File upload UI obeys LFS filters

Uses Lfs::FileModificationHandler to coordinate LFS detection, creation of LfsObject, etc

Caveats:
 1. This isn't used by the multi-file editor / Web IDE
 2. This isn't used on rename. We'd need to be able to download LFS files
    and add them to the commit if they no longer match so not as simple.
 3. We only check the root .gitattributes file, so this should be improved
    to correctly check for nested .gitattributes files in subfolders.

app/services/files/create_service.rb

 module Files
   class CreateService < Files::BaseService
     def create_commit!
+      handler = Lfs::FileModificationHandler.new(project, @branch_name)
+
+      handler.new_file(@file_path, @file_content) do |content_or_lfs_pointer|
+        create_transformed_commit(content_or_lfs_pointer)
+      end
+    end
+
+    private
+
+    def create_transformed_commit(content_or_lfs_pointer)
       repository.create_file(
         current_user,
         @file_path,
-        @file_content,
+        content_or_lfs_pointer,
         message: @commit_message,
         branch_name: @branch_name,
         author_email: @author_email,

app/services/lfs/file_modification_handler.rb

+module Lfs
+  class FileModificationHandler
+    attr_reader :project, :branch_name
+
+    delegate :repository, to: :project
+
+    def initialize(project, branch_name)
+      @project = project
+      @branch_name = branch_name
+    end
+
+    def new_file(file_path, file_content)
+      if project.lfs_enabled? && lfs_file?(file_path)
+        lfs_pointer_file = Gitlab::Git::LfsPointerFile.new(file_content)
+        lfs_object = create_lfs_object!(lfs_pointer_file, file_content)
+        content = lfs_pointer_file.pointer
+
+        success = yield(content)
+
+        link_lfs_object!(lfs_object) if success
+      else
+        yield(file_content)
+      end
+    end
+
+    private
+
+    def lfs_file?(file_path)
+      repository.attributes_at(branch_name, file_path)['filter'] == 'lfs'
+    end
+
+    def create_lfs_object!(lfs_pointer_file, file_content)
+      LfsObject.find_or_create_by(oid: lfs_pointer_file.sha256, size: lfs_pointer_file.size) do |lfs_object|
+        lfs_object.file = CarrierWaveStringFile.new(file_content)
+      end
+    end
+
+    def link_lfs_object!(lfs_object)
+      project.lfs_objects << lfs_object
+    end
+  end
+end

changelogs/unreleased/jej-upload-file-tracks-lfs.yml

+---
+title: File Upload UI can create LFS pointers based on .gitattributes
+merge_request: 16412
+author:
+type: fixed

lib/carrier_wave_string_file.rb

+class CarrierWaveStringFile < StringIO
+  def original_filename
+    ""
+  end
+end

lib/gitlab/git/lfs_pointer_file.rb

+module Gitlab
+  module Git
+    class LfsPointerFile
+      def initialize(data)
+        @data = data
+      end
+
+      def pointer
+        @pointer ||= <<~FILE
+          version https://git-lfs.github.com/spec/v1
+          oid sha256:#{sha256}
+          size #{size}
+        FILE
+      end
+
+      def size
+        @size ||= @data.bytesize
+      end
+
+      def sha256
+        @sha256 ||= Digest::SHA256.hexdigest(@data)
+      end
+    end
+  end
+end

spec/lib/gitlab/git/lfs_pointer_file_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Git::LfsPointerFile do
+  let(:data) { "1234\n" }
+
+  subject { described_class.new(data) }
+
+  describe '#size' do
+    it 'counts the bytes' do
+      expect(subject.size).to eq 5
+    end
+
+    it 'handles non ascii data' do
+      expect(described_class.new("ääää").size).to eq 8
+    end
+  end
+
+  describe '#sha256' do
+    it 'hashes the content correctly' do
+      expect(subject.sha256).to eq 'a883dafc480d466ee04e0d6da986bd78eb1fdd2178d04693723da3a8f95d42f4'
+    end
+  end
+
+  describe '#pointer' do
+    it 'starts with the LFS version' do
+      expect(subject.pointer).to start_with('version https://git-lfs.github.com/spec/v1')
+    end
+
+    it 'includes sha256' do
+      expect(subject.pointer).to match(/^oid sha256:[0-9a-fA-F]{64}/)
+    end
+
+    it 'ends with the size' do
+      expect(subject.pointer).to end_with("\nsize 5\n")
+    end
+  end
+end

spec/services/files/create_service_spec.rb

+require "spec_helper"
+
+describe Files::CreateService do
+  let(:project) { create(:project, :repository) }
+  let(:repository) { project.repository }
+  let(:user) { create(:user) }
+  let(:file_content) { 'Test file content' }
+  let(:branch_name) { project.default_branch }
+  let(:start_branch) { branch_name }
+
+  let(:commit_params) do
+    {
+      file_path: file_path,
+      commit_message: "Update File",
+      file_content: file_content,
+      file_content_encoding: "text",
+      start_project: project,
+      start_branch: start_branch,
+      branch_name: branch_name
+    }
+  end
+
+  subject { described_class.new(project, user, commit_params) }
+
+  before do
+    project.add_master(user)
+  end
+
+  describe "#execute" do
+    context 'when file matches LFS filter' do
+      let(:file_path) { 'test_file.lfs' }
+      let(:branch_name) { 'lfs' }
+
+      context 'with LFS disabled' do
+        it 'skips gitattributes check' do
+          expect(repository).not_to receive(:attributes_at)
+
+          subject.execute
+        end
+
+        it "doesn't create LFS pointers" do
+          subject.execute
+
+          blob = repository.blob_at('lfs', file_path)
+
+          expect(blob.data).not_to start_with('version https://git-lfs.github.com/spec/v1')
+          expect(blob.data).to eq(file_content)
+        end
+      end
+
+      context 'with LFS enabled' do
+        before do
+          allow(project).to receive(:lfs_enabled?).and_return(true)
+        end
+
+        it 'creates an LFS pointer' do
+          subject.execute
+
+          blob = repository.blob_at('lfs', file_path)
+
+          expect(blob.data).to start_with('version https://git-lfs.github.com/spec/v1')
+        end
+
+        it "creates an LfsObject with the file's content" do
+          subject.execute
+
+          expect(LfsObject.last.file.read).to eq file_content
+        end
+
+        it 'links the LfsObject to the project' do
+          expect do
+            subject.execute
+          end.to change { project.lfs_objects.count }.by(1)
+        end
+      end
+    end
+  end
+end