
+
+    def source
+      if merge_request.squash
+        squash_sha!
+      else
+        merge_request.diff_head_sha
+      end
+    end
+
+    private
+
+    def handle_merge_error(*args)
+      # No-op
+    end
+
+    def commit_message
+      params[:commit_message] ||
+        merge_request.default_merge_commit_message
+    end
+
+    def squash_sha!
+      strong_memoize(:squash_sha) do
+        params[:merge_request] = merge_request
+        squash_result = ::MergeRequests::SquashService.new(project, current_user, params).execute
+
+        case squash_result[:status]
+        when :success
+          squash_result[:squash_sha]
+        when :error
+          raise ::MergeRequests::MergeService::MergeError, squash_result[:message]
+        end
+      end
+    end
+  end
+end