Don't transform empty .ipynb diffs

Prevents displaying the transformed ipynb file diffs when we see that
after clean up no changes are left to be displayed.

Changelog: fixed

app/presenters/blob_presenter.rb

@@ -125,7 +125,7 @@ class BlobPresenter < Gitlab::View::Presenter::Delegated
   end
 
   def transformed_blob_data
-    @transformed_blob ||= if blob.path.ends_with?('.ipynb')
+    @transformed_blob ||= if blob.path.ends_with?('.ipynb') && blob.transformed_for_diff
                             IpynbDiff.transform(blob.data,
                                                 raise_errors: true,
                                                 options: { include_metadata: false, cell_decorator: :percent })

lib/gitlab/diff/file.rb

@@ -456,19 +456,27 @@ module Gitlab
         from = old_blob_lazy&.data
         to = new_blob_lazy&.data
 
-        new_diff = IpynbDiff.diff(from, to,
-                                  diff_opts: { context: 5, include_diff_info: true },
-                                  transform_options: { cell_decorator: :percent },
-                                  raise_if_invalid_notebook: true)
-
-        diff.diff = new_diff.scan(/.*\n/)[2..-1].join('') if new_diff
+        transformed_diff = IpynbDiff.diff(from, to,
+                                          diff_opts: { context: 5, include_diff_info: true },
+                                          transform_options: { cell_decorator: :percent },
+                                          raise_if_invalid_notebook: true)
+        new_diff = strip_diff_frontmatter(transformed_diff)
+
+        if new_diff
+          diff.diff = new_diff
+          new_blob_lazy.transformed_for_diff = true if new_blob_lazy
+          old_blob_lazy.transformed_for_diff = true if old_blob_lazy
+        end
 
         Gitlab::AppLogger.info({ message: new_diff ? 'IPYNB_DIFF_GENERATED' : 'IPYNB_DIFF_NIL' })
-
       rescue IpynbDiff::InvalidNotebookError => e
         Gitlab::ErrorTracking.track_exception(e, issue_url: 'https://gitlab.com/gitlab-org/gitlab/-/issues/344676')
       end
 
+      def strip_diff_frontmatter(diff_content)
+        diff_content.scan(/.*\n/)[2..-1]&.join('') if diff_content.present?
+      end
+
       def alternate_viewer_class
         return unless viewer.instance_of?(DiffViewer::Renamed)
 

lib/gitlab/git/blob.rb

@@ -24,7 +24,7 @@ module Gitlab
       LFS_POINTER_MIN_SIZE = 120.bytes
       LFS_POINTER_MAX_SIZE = 200.bytes
 
-      attr_accessor :size, :mode, :id, :commit_id, :loaded_size, :binary
+      attr_accessor :size, :mode, :id, :commit_id, :loaded_size, :binary, :transformed_for_diff
       attr_writer :name, :path, :data
 
       def self.gitlab_blob_truncated_true
@@ -127,6 +127,7 @@ module Gitlab
         # Retain the actual size before it is encoded
         @loaded_size = @data.bytesize if @data
         @loaded_all_data = @loaded_size == size
+        @transformed_for_diff = false
 
         record_metric_blob_size
         record_metric_truncated(truncated?)

spec/lib/gitlab/diff/file_spec.rb

@@ -52,7 +52,7 @@ RSpec.describe Gitlab::Diff::File do
   end
 
   describe 'initialize' do
-    context 'when file is ipynb' do
+    context 'when file is ipynb with a change after transformation' do
       let(:commit) { project.commit("f6b7a707") }
       let(:diff) { commit.raw_diffs.first }
       let(:diff_file) { described_class.new(diff, diff_refs: commit.diff_refs, repository: project.repository) }
@@ -77,6 +77,20 @@ RSpec.describe Gitlab::Diff::File do
         end
       end
     end
+
+    context 'when file is ipynb, but there only changes that are removed' do
+      let(:commit) { project.commit("2b5ef814") }
+      let(:diff) { commit.raw_diffs.first }
+      let(:diff_file) { described_class.new(diff, diff_refs: commit.diff_refs, repository: project.repository) }
+
+      before do
+        stub_feature_flags(jupyter_clean_diffs: true)
+      end
+
+      it 'does not recreate the diff' do
+        expect(diff_file.diff.diff).to include('execution_count')
+      end
+    end
   end
 
   describe '#diff_lines' do

spec/presenters/blob_presenter_spec.rb

@@ -126,6 +126,10 @@ RSpec.describe BlobPresenter do
       let(:blob) { repository.blob_at('f6b7a707', 'files/ipython/markdown-table.ipynb') }
       let(:git_blob) { blob.__getobj__ }
 
+      before do
+        allow(git_blob).to receive(:transformed_for_diff).and_return(true)
+      end
+
       it 'uses md as the transformed language' do
         expect(Gitlab::Highlight).to receive(:highlight).with('files/ipython/markdown-table.ipynb', anything, plain: nil, language: 'md')
 

spec/support/helpers/test_env.rb

@@ -53,7 +53,7 @@ module TestEnv
     'wip'                                => 'b9238ee',
     'csv'                                => '3dd0896',
     'v1.1.0'                             => 'b83d6e3',
-    'add-ipython-files'                  => 'c10c411',
+    'add-ipython-files'                  => '2b5ef814',
     'add-pdf-file'                       => 'e774ebd',
     'squash-large-files'                 => '54cec52',
     'add-pdf-text-binary'                => '79faa7b',