Cache diff highlight in discussions

This commit handles note diffs caching, which considerably improves
the performance on merge requests with lots of comments.
Important to note that the caching approach taken here is different
from `Gitlab::Diff::HighlightCache`. We do not reset the whole cache
when a new push is sent or anything else. That's because discussions
diffs are persisted and do not change.

2 files changed, 163 insertions, 0 deletions

diff --git a/spec/lib/gitlab/discussions_diff/file_collection_spec.rb b/spec/lib/gitlab/discussions_diff/file_collection_spec.rb
new file mode 100644
index 00000000000..0489206458b
--- /dev/null
+++ b/spec/lib/gitlab/discussions_diff/file_collection_spec.rb
@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::DiscussionsDiff::FileCollection do
+  let(:merge_request) { create(:merge_request) }
+  let!(:diff_note_a) { create(:diff_note_on_merge_request, project: merge_request.project, noteable: merge_request) }
+  let!(:diff_note_b) { create(:diff_note_on_merge_request, project: merge_request.project, noteable: merge_request) }
+  let(:note_diff_file_a) { diff_note_a.note_diff_file }
+  let(:note_diff_file_b) { diff_note_b.note_diff_file }
+
+  subject { described_class.new([note_diff_file_a, note_diff_file_b]) }
+
+  describe '#load_highlight', :clean_gitlab_redis_shared_state do
+    it 'writes uncached diffs highlight' do
+      file_a_caching_content = diff_note_a.diff_file.highlighted_diff_lines.map(&:to_hash)
+      file_b_caching_content = diff_note_b.diff_file.highlighted_diff_lines.map(&:to_hash)
+
+      expect(Gitlab::DiscussionsDiff::HighlightCache)
+        .to receive(:write_multiple)
+        .with({ note_diff_file_a.id => file_a_caching_content,
+                note_diff_file_b.id => file_b_caching_content })
+        .and_call_original
+
+      subject.load_highlight([note_diff_file_a.id, note_diff_file_b.id])
+    end
+
+    it 'does not write cache for already cached file' do
+      subject.load_highlight([note_diff_file_a.id])
+
+      file_b_caching_content = diff_note_b.diff_file.highlighted_diff_lines.map(&:to_hash)
+
+      expect(Gitlab::DiscussionsDiff::HighlightCache)
+        .to receive(:write_multiple)
+        .with({ note_diff_file_b.id => file_b_caching_content })
+        .and_call_original
+
+      subject.load_highlight([note_diff_file_a.id, note_diff_file_b.id])
+    end
+
+    it 'does not err when given ID does not exist in @collection' do
+      expect { subject.load_highlight([999]) }.not_to raise_error
+    end
+
+    it 'loaded diff files have highlighted lines loaded' do
+      subject.load_highlight([note_diff_file_a.id])
+
+      diff_file = subject.find_by_id(note_diff_file_a.id)
+
+      expect(diff_file.highlight_loaded?).to be(true)
+    end
+
+    it 'not loaded diff files does not have highlighted lines loaded' do
+      subject.load_highlight([note_diff_file_a.id])
+
+      diff_file = subject.find_by_id(note_diff_file_b.id)
+
+      expect(diff_file.highlight_loaded?).to be(false)
+    end
+  end
+end
diff --git a/spec/lib/gitlab/discussions_diff/highlight_cache_spec.rb b/spec/lib/gitlab/discussions_diff/highlight_cache_spec.rb
new file mode 100644
index 00000000000..fe26ebb8796
--- /dev/null
+++ b/spec/lib/gitlab/discussions_diff/highlight_cache_spec.rb
@@ -0,0 +1,102 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::DiscussionsDiff::HighlightCache, :clean_gitlab_redis_cache do
+  describe '#write_multiple' do
+    it 'sets multiple keys serializing content as JSON' do
+      mapping = {
+        3 => [
+          {
+            text: 'foo',
+            type: 'new',
+            index: 2,
+            old_pos: 10,
+            new_pos: 11,
+            line_code: 'xpto',
+            rich_text: '<blips>blops</blips>'
+          },
+          {
+            text: 'foo',
+            type: 'new',
+            index: 3,
+            old_pos: 11,
+            new_pos: 12,
+            line_code: 'xpto',
+            rich_text: '<blops>blips</blops>'
+          }
+        ]
+      }
+
+      described_class.write_multiple(mapping)
+
+      mapping.each do |key, value|
+        full_key = described_class.cache_key_for(key)
+        found = Gitlab::Redis::Cache.with { |r| r.get(full_key) }
+
+        expect(found).to eq(value.to_json)
+      end
+    end
+  end
+
+  describe '#read_multiple' do
+    it 'reads multiple keys and serializes content into Gitlab::Diff::Line objects' do
+      mapping = {
+        3 => [
+          {
+            text: 'foo',
+            type: 'new',
+            index: 2,
+            old_pos: 11,
+            new_pos: 12,
+            line_code: 'xpto',
+            rich_text: '<blips>blops</blips>'
+          },
+          {
+            text: 'foo',
+            type: 'new',
+            index: 3,
+            old_pos: 10,
+            new_pos: 11,
+            line_code: 'xpto',
+            rich_text: '<blips>blops</blips>'
+          }
+        ]
+      }
+
+      described_class.write_multiple(mapping)
+
+      found = described_class.read_multiple(mapping.keys)
+
+      expect(found.size).to eq(1)
+      expect(found.first.size).to eq(2)
+      expect(found.first).to all(be_a(Gitlab::Diff::Line))
+    end
+
+    it 'returns nil when cached key is not found' do
+      mapping = {
+        3 => [
+          {
+            text: 'foo',
+            type: 'new',
+            index: 2,
+            old_pos: 11,
+            new_pos: 12,
+            line_code: 'xpto',
+            rich_text: '<blips>blops</blips>'
+          }
+        ]
+      }
+
+      described_class.write_multiple(mapping)
+
+      found = described_class.read_multiple([2, 3])
+
+      expect(found.size).to eq(2)
+
+      expect(found.first).to eq(nil)
+      expect(found.second.size).to eq(1)
+      expect(found.second).to all(be_a(Gitlab::Diff::Line))
+    end
+  end
+end