Apply patches when creating MR via email

This allows users to add patches as attachments to merge request
created via email.

When an email to create a merge request is sent, all the attachments
ending in `.patch` will be applied to the branch specified in the
subject of the email. If the branch did not exist, it will be created
from the HEAD of the repository.

When the patches could not be applied, the error message will be
replied to the user.

The patches can have a maximum combined size of 2MB for now.

3 files changed, 83 insertions, 0 deletions

diff --git a/lib/gitlab/git/patches/collection.rb b/lib/gitlab/git/patches/collection.rb
new file mode 100644
index 00000000000..ad6b5d32abc
--- /dev/null
+++ b/lib/gitlab/git/patches/collection.rb
@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Git
+    module Patches
+      class Collection
+        MAX_PATCH_SIZE = 2.megabytes
+
+        def initialize(one_or_more_patches)
+          @patches = Array(one_or_more_patches).map do |patch_content|
+            Gitlab::Git::Patches::Patch.new(patch_content)
+          end
+        end
+
+        def content
+          @patches.map(&:content).join("\n")
+        end
+
+        def valid_size?
+          size < MAX_PATCH_SIZE
+        end
+
+        # rubocop: disable CodeReuse/ActiveRecord
+        # `@patches` is not an `ActiveRecord` relation, but an `Enumerable`
+        # We're using sum from `ActiveSupport`
+        def size
+          @size ||= @patches.sum(&:size)
+        end
+        # rubocop: enable CodeReuse/ActiveRecord
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/git/patches/commit_patches.rb b/lib/gitlab/git/patches/commit_patches.rb
new file mode 100644
index 00000000000..c62994432d3
--- /dev/null
+++ b/lib/gitlab/git/patches/commit_patches.rb
@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Git
+    module Patches
+      class CommitPatches
+        include Gitlab::Git::WrapsGitalyErrors
+
+        def initialize(user, repository, branch, patch_collection)
+          @user, @repository, @branch, @patches = user, repository, branch, patch_collection
+        end
+
+        def commit
+          repository.with_cache_hooks do
+            wrapped_gitaly_errors do
+              operation_service.user_commit_patches(user, branch, patches.content)
+            end
+          end
+        end
+
+        private
+
+        attr_reader :user, :repository, :branch, :patches
+
+        def operation_service
+          repository.raw.gitaly_operation_client
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/git/patches/patch.rb b/lib/gitlab/git/patches/patch.rb
new file mode 100644
index 00000000000..fe6ae1b5b00
--- /dev/null
+++ b/lib/gitlab/git/patches/patch.rb
@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Git
+    module Patches
+      class Patch
+        attr_reader :content
+
+        def initialize(content)
+          @content = content
+        end
+
+        def size
+          content.bytesize
+        end
+      end
+    end
+  end
+end