Speedup DiffNote#active? on discussions, preloading noteables and avoid touching git repository to return diff_refs when possible

- Preloading noteable we share the same noteable instance when more than one 
discussion refers to the same noteable.
- Any other call to that object that is cached in that object will be for any 
discussion.
- In those cases where merge_request_diff has all the sha stored to build a diff_refs get that 
diff_refs using directly those sha instead accessing to the git repository to first get the 
commits and later the sha.

2 files changed, 26 insertions, 2 deletions

diff --git a/spec/models/diff_note_spec.rb b/spec/models/diff_note_spec.rb
index af8e890ca95..1fa96eb1f15 100644
--- a/spec/models/diff_note_spec.rb
+++ b/spec/models/diff_note_spec.rb
@@ -119,7 +119,7 @@ describe DiffNote, models: true do
 
       context "when the merge request's diff refs don't match that of the diff note" do
         before do
-          allow(subject.noteable).to receive(:diff_refs).and_return(commit.diff_refs)
+          allow(subject.noteable).to receive(:diff_sha_refs).and_return(commit.diff_refs)
         end
 
         it "returns false" do
@@ -168,7 +168,7 @@ describe DiffNote, models: true do
 
         context "when the note is outdated" do
           before do
-            allow(merge_request).to receive(:diff_refs).and_return(commit.diff_refs)
+            allow(merge_request).to receive(:diff_sha_refs).and_return(commit.diff_refs)
           end
 
           it "uses the DiffPositionUpdateService" do
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index a0e3c26e542..21d22c776e9 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -686,4 +686,28 @@ describe MergeRequest, models: true do
       subject.reload_diff
     end
   end
+
+  describe "#diff_sha_refs" do
+    context "with diffs" do
+      subject { create(:merge_request, :with_diffs) }
+
+      it "does not touch the repository" do
+        subject # Instantiate the object
+
+        expect_any_instance_of(Repository).not_to receive(:commit)
+
+        subject.diff_sha_refs
+      end
+
+      it "returns expected diff_refs" do
+        expected_diff_refs = Gitlab::Diff::DiffRefs.new(
+          base_sha:  subject.merge_request_diff.base_commit_sha,
+          start_sha: subject.merge_request_diff.start_commit_sha,
+          head_sha:  subject.merge_request_diff.head_commit_sha
+        )
+
+        expect(subject.diff_sha_refs).to eq(expected_diff_refs)
+      end
+    end
+  end
 end